Historical Day Lengths on Earth

Historical Day Lengths on Earth

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I recently read something about a major earthquake in Japan having possibly shortened Earth's day due to an increase in rotation. Here's an example:

When I read some more about Earth's day length, I learned that the length of Earth's day is believed to have been shorter in the past and getting longer over time. Here's an example from this site:

What was the length of the solar day 73 million years ago?

As some may know from an introductory psychology course, humans tend to settle into a day that is longer than 24 hours when placed in an environment with no external time or light cues. For most people, their circadian system is able to adjust their day to the 24-hour Earth day. Some blind people lose this ability, as seen from commercials about medication to help them gain a 24-hour rhythm. Here is some background about that:

At the risk of asking an unacceptable "opinion" question, how convincing is the evidence of historical Earth day lengths? Is the evidence at a point where shorter Earth days in the past are an accepted certainty? Or are things still at a point of being a best guess without really knowing? It would seem rather odd for our nervous systems to have evolved with a preference for days longer than 24 hours. It is certainly possible, but I would appreciate some input here before spending time contemplating why evolution might have worked that way. I'm only a hobbyist, so there would be no need to go into too much depth for me specifically. Thanks in advance.

Edit: The current answers and comments are interesting and appreciated, but to clarify, the intended focus is the extent and quality of the astronomical evidence that could be used in a discussion (elsewhere) about circadian issues.

From a physics point of view, there's no obvious mechanism that could provide significant, consistent increases in the earth's rotation speed.

The moon's gravitation appears to be the main driver of changes to the earth's rotation. And since it revolves more slowly than the earth's rotation, it tends to pull energy from the rotation, lengthening the day. This is completely consistent with the evidence that points to ancient years on earth having more (and therefore shorter) days than is true in the modern era. This is also consistent with the observed gradual recess of the moon away from the earth.

It is also possible to directly measure the length of a day and see that days on earth are significantly longer than they were in 1900. Again, consistent with a slowing rotation.

The implications of this for the evolution of sleep cycles would indeed be better in another location (perhaps Biology SE). But I'd suggest that this is a very slow process.

Remember that this is something that increases the length of the average day. But there are lots of other variations between days during a year. An organism might for instance synch to something like sunrise. But in spring, sunrises come more than a minute earlier than 24 hours apart each day, and in fall they come more than a minute later than average each day.

Meanwhile, it was probably more than a million years ago that the average day was shorter by a full minute. So the increase in day length over time is definitely happening, but at a rate that is almost irrelevant to biology.

Our biological clocks are ancient. One theory about the >24 hour period is that the clocks evolved in the sea, where the tidal period can be more important than the solar day length.

The tides are primarily caused by the Moon, and in the modern era the Moon rises roughly 50 minutes later each day, plus or minus 20 minutes, due to the eccentricity, inclination, and general complexity of the lunar orbit. The actual tidal changes at a given location are quite complex, but that lunar period gives a reasonable first approximation. So it's actually a good strategy to have a clock with a period of 24h 50m but which can easily be adjusted to be in synch with sensory input.

That 24h 50m lunar period would have been different hundreds of millions of years ago, when our ancestors were in the sea. But I'm not sure how different, since although the solar day length was shorter, so was the lunar orbital period, and so the Moon' daily motion relative to the Sun was greater.

It can be observed. The astronomical constants at 2000AD_500AD _ 1604AD are given below. The Àryabhatíya of Àryabhata, the oldest precise astronomical constant?

Here are some tables.This table is from Jacobs.

Table 1. Comparison of The Àryabhatiya of Àryabhata and Astronomic values.(orbit times are slowing down ever so slightly) Astronomy Constants AD2000 _ AD 500 _ 1604 BC

Rotations per solar orbit 366.25636031_ 366.2563589_ 366.25635656

Days per solar orbit 365.25636031_ 365.2563589_ 365.25635656

Days per lunar orbit 27.32166120_ 27.3216638_ 27.32166801

Rotations per lunar orbit 27.39646289_ 27.39646514_ 27.39646936

See the number of decimal places. It goes on like this to the Rg veda. India had a passion for astronomy and was very good at it!

Other links. Amartya Kumar Dutta - Aryabhata and Axial Rotation of Earth

Orbits decay ever so slightly in 1000 year time scales!

Systemic geophysical investigations into past variation of the earth's LOD (length of day) date at least from the 1960's (Wells, J. W. 1963. Coral growth and geochronometry. Nature. 197:948-950.) Growth indicators--persistent lines or layers deposited at intervals ranging from 6-hour tide cycles to annual--in mollusks, corals, and other invertebrates, both modern and fossilized, give us an objective record of major aspects of the Earth-Moon system, including LOD. Geology offers other corroborating techniques that suggest a monotonic but slightly varying deceleration of the Earth's rotation, and therefore lengthening of day, of about 20 minutes per 100 million years.

Given the relatively slow changes in LOD, the OP can reasonably wonder why mammals don't adhere more closely to a 24-hour or slightly longer daily cycle. The only good answers for the variability of circadian rhythms in mammals are empirical.

The History of Earth Day

Every year on April 22, Earth Day marks the anniversary of the birth of the modern environmental movement in 1970.

Let’s take a look at the last half-century of mobilization for action:


Earth Day 1970 gave a voice to an emerging public consciousness about the state of our planet —

In the decades leading up to the first Earth Day, Americans were consuming vast amounts of leaded gas through massive and inefficient automobiles. Industry belched out smoke and sludge with little fear of the consequences from either the law or bad press. Air pollution was commonly accepted as the smell of prosperity. Until this point, mainstream America remained largely oblivious to environmental concerns and how a polluted environment threatens human health.

However, the stage was set for change with the publication of Rachel Carson’s New York Times bestseller Silent Spring in 1962. The book represented a watershed moment, selling more than 500,000 copies in 24 countries as it raised public awareness and concern for living organisms, the environment and the inextricable links between pollution and public health.

Earth Day 1970 would come to provide a voice to this emerging environmental consciousness, and putting environmental concerns on the front page.


As the millennium approached, Hayes agreed to spearhead another campaign, this time focused on global warming and a push for clean energy. With 5,000 environmental groups in a record 184 countries reaching out to hundreds of millions of people, Earth Day 2000 built both global and local conversations, leveraging the power of the Internet to organize activists around the world, while also featuring a drum chain that traveled from village to village in Gabon, Africa. Hundreds of thousands of people also gathered on the National Mall in Washington, DC for a First Amendment Rally.

30 years on, Earth Day 2000 sent world leaders a loud and clear message: Citizens around the world wanted quick and decisive action on global warming and clean energy.


As in 1970, Earth Day 2010 came at a time of great challenge for the environmental community to combat the cynicism of climate change deniers, well-funded oil lobbyists, reticent politicians, a disinterested public, and a divided environmental community with the collective power of global environmental activism. In the face of these challenges, Earth Day prevailed and EARTHDAY.ORG reestablished Earth Day as a major moment for global action for the environment.

Over the decades, EARTHDAY.ORG has brought hundreds of millions of people into the environmental movement, creating opportunities for civic engagement and volunteerism in 193 countries. Earth Day engages more than 1 billion people every year and has become a major stepping stone along the pathway of engagement around the protection of the planet.


Today, Earth Day is widely recognized as the largest secular observance in the world, marked by more than a billion people every year as a day of action to change human behavior and create global, national and local policy changes.

Now, the fight for a clean environment continues with increasing urgency, as the ravages of climate change become more and more apparent every day.

As the awareness of our climate crisis grows, so does civil society mobilization, which is reaching a fever pitch across the globe today. Disillusioned by the low level of ambition following the adoption of the Paris Agreement in 2015 and frustrated with international environmental lethargy, citizens of the world are rising up to demand far greater action for our planet and its people.

The social and cultural environments we saw in 1970 are rising up again today — a fresh and frustrated generation of young people are refusing to settle for platitudes, instead taking to the streets by the millions to demand a new way forward. Digital and social media are bringing these conversations, protests, strikes and mobilizations to a global audience, uniting a concerned citizenry as never before and catalyzing generations to join together to take on the greatest challenge that humankind has faced.

By tapping into some of the learnings, outcomes, and legacy of the first Earth Day, EARTHDAY.ORG is building a cohesive, coordinated, diverse movement, one that goes to the very heart of what EARTHDAY.ORG and Earth Day are all about — empowering individuals with the information, the tools, the messaging and the communities needed to make an impact and drive change.

We invite you to be a part of Earth Day and help write many more chapters—struggles and victories—into the Earth Day book.

Historical Day Lengths on Earth - Astronomy

What is the length of day at the equator? I recently told a friend with great certainty that at the equator the sun rose and set at exactly 12 hour intervals at roughly 6 a.m. and 6 p.m. (depending on the time zone) every day of the year. Now I'm not so sure.

You're right. It is always exactly 12 hours. There are a couple of different ways you can convince yourself that this is so, but I think the easiest one is this symmetry argument:

In the Northern Hemisphere, the length of the day is longer during the months when the North Pole is tilted towards the Sun and shorter during the months when it's tilted away from the Sun. The reverse is true for the Southern Hemisphere. The Equator is exactly halfway in between the poles. So it wouldn't make any sense for a day on the equator to be longer when one of the poles is tilted towards the Sun, and shorter when the other one is.

This page was last updated on June 27, 2015.

About the Author

Christopher Springob

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies. He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.

How Long Is a Day?

Can’t “day” just mean a period of time? Darius and Karin Viet explain.

I wanted to comment on an article or part of a book that I read. Specifically, this

If the days of creation are really geologic ages of millions of years, then the gospel message is undermined at its foundation because it puts death, disease, thorns, and suffering before the Fall. The effort to define days as geologic ages results from an erroneous approach to Scripture reinterpreting the Word of God on the basis of the fallible theories of sinful people.”

This is wrong. Really wrong. This is possibly the worst thing I have ever read relating to the bible. I do not know what bible this person read, or how they read it, but they did it wrong. To start, the Fall happened eons before the creation of earth. Second, who made this guy the end all be all of how to interpret the bible ? Third (this is very important), when the bible was originally written, it didn’t have the word day in it. Any one who paid attention in seminary knows that the original Greek word meant “period of time,” which could be one nano second, or a billion years. Those of us who choose to define “period of time” as a billion years would actually seem more correct, because if the bible says Earth was made in six “periods of time,” and science tells us that the earth was created in 6 billion years, then one could conclude that because of the infinity of God , a billion years to him would seem like a day to us.

I hope that the creators of this site realize that it was God who created science, and to completely disregard what God created is an affront to Him and everything He has ever done for us.

Thank you for contacting AiG. We’d like to address your concerns in this reply.

First, why is there no general email for comments on the website or articles?

You may make comments at the “Inquiries and Comments” section of the website.

As such, I wanted to comment on an article or part of a book that I read. Specifically, this “If the days of creation are really geologic ages of millions of years, then the gospel message is undermined at its foundation because it puts death, disease, thorns, and suffering before the Fall. The effort to define days as geologic ages results from an erroneous approach to Scripture reinterpreting the Word of God on the basis of the fallible theories of sinful people.”

The article you are referencing is “Could God Really Have Created Everything in Six Days?,” a chapter in The New Answers Book 1. In this chapter, Ken Ham shows how reading the creation account in a straightforward way, applying the proper hermeneutic for interpreting historical narrative, and considering the other passages of Scripture on this subject will lead one to conclude that the word for “day” in the creation account is a 24-hour day.

This is wrong. Really wrong. This is possibly the worst thing I have ever read relating to the bible . I do not know what bible this person read, or how they read it, but they did it wrong. To start, the Fall happened eons before the creation of earth.

How do you know that “the Fall happened eons before the creation of the earth”? Our beliefs are based on one of two starting points. We either start with the infallible Word of God or the fallible opinions of man. According to the infallible Word of God, the Fall occurred on earth when Adam and Eve ate from the tree of the knowledge of good and evil. So the Fall obviously happened after the creation of the earth.

Second, who made this guy the end all be all of how to interpret the bible ?

Ken Ham is certainly not the end all be all of how to interpret the Bible. The Bible itself sets the standard for its interpretation. We invite you to consider Part 1 and Part 2 of this article on Bible interpretation principles. Part 2 specifically shows how Genesis 1–11 should be interpreted as historical narrative, according to the following principles: carefully observe the text, context is key, clarity of Scripture, compare Scripture with Scripture, classification of the text, and the church’s historical view.

Third (this is very important), when the bible was originally written, it didn’t have the word day in it. Any one who paid attention in seminary knows that the original Greek word meant “period of time,” which could be one nano second, or a billion years.

Respectfully, the original word is a Hebrew, not Greek, word, since the Old Testament was written primarily in Hebrew (with a few sections in Aramaic). The New Testament was written in Greek. While the Hebrew word for “day” can refer to an indeterminate period of time or the daylight portion of the day, the context gives us the proper interpretation. Dr. Terry Mortenson said, “Everywhere else in the Old Testament, when the Hebrew word for ‘day’ ( יוֹם , yom) appears with ‘evening’ or ‘morning’ or is modified by a number (e.g., ‘sixth day’ or ‘five days’), it always means a 24-hour day.”1

You are correct that many Christian seminaries don’t hold to a literal interpretation of Genesis. That’s because they have compromised with worldly ideas instead of standing firm on God’s Word, as shown by the book Already Compromised.

Those of us who choose to define “period of time” as a billion years would actually seem more correct, because if the bible says Earth was made in six “periods of time,” and science tells us that the earth was created in 6 billion years, then one could conclude that because of the infinity of God , a billion years to him would seem like a day to us.

First, claiming that “science tells us” is a logical fallacy of reification. We interpret the evidence of past events according to our worldviews. Second, you may be referencing 2 Peter 3:8 , which says, “ with the Lord one day is as a thousand years, and a thousand years as one day. ” Ken Ham addressed this verse, showing how it has nothing to do with the creation account and uses the word “as” because it is making a comparison, not a literal statement that a day equals a thousand years. This verse does not refer to the creation account but instead shows how God is not limited by time, so even though Christ’s return is delayed, the interval of time since His ascension is nothing to God .

I hope that the creators of this site realize that it was God who created science, and to completely disregard what God created is an affront to Him and everything He has ever done for us.

You are right that God is the Author of scientific laws. However, He is also the Author of the Bible, His special revelation, in which He tells us He created the earth—in six days ( Genesis 1 Exodus 20:11 ). Jesus Himself affirmed a young earth. We should trust God’s complete and inerrant Word instead of the fallible opinions of man. This doesn’t mean Christians are anti-science. Rather, when interpreting scientific evidence, a Christian’s starting point should be the Word of the One who created everything.

The biblical six-day position is essential to upholding the integrity of Scripture and leads to a clear understanding of the gospel: the sin of Adam and Eve brought death, but God sent His Son to sacrifice His life for sinners who turn in faith to the Lord Jesus.

Polish astronomer Copernicus is born

On February 19, 1473, Nicolaus Copernicus is born in Torun, a city in north-central Poland on the Vistula River. The father of modern astronomy, he was the first modern European scientist to propose that Earth and other planets revolve around the sun.

Copernicus was born into a family of well-to-do merchants, and after his father’s death, his uncle–soon to be a bishop–took the boy under his wing. He was given the best education of the day and bred for a career in canon (church) law. At the University of Krakow, he studied liberal arts, including astronomy and astrology, and then, like many Polish people of his social class, was sent to Italy to study medicine and law.

While studying at the University of Bologna, he lived for a time in the home of Domenico Maria de Novara, the principal astronomer at the university. Astronomy and astrology were at the time closely related and equally regarded, and Novara had the responsibility of issuing astrological prognostications for Bologna. Copernicus sometimes assisted him in his observations, and Novara exposed him to criticism of both astrology and aspects of the Ptolemaic system, which placed Earth at the center of the universe.

Copernicus later studied at the University of Padua and in 1503 received a doctorate in canon law from the University of Ferrara. He returned to Poland, where he became a church administrator and doctor. In his free time, he dedicated himself to scholarly pursuits, which sometimes included astronomical work. By 1514, his reputation as an astronomer was such that he was consulted by church leaders attempting to reform the Julian calendar.

The cosmology of early 16th-century Europe held that Earth sat stationary and motionless at the center of several rotating, concentric spheres that bore the celestial bodies: the sun, the moon, the known planets, and the stars. From ancient times, philosophers adhered to the belief that the heavens were arranged in circles (which by definition are perfectly round), causing confusion among astronomers who recorded the often eccentric motion of the planets, which sometimes appeared to halt in their orbit of Earth and move retrograde across the sky.

In the second century A.D., the Alexandrian geographer and astronomer Ptolemy sought to resolve this problem by arguing that the sun, planets, and moon move in small circles around much larger circles that revolve around Earth. These small circles he called epicycles, and by incorporating numerous epicycles rotating at varying speeds he made his celestial system correspond with most astronomical observations on record.

The Ptolemaic system remained Europe’s accepted cosmology for more than 1,000 years, but by Copernicus’ day accumulated astronomical evidence had thrown some of his theories into confusion. Astronomers disagreed on the order of the planets from Earth, and it was this problem that Copernicus addressed at the beginning of the 16th century.

Sometime between 1508 and 1514, he wrote a short astronomical treatise commonly called the Commentariolus, or “Little Commentary,” which laid the basis for his heliocentric (sun-centered) system. The work was not published in his lifetime. In the treatise, he correctly postulated the order of the known planets, including Earth, from the sun, and estimated their orbital periods relatively accurately.

For Copernicus, his heliocentric theory was by no means a watershed, for it created as many problems as it solved. For instance, heavy objects were always assumed to fall to the ground because Earth was the center of the universe. Why would they do so in a sun-centered system? He retained the ancient belief that circles governed the heavens, but his evidence showed that even in a sun-centered universe the planets and stars did not revolve around the sun in circular orbits. Because of these problems and others, Copernicus delayed publication of his major astronomical work, De revolutionibus orbium coelestium libri vi, or “Six Books Concerning the Revolutions of the Heavenly Orbs,” nearly all his life. Completed around 1530, it was not published until 1543–the year of his death.

In the work, Copernicus’ groundbreaking argument that Earth and the planets revolve around the sun led him to make a number of other major astronomical discoveries. While revolving around the sun, Earth, he argued, spins on its axis daily. Earth takes one year to orbit the sun and during this time wobbles gradually on its axis, which accounts for the precession of the equinoxes. Major flaws in the work include his concept of the sun as the center of the whole universe, not just the solar system, and his failure to grasp the reality of elliptical orbits, which forced him to incorporate numerous epicycles into his system, as did Ptolemy. With no concept of gravity, Earth and the planets still revolved around the sun on giant transparent spheres.

In his dedication to De revolutionibus—an extremely dense scientific work𠅌opernicus noted that “mathematics is written for mathematicians.” If the work were more accessible, many would have objected to its non-biblical and hence heretical concept of the universe. For decades, De revolutionibus remained unknown to all but the most sophisticated astronomers, and most of these men, while admiring some of Copernicus’ arguments, rejected his heliocentric basis. It was not until the early 17th century that Galileo and Johannes Kepler developed and popularized the Copernican theory, which for Galileo resulted in a trial and conviction for heresy. Following Isaac Newton’s work in celestial mechanics in the late 17th century, acceptance of the Copernican theory spread rapidly in non-Catholic countries, and by the late 18th century it was almost universally accepted.

A brief history of Radio Astronomy

To start with, a very short history of radio astronomy would be helpful. Radio astronomy was born in the early 1930s when Karl Jansky, working for Bell Laboratories, was trying to determine the origin of a source of noise that was showing up in receivers operating in the 20 MHz region of the radio spectrum.

Jansky built a steerable antenna and began searching for the source of the noise by taking directional measurements. To his surprise, he discovered that this noise was from extraterrestrial sources. Jansky, enthused by his discovery, published his work, however the majority of astronomers at the time were decidedly underwhelmed by this discovery and for the most part dismissed it as either irrelevant or simply curious. There were a few inventive individuals who saw the potential for this noise from space.

One of them, Grote Reber, an electronics engineer and avid radio armature, had reviewed Jansky's original discovery and speculated that the signals were of thermal origin (caused by very hot objects), and as such they should be easier to detect at higher frequencies. Since Jansky's original work was done at 20 MHz (about 15 metre wavelength) and a beam width of about 25 degrees, Reber wanted to narrow the effective beam width to obtain finer detail. Reber reasoned that he should build his first receiver and antenna to operate at 3000 MHz (10cm wavelength) an extraordinary frequency at that time. With his own resources and enthusiasm, Reber built the first parabolic reflector radio telescope. Since this was deemed a private 'extracurricular' activity, Reber received no sponsorship or support. Besides being the first of its kind, it was also a huge structure. Basically built by a single individual, it was 9.5 metres (31 feet or 3 stories) in diameter.

The term 'Radio Telescope' had not been coined at the time, however Reber gets the credit for building the first one. Although he did not prove his original hypothesis, his work went on to detail the first radio map of the galactic plane and large portions of the sky. Reber published his work "Cosmic Static" in the late 1930's.

It was the search for static or noise that led to the development of the radio telescope, and it is essentially noise from the universe that the radio telescope detects. Buried in this roiling confusion is information that is specific in nature to astronomical objects and phenomena. This noise bears witness to the physical characteristics of the universe. The information is presented as a mixture of signal properties such as frequency, phase, amplitude and in some cases repetitive patterns. Also present is information that can be mathematically assembled into 'radio pictures' of these cosmic objects. Some signals arrive from finely defined sources that can be, by and large, considered as point sources (quasars and pulsars for example).

Other sources cover vast areas and can be thought of as wide field objects. These are clouds of dust and gas, star 'nurseries', galaxies and a plethora of other interesting goodies. To obtain information from these sources, the radio telescope must receive not only specific information but also all the 'noise' from these objects and their surroundings then reject what isn't wanted and record the results.

Radio frequency signals of extraterrestrial origin are extremely weak. As an example, if all the signal energy ever received from all the radio telescopes ever built (viewing objects other than the sun) were combined, there would not be enough total energy to melt a single snowflake.

The radio telescope must first concentrate signals gathered over a wide area and focus them into a small area. This is the same principle on which the reflecting optical telescope operates. The term "radio optics" refers to this similarity. Since the term 'light' really means electromagnetic radiation, all the same basic equations, theories and principles are applicable to radio, infrared or visible light. The big difference is that optical telescopes operate at extremely high frequencies and microscopic wavelengths, while their cousins the radio telescopes work at lower frequencies and longer wavelengths.

Resolution, which can also be expressed as beam width, is a function of the wavelength of the signal and the diameter of the reflector. At optical frequencies (blue-green light 600,000 GHz or a wavelength of .0005 mm) a 1 meter diameter "perfect" mirror will have a beam width of about .00003 degrees. The same mirror operating at radio frequencies (30 GHz for example with a wavelength of 1 cm) will have a beam width of about 6 degrees. As can be seen, the beam width for the radio telescope is about 200,000 times wider, thus yielding lower resolution observations. At first the solution to this was to build bigger and bigger reflectors, giving narrower beam widths and higher resolutions.

By the late 1950's reflectors of 100 metres (300 feet) across were being built. At diameters larger than this, a steerable reflector becomes far too heavy and cumbersome to be effectively used. The big problem is that the surface warps and deforms due to gravity and thus the effectiveness of the reflector is compromised. The one advantage of large reflectors is that with their very large gathering surface area they offer significant signal strength the down side of this is that they are very expensive to operate, maintain, and build.

Even with the large areas, one still must remember that the beam width is still wide compared to optical instruments. A 100 metre diameter radio telescope, operating at 10 cm wavelength, still only has the individual resolving ability of an optical mirror of about 5 mm (less than 1/4 inch). Even with such seemingly myopic resolution, the sheer size of these instruments allows for detection of weak sources billions of light-years away. In a later article I will discuss interferometry, a technique by which multiple radio telescopes can be combined to give the effective resolution of a single telescope many miles across. This process changes the apparently fuzzy world of the radio telescope to one of crystal clarity. Modern radio telescope arrays such as the VLA in New Mexico and the Caltech OVRO millimetre array have resolving abilities far beyond even the Hubble telescope.

The temperature of the radio telescope, its reflector, and its receiver are all sources of noise with which the observer must contend. Since everything with a temperature above absolute zero gives off electromagnetic noise in one form or another, and the fact that what a radio telescope 'sees' is essentially electromagnetic noise, the radio telescope needs to be highly selective and reject as much superfluous noise as possible.

One method of counteracting noise is to cool the receiving electronics to a temperature just a few degrees above absolute zero. This eliminates thermally generated noise in the electronics. Once this noise has been removed, the amplified signal of interest is then selectively amplified again, converted to more manageable frequency bands, divided into a series of adjacent channels and finally processed to detect the relative power or energy of the source along with frequency and phase detection.

Because a radio telescope is so sensitive, other methods of reducing noise are used. One is to reduce reflected and thermal noise from the ground. This is why many radio telescopes have a Cassegrain configuration (a secondary mirror reflects the signals back through a hole in the centre of the main reflector). Since the receiving electronics input focus points to the sky, picking up thermal and reflected noise from the ground is avoided.

The final method is to reduce the contributed noise from terrestrial sources. This translated means move the telescope away from the high density cities to some remote location where the local denizens, i.e. rabbits, moss, and life forms found under rocks, do not pollute the radio spectrum. This also usually means placing the telescope in a valley surrounded by mountains so that the terrain blocks a great deal of unwanted radio noise. Add to this the help of the local authorities to declare the surrounding area of the telescope as a 'radio free' zone and you have a reasonably quiet observing site. Finally when all this is combined, the effective noise temperature of an entire radio telescope system can be reduced to only a few tens of degrees above absolute zero, (quite an improvement when considered that typical room temperature is about 300 Kelvin).

A signal arriving from a celestial source has now been gathered by a large reflector, concentrated into a small area and fed to a low noise electronic receiver that is isolated from strong external sources, quiet in its own operation and highly selective. The next part of the process is to store the information for subsequent processing. Since many of the radio source signals are so weak, it is often necessary for a telescope to stay fixed on a target for extended lengths of time to insure sufficient information has been gathered. The result of these long 'exposure times' (to borrow a phrase from photography), results in huge amounts of data. In the early days of radio astronomy, information was recorded on paper, which chart recorders spewed out by the mile, and consequently the astronomer had to inspect visually, by the mile. This was an arduous process and sometimes required months to extract the information.

In the 1960s magnetic tape was substituted for paper and computers were given the task of correlating the information. Today with inexpensive desktop computers, flash analogue to digital converters, and billion operation per second digital signal processing chips, much of the information obtained can be processed in real time. It is the results of the computations on the raw signal data that carries the ultimate useful information. With faster and faster real time processing, the storage of information has shifted from saving the raw incoming signals to saving the derivatives and ultimately to saving only the specific information. This not only reduces the total storage required (raw signals require magnitudes more storage) but allows for faster retrieval of pertinent information since the data has been prefiltered and formatted.

Last, but not least, is the interpretation of the data into a meaningful format. Despite our ability to interpret numbers and form abstract conclusions, we human beings are visually oriented. The information from a radio telescope can indeed be turned into a picture that is easy to understand. However, along with this visual presentation comes volumes of additional information that, when analysed, reveals the secret workings of much of the universe. This information is often intangible to our senses. Properties such as phase, coherence, polarisation and subtle frequency variations cannot be discerned from a simple picture. Additional signal processing and receiving techniques must be used to reveal these characteristics. Often, the presentation of these other qualities will be in a visual or pictorial format, but the colours and intensities will demonstrate properties not normally visible. These 'false colour' images present to the mind visualisations of concepts and properties heretofore unobservable.

The radio telescope, while not as basically easy to use as a simple optical instrument, actually reveals much more information to the observer. With its ability to cover a much wider portion of the electromagnetic spectrum, the radio telescope shows much more of the inner workings of the universe. The intrinsic composition of interstellar clouds, the birth of stars, and the properties of stars whose lives have passed, are all observable with the radio telescope where these mysteries are masked to the optical instruments. Now with the combination of highly accurate optical and radio imaging, the cosmos is beginning to become comprehensible.

Jim Fredsti is a Research Engineer at
Owens Valley Radio Observatory,
California Institute of Technology,
Big Pine, California, USA.

This article is the second in a series on Radio Astronomy, bookmark this page as the following articles will be uploaded shortly. To return to the first article: first radio astronomy article.

Transit of Venus and the Distance to the Sun

Fig: 1: Earth (blue), Venus (grey) and the Sun (orange), not drawn to scale. If Venus’s orbit (black dashed circle sitting inside grey rectangle) were aligned perfectly with Earth’s orbit (blue dashed circle sitting inside light blue rectangle), then every time Venus passed between Earth and the Sun there would be a transit — Venus would appear from Earth to move across the face of the Sun.

Much has been written and is still being written about the 2012 transit across the Sun by Venus. You can read in many places (here’s a good one) about how rare this transit is, and why it is so rare: naively Venus, which circles the sun more rapidly than does the Earth, should pass between the Earth and the Sun once every orbit (Figure 1), but because the orbits of the two planets are not well aligned (Figure 2) Venus often appears to pass above or below the Sun from the Earth’s perspective.

Rather than rehash what so many have written about, I wanted to add a few little details that aren’t so easy to find on the internet.

You may have read that with a technique based on earlier reasoning from 1678 – 1716 by astronomer Edmund Halley (of Halley’s Comet fame) and James Gregory before him, the transit of Venus in 1761 was used to determine the distance from the Earth to the Sun (and to Venus and all the other planets) with to a precision of about 2 percent, by far the highest precision yet obtained. (It had been hoped the measurement would be about ten times more precise, but an unexpected optical effect, called the “black drop effect”, whose cause still generates controversy, interfered.) But you may not have read that this measurement was based — as are so many measurements of distance in astronomy, out to the relatively nearby stars — on the principle of parallax, the same geometrical fact used by our eyes and brains to produce depth perception, our ability to tell how far away objects are just by looking at them.

Fig. 2: Earth (blue), Venus (grey) and the Sun (orange), not drawn to scale. Venus orbit (black circle within grey rectangle) is tilted relative to Earth’s orbit (blue circle within light blue rectangle.) The degree of tilt is exaggerated here. Since Earth and Venus orbit the sun at different rates, they may pass each other anywhere along their orbits. Top: Most of the time, when they pass each other, Venus lies below or above (green line) the line between the Earth and the Sun (red line) and no transit occurs. Bottom: Only on the rare occasions that the line connecting the Earth and the Sun is also the line where the two orbital planes intersect does Venus lie on or near the same line, leading to a transit.

Without parallax, it isn’t hard to figure out how far Venus is relative to the Sun — that is, to determine the ratio of the radius of Venus’s orbit LV to the radius of Earth’s orbit LE. That’s why it was widely understood quite early in Renaissance astronomy what the relative distances were from the planets to the Earth and to the Sun. But to determine LV and LE separately requires a parallax measurement, and a transit of Venus can provide a good one. The transits of Venus in the 1760s provided a rather precise measurement of LE – LV , the “absolute” distance from Earth to Venus and that allowed LE and LV and the distances to all the other planets to become known, to a precision of a couple of percent. (There was an earlier measurement of the distance from Earth to Mars that was precise to about ten percent, made in the late 1600s this too was based on parallax, but that’s another story.)

One preliminary point: The Earth and Venus, and even the Sun, are so small compared to the distances between them that drawing pictures that are really accurate is basically impossible. When making pictures that illustrate what is going on, it is always necessary to make the planets look bigger than they are, relative to the distances between them, just so you can understand the important conceptual points being made. Do keep this in mind! None of my pictures below are to scale — they can’t be.

The Relative Size of Venus’ Orbit Compared to Earth’s Orbit

Fig. 3: The orbits of Earth (blue) and Venus (grey) around the Sun, approximating the orbits of Earth and Venus as circular and aligned. The Earth orbits the Sun, of course, but at any moment we may choose to draw the Earth as off to the left of the Sun. At that moment Venus may be anywhere in its orbit. Over time, the angle between the Sun and Venus, grows and shrinks, with a maximum that is the angle between the violet and orange lines. Notice this angle is necessarily less than 90 degrees, because Venus’s orbit has a smaller radius than Earth’s.

To understand the basic reason why it is easy to determine LV/LE , we’re going to assume, for the purposes of making an estimate and seeing the basic principles involved, that the orbits of Earth and Venus around the sun are circular and that they are aligned — that they lie in the same plane (shown in slanted perspective in Figure 1, and from directly “above” in Figure 3). In fact the orbits of Earth and Venus are slightly elliptical and they are not perfectly aligned (Figure 2), and as noted earlier this explains why transits of Venus are rare. But the ellipticity and non-alignment are minor points for the following argument, so we can ignore them initially, and put them back in later to get more accurate answers (which I would do, carefully, if I were teaching a class for future experts, but won’t do here as it doesn’t add much conceptual understanding.)

What we’re doing here is a physicist’s classic technique make an approximation that is sufficient for current purposes, and don’t work harder than necessary. It’s a very powerful way of thinking about science, and about knowledge in general — any question you ask only needs to be answered to a certain degree of precision, so use the simplest technique that gets you the answer to that level of precision. This method’s been used for centuries, to great effect, and it applies far beyond physics.

So we’ll make the approximation that the orbits are circular and aligned, and the answers we’ll get will be approximately correct, to a few percent. That will be enough to illustrate the basic conceptual principles involved, which is my current goal. You can trust me that it’s possible to make a much more accurate and precise calculation, or you can become an expert and figure it out for yourself. But the approximation used here will not only give a pretty good answer but will be enough to show you why it is easy to figure out the ratio of LV to LE, but not to figure out LE or LV separately.

During the year, as Earth and Venus orbit the Sun at different rates, the relative positions of Earth and Venus change relative to the Sun. If, on a particular date (day, month and year), I choose to draw a picture with the Sun at the center and the Earth off to the left, as in Figure 2, then Venus may be anywhere in its orbit, depending on the date. And that means, relative to Earth’s point of view, the angle between Venus and the Sun in the sky will vary, depending on the date. This is shown in Figure 3, where the angle is called γ. The angle is easily measured find Venus is in the sky just after sunset or just before sunrise, and measure the angle between Venus and the Sun see Figure 4.

Fig. 4: An easy way to measure the angles shown in Figure 3 is to look at Venus just after sunset (or before sunrise) when one can see on the sky how far Venus lies from the Sun. The position of Venus will change at each successive sunset, first growing to a large angle, then falling back toward the sun.

What you can see from Figure 3 is that γ has a maximum size, shown by the angle between the orange line and the violet line. As it travels in its orbit, Venus will appear in a different location at each sunset for a while, night after night, it will be in a higher location above the horizon, and then eventually begin to fall back toward the horizon. By watching Venus night after night, just after sunset, and measuring γ night after night, we can determine the maximum value of γ, which I’ll call γmax.

It’s obvious from Figure 3 that (as drawn in Figure 4) γmax is less than 90 degrees, because the violet line must lie between the orange and red lines, which are perpendicular. You can see geometrically that this is a consequence of the fact that Venus is always closer to the Sun than is the Earth. These angles explain why Venus is always visible either just after sunset or just before sunrise (except for the few days when it lies in front of or behind the Sun.) Venus can never be directly overhead after dark, for this would require it to lie to the left of the red line, which it can never do.

Fig. 5: When Venus reaches its maximum angle from the Sun from Earth’s perspective, the two planets and the Sun form a right-angle triangle, from which the ratio of Venus’s orbital radius to that of Earth can be easily determined. However, neither the radius of Earth’s orbit nor that of Venus may be separately determined by this method. (This remains true even in the more realistic case where the orbits are slightly non-circular and slightly tilted relative to one another.)

Now — we can determine the ratio of the radii of the two orbits — of LV to LE — using γmax. It’s simple geometry, Figure 5. The point is that when Venus is at its maximum angle from the Sun, the line from the Sun to Venus is perpendicular to the line from the Earth to Venus, and so the lines joining the three objects form a right-angle triangle. From this we obtain, using standard trigonometry, that

And from this (and other simple geometric arguments) we get the ratios of all the distances to the other planets.

Again, this isn’t exactly right, for the reasons mentioned at the start the planetary orbits are ellipses, and the ellipses don’t lie in the same plane. In other words, LE and LV aren’t exactly constant over the year, and γmax is actually something more complicated that has to be thought about in three dimensions, as in Figure 2, not two dimensions as in Figures 1, 3 and 5. But with precise measurements over many years of the positions of Venus and the Sun in the sky, it is possible to determine the precise orbits of Venus and the Earth around the Sun, and improve the argument. The main point is the same all of the measurements of the location of Venus and the Sun in the sky allow only a measurement of the relative sizes of the orbits of Venus and Earth. But the overall size — the actual values of LE and LV — cannot be determined. A different method is needed.

The Transit of Venus, Parallax, and the Distance to the Sun

If you aren’t familiar with parallax already, or just want a review, you can read my article on parallax.

The reason that a transit of Venus allows for a measurement of the absolute size of the Earth’s orbit and Venus’s orbit is that the transit of Venus can be observed with precision at different locations on the Earth, giving two different high-precision perspectives on the apparent location of Venus relative to the Sun, taken from positions that are separated by a known distance. This parallax measurement (a bit of a tricky one) in turn allows the absolute distance from Earth to Venus to be determined from the parallax angle and the distance between the two observing points on Earth, just as the different views of an object from our left and right eyes allows our brains to provide us with depth perception — a sense for how far away the object is.

Fig. 6: From a large Observing planet (blue), a perfectly aligned transit of a smaller Transiting planet (grey) across a star (orange). Top: seen from the “side”, the apparent location in the sky of the Transiting planet on the surface of the star for an observer on the equator of the Observing planet lies on the equator of the star but for an observer at the south pole of the Observing planet, the Transiting planet appears to be north of the star’s equator by an angle alpha. If the radius R of the large planet is known, this determines D, the distance between the two planets, by simple trigonometry. Bottom: the transits as seen by the observers at the equator (red) and the south pole (violet)

To illustrate the point, let me draw how this would work with large planets so you can see in a figure what’s going on. In Figure 6 I show a planet where a transit is to be Observed (later to be the Earth), and a planet that’s Transiting (later to be Venus), in front of a star. And I’m going to imagine the simplified situation (just to make the geometry more obvious and the basic point easier to visualize at first) where the planets and the star are perfectly aligned (which will not be the case for this year’s transit) so that from the point of view of someone on the equator of the observing planet, the transiting planet will appear to move along the equator of the star. This is shown from the “side” at the top of Figure 6 note the red line from the equator of the observing planet to the star, passing through the equator of the transiting planet.

With this perfect alignment, an observer on the equator of the outer planet will see the inner planet traverse the equator of the star. This is shown as the red curve in the lower part of Figure 6. But an observer at the south pole of the outer planet will see the inner planet traverse the star on a path (violet line) that lies north of the star’s equator. (The reverse would be true at the north pole.) If the angle α on the sky between the paths taken by the transiting planet, as seen from the equator and the pole of the observing planet, is measured, and the radius R of the observing planet is known, then we can draw a right-angle triangle that connects the transiting planet, the center of the observing planet, and the pole of the observing planet, whose small angle is α. Simple trigonometry then tells us that the two planets are a distance D apart during the transit, where

Fig. 7: As in Figure 6, but drawn somewhat more realistically for the case of Earth, Venus and the Sun, emphasizing the tremendous distances, tiny planets, and minuscule angles involved. Again this is shown in the unrealistic case where the orbits of Venus and Earth (and Earth’s axis of rotation) are perfectly aligned.

Now the same applies for the Earth, Venus and the Sun, except that the Earth and Venus are so small, and the distances between them and the Sun so vast, that it turns out that the angle α would be only about 1/20th of a degree! (That’s quite tiny but also quite measurable, but to measure the distance to the Sun precisely, as 18th century astronomers hoped to do, would require a very precise measurement of this tiny angle, which is not so easy.) That’s far too small an angle for me to draw, so you have to trust me that what actually is happening is just a very extreme version of what I drew in Figure 6, with the planets and the star (the Sun, of course) much smaller than I drew them, relative to the distances. Even what is shown in Figure 7 still makes the planets look far larger than they are. But the idea is the same: the distance DEV between the Earth and Venus during the transit can be determined be measuring the parallax angle α, (bottom of Figure 7 note the sun’s angular diameter is about 1/2 of a degree).

Now there are all sorts of unanswered questions here.

  • I’ve told you how to measure DEV, the distance from the Earth to Venus during the transit. But wasn’t the goal to measure LE and LV, the distance from Earth to the Sun and from Venus to the Sun?
  • Nobody went to the Earth’s south pole to watch Venus transit the sun in 1761 or 1769.
  • I assumed the Earth, Venus and the Sun were perfectly aligned, so that a point on the equator of the Earth would see Venus moving across the equator of the Sun. But that certainly isn’t the case, and isn’t even that close to being the case during a typical transit (and it won’t be in 2012, in particular).
  • The angle α is small enough that it is hard to measure precisely — especially since, in the days before photography and instantaneous communication, and lacking a clear indication of where is the Sun’s north pole, ensuring a precise comparison of two measurements of Venus’s path made at two very different locations on the earth would have been very challenging indeed. Yet the original goal was to measure the angle to better than 1 part in 500 (though this was set back to 1 part in 50 by the “black drop effect” mentioned earlier.)

Ok, so how do we get around these issues?

First, how do we go from a measurement of DEV to a measurement of what we want, LE and LV? That’s easy, because we already know all the ratios — in particular we already know LE/LV (approximately, from Figure 4, or more accurately, if we do our astronomy more carefully) from the maximum angle γmax between the Venus and the Sun as seen from Earth. We also have DEV = LE – LV = LE (1-LV/LE), from Figure 7. So we can get an (approximate) measure of LE by using

where α is the parallax angle measured during the transit and γmax is the maximum possible angle between Venus and the Sun (Figure 5). Doing it more precisely involves more elaborate geometry, but the basic ideas are the same.

Second, even if the alignment between the planets and the Sun were perfect, the two measurements of Venus’s path don’t have to be made at the equator and either pole of the Earth. They can be made at any two latitudes on the Earth. The geometry gets a tiny bit more complicated, but not much, and the principles remain the same. See Figure 8.

Fig. 8: Even when alignment is imperfect, the same basic principles as in Figure 6 apply, just with more complicated trigonometry observed from points at different latitudes of the Observing planet, the Transiting planet follows two different paths across the sun, due to parallax, and from this observation the distance between the two planets can be obtained.

Third, even without perfect alignment, there will still be a small parallax angle that arises when the measurements are made from two different points on the earth, and if that angle can be measured well, it can be turned (through somewhat more complicated equations) into a measurement of D. This point is also illustrated in Figure 8, at bottom.

Now the fourth issue — the challenge, especially historically, of measuring the angular shift α in the path of the Venus’s transit, is one which leads us to an alternative attempt to try to measure timing — either the duration of the transit, or just the time at the start or the end of the transit — rather than angles. The first was apparently proposed by Halley, based on earlier ideas of Gregory, and the latter was suggested as a further refinement by Delisle. (Halley’s method did not require different locations to have synchronized time Delisle’s later method did require it, and thus relied on more advanced clock technology.) It’s much easier to make a precise measurement of duration — even back in the 17th or 18th century — or of the moment of the beginning and end of the eclipse — than to precisely measure the location of Venus relative to the Sun’s disk, especially without photography. You can see that the purple and red paths of Venus crossing the sun are of slightly different lengths, because of the fact that they are not crossing the sun at the same location, and that means that duration of the transit will be different by an amount related to the parallax angle. Unfortunately things are more complicated than they look at first — because the Earth is rotating, and moving around the Sun, which means that a given observer is moving a considerable distance while Venus is transiting the Sun. So it takes some real effort (somewhat complicated and tricky, though very easy with modern computers) to determine the difference in the timing of the start and end of the transit that two different observers on the Earth will detect, depending on the distance to the Sun. Halley, at the turn of the 18th century, already understood all of the geometric principles involved (and if you subtract the dated English phraseology and style from his text, you may be impressed by the modern-sounding sophistication of his statements, and you will see that scientists of three hundred years ago were in many ways much like scientists today, possessed of the same intellect and lacking only scientific technology of the present.)

Fig. 9: What a real transit of Venus may look like as observed from different points on the Earth: misaligned with the Sun’s center and with the Earth’s orbital plane. It is easy to see that the lengths of the red line and the violet line that lie in front of the Sun are different this causes the length of the transit to be slightly shorter or longer for different observers on the Earth, in turn allowing an indirect and more precise measurement of the parallax angle.

All this is to say that parallax — that difference in apparent location that observers at a particular time but at different locations on Earth will ascribe to Venus relative to the Sun — was historically an important method by which the overall size of the solar system was determined. Today there are more powerful methods available, but you can enjoy knowing that what you see in the skies today has great historical significance… or you can simply enjoy the vision of Venus proceeding in its stately motion around our star.

Graphical Analyses of the Lengths of the Seasons

For each chart below you may click on the thumbnail image to view it as a PNG (Portable Network Graphics image) your web browser, or click on the PDF icon to view a higher-quality image of the chart using a PDF reader such as the freely available Adobe Acrobat Reader. All of the images and PDFs are well below 100 KB in size.

All of the following lengths of the seasons charts depict numerical integrations of the season lengths in terms of atomic days, whereas inhabitants of Earth actually experience the seasons as mean solar days. The variation in lengths of the seasons (measured in days), however, is much greater than the changing length of the mean solar day due to tidal forces (measured in milliseconds), so even if the lengths of seasons were replotted in terms of mean solar days there would not be any visually discernible differences.

Nevertheless it is easy to numerically discern the long-term change. The long-term mean season length given here in terms of atomic time is 91 days 7h 27m 12s. At the assumed rate of tidal slowing (mean solar day longer by 1.75 atomic milliseconds per century) the mean solar time season in 100000 BC was about 91 days 7h 29m 55s whereas in 100000 AD the mean solar time season will be about 91 days 7h 24m 35s.

The small wiggles in the plotted lines are not graphic artifacts, but are variations of several minutes from year-to-year caused mainly by gravitational interactions with Moon and to a lesser but non-negligible extent Venus and Jupiter.

At year 2007 AD, Spring = about 92+ 3 /4 days and getting shorter, Summer = about 93+ 2 /3 days and getting longer, Autumn = about 89+ 5 /6 days and getting longer, Winter = slightly less than 89 days and getting shorter, with an average season length in this data set of about 91 days 7h 27m 15s. The significance of the changing season lengths will become obvious in chart #2, next below.

The Northern Hemisphere dominates global weather patterns because it contains most of the land area. With its summer being the longest season and getting longer, while winter is the shortest season and getting shorter, for the next several millennia there will be an unavoidable cumulative trend toward global warming that is being further amplified by greenhouse gases, deforestation, desertification, and heat production by human activities.

In the present era perihelion is about a month ahead of mid-Winter, which it will reach around year 3850, so the length of Winter is approaching an extreme minimum and the length of Summer is approaching an extreme maximum, with correspondingly milder than usual temperatures during both seasons (for the northern hemisphere). The length of Spring will continue its past 2000 year steady decline for about another 4000 years until perihelion approaches the Spring equinox, after which the Vernal Equinoctial Year length will get shorter.

Study this chart carefully, in comparison with the description above of the effect of the perihelion cycle on season lengths, otherwise the longer-term charts below won't make much sense!

In this longer-range view, spanning nearly 3 perihelion cycles, the relationship between Earth orbital eccentricity (lavender curve, secondary y-axis) and the variations of season lengths is evident.

Here we can easily see that the length of the full perihelion cycle, indicated by the intervals between the color-coded vertical gridlines, varies with the mean Earth orbital eccentricity. Perihelion advances at a faster rate as the orbital eccentricity decreases.

Note that when Spring and Autumn are equal at the median, Winter and Summer are at their respective minimum and maximum extremes, but when Winter and Summer are equal at the median, Spring and Autumn are at their respective minimum and maximum extremes, as per chart #2 above.

Why Are Summer Days Long and Winter Days Short?

The Earth's tilt on its axis is what causes the change in the seasons and explains why summer days are longer than winter days. The Earth orbits in an ellipse around the Sun, and because of this, it draws closer to the Sun at some points than at others. It is the direction of the Earth's tilt in its axis that determines the length of days and nights.

An ellipse is an oval shape rather than a circle. In the summertime in the northern hemisphere, the Earth is farther from the Sun because of the ellipse in its orbit, but the angle of the Earth's tilt points the hemisphere towards the Sun, making the days longer. The Sun's angle is also higher during the summer months than the winter months. In the winter, the Earth's orbit draws it closer to the Sun, but the Earth's axis tilts away from the sun, making the days shorter in the northern hemisphere. The summer solstice marks the first day of the summer and the longest day of the year. This is because the North Pole is pointed the closest to the Sun than any other day of the winter. The reverse is true during the winter solstice when the North Pole is tilted the farthest from the Sun.

History of Earth in 24-hour clock

I’m not sure where this is originally from, but I found it on an intro to geology course page. What happens when midnight comes around again?



The only thing that bothers me is why this is a 24h clock… . )

So that it can be set to the tune of “Two minutes to midnight” by Iron Maiden.

I mean the clock says it’s a 24 hour clock, but the labels on the outside only go from 0 to 12…

Because it uses 24 hours to get from the origin of the Earth to now. Midnight to noon is 12 hours, and noon to midnight is another twelve. Look closer – the labels go from 0 to 12, then 0 to 12 again.

Good point – it would have been easier to comprehend if it used “Zulu time”, e.g. single celled algae would be at 14:08

Zulu is just GMT. It is a time zone, not a format of time. Not trying to be a dick, just informative. :-)

True a 24 hour clock should be in “World Time” or as we call it in the US “Military Time”

Who knows? We’ll probably never make it past 12:15 AM anyway. And I think I’m being awfully generous in that assessment.

And if they insist on making it a 24hour clock why do the labels show 12 hour notation? They don’t even include an am/pm indicator.

It does say AM on the left side and PM on the right side

Original concept is Carl Sagan’s Cosmic Calendar. But the Tree Of Life ( is my favorite take on insignificance of humans in time.

We will find out what happens at midnight come 12.21.2012

Midnight is NOW. So if the earth began 24 hrs ago, we have been here for 1m 17s. At least as of the day the illustration was created

Thank you so much for getting this. The diagram was fascinating, but then I couldn’t believe the author’s mistake when I read “What happens when midnight comes around again?”

Among all the comments that missed it, thank you for being the only one to realize that midnight is the present.

yeah, I thought it was kinda obvious…

These fossils at 5:36am what where they of if the first single cell algae started at 2:08pm? I am guessing bacteria or similar. Took a while to get from bacteria to algae no? Mind blowing stuff.

There’s definitely nitpicking to be had (@NickP points out the 24 hour vs am/pm notation discrepancy), and I would ask why the clock has hands**, but overall I have to say that it packs a lot of useful information into a small space. Certainly I am unable to come up with a better way to represent this information.

**This is a silly point, since without the hands, the entire “clock” metaphor falls apart.

The second day is just like the first day except that kids eat free at Perkins.

Dr. William “Bill” Schopf of UCLA (Earth and Space Sciences) came up with this clock concept as a graduate student in the 60’s. I think he still teaches at UCLA, you should go ask him about it!

compare to the Doomsday Clock of the Bulletin of the Atomic Scientists:

True that @Joel Goldstick and that is where I have a question. We have been here for 1m and 17s = 77seconds. That is 77seconds of 86400 seconds in 24 hours. This is around 0.089%. If the earth age is 4.5 billion then earliest humans should be around 4 millions years. I looked up and earliest earliest human fossils were found 2.5million years.

If only this info-graphic has told me 24hours = how many years?

The Singularity is at midnight, after that, we go in reverse.

Whoa…I thought God created Earth (oh…and everything else in the time-space continuum) only about 6,000 years ago. The History of Earth Clock seems to imply that Earth has been around a lot longer. This is all so confusing.

Don’t tell me – midnight on this clock is Dec 12, 2012?

I’m sorry but this infographic could have been better server is a different format. I’m really not a fan of this clock. First it’s confusing because it’s a 24 hour clock but the times are given in military time e.g. Sexual Reproduction is at 16:08. Also a clock implies that we’re going to be starting the whole process again. Is that what you’re suggesting? I say a good old-fashioned timeline would be better servered here.

my god are you all thick. its the age of the earth represented as one day. there is no tomorrow in it, 12 midnight is now.

Watch the video: Μίκυ Μάους μικρο-ιστορίες - Μια κουφή μέρα (May 2022).