Astronomy

What is the right ascension overhead at Greenwich at noon on 21 march

What is the right ascension overhead at Greenwich at noon on 21 march


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My question is seen in the title: What is the right ascension overhead at Greenwich at noon on 21 march

I'm a little bit confused since the right ascension is set by the vernal equinox and don't understand how this is related to Greenwich.


At the vernal equinox, the sun crosses the celestial equator, and by definition, it has an RA of 0h (at the current epoch). At mid-day on that day, the sun will be more or less due south, and as the line of zero RA forms a great circle, the RA "overhead" will also be zero.

The sun won't be exactly zero RA except at the moment of the equinox, and also the length of the solar day varies throughout the year, which results in the time at which the sun is due south is not quite 12 noon by clock time.

All the above is independent of whether or not you are observing from Greenwich.


Terrestrial Coordinates

Lines of Latitude can be run north and south of the Equator. These are a series of Small Circles and are shown on the left sphere below. Small circles are circles drawn on a sphere that are smaller than the sphere.

Zero Latitude is the Equator. The North Pole is at Latitude 90 degrees North (written as 90°N). The South Pole is at 90 degrees South (90°S). To the north of the Equator are lines of Latitude running from 0° to 90°N. To the south are lines of Latitude running from 0° to 90°S. All lines of Latitude are parallel to each other.

For reasons that we will discuss later, there are four other special lines of Latitude on the Earth. These are given names and are listed below.

23.5°N Tropic of Cancer
66.5°N Arctic Circle
23.5°S Tropic of Capricorn
66.5°S Antarctic Circle

These four lines of Latitude divide the Earth into climatic zones. The region between the two tropics (23.5°N to 23.5°S) is known as The Tropics . The region north of the Arctic Circle (66.5°N to the North Pole at 90°N) and the region south of the Antarctic Circle (66.5°S to the South Pole at 90°S) are called The Polar Regions . The intermediate areas (23.5°N to 66.5°N and 23.5°S to 66.5°S) are called The Temperate Zones and are often split into the North Temperate Zone and the South Temperate Zone . Most of the Earth's population lives in the temperate zones.

Longitude

Unlike Latitude which has two fixed points (the poles) and a fixed great circle (the Equator), Longitude has no natural zero. All lines of Longitude are identical. The zero line of Longitude is called the Zero Meridian or Prime Meridian . Its position must be decided by international agreement.

The Earth's Prime Meridian was fixed by astronomers and geographers to run through Greenwich Observatory in South East London, England. This observatory was the centre of cartography (map making), time keeping and stellar observation. The zero meridian is now usually called the Greenwich Meridian .

Longitude runs East and West from Greenwich (0° to 180°E and 0° to 180°W). The 180° line of Longitude is called the International Date Line . It passes mainly through the Pacific Ocean.

With these two coordinates (Latitude and Longitude), we can uniquely fix sites on the Earth. Five selected cities are listed below.

City Country Latitude Longitude
LondonUnited Kingdom 51.5°N
Buenos AiresArgentina 35.0°S58.5°W
ReykjavikIceland 64.0°N22.0°W
SingaporeSingapore 1.0°N104°E
Los AngelesUnited States of America 34.0°N118.5°W

From the above table, it is easy to see that Singapore is the closest of the five cities to the Equator. Reykjavik is the furthest from the Equator and therefore closest to one of the poles. Buenos Aires and Los Angeles are roughly the same distance from the Equator: one in the southern hemisphere, the other in the northern.

Both Latitude and Longitude are measured in degrees. One complete turn is 360° half a turn is 180° and a right angle (quarter turn) is 90°. On the Earth, one degree of Latitude (or Longitude along the equator) is equal to approximately 111km.

The Earth is divided into time zones. 15° is equal to 1 hour. The Greenwich Meridian is the zero for Earth's time zones, again by international agreement.


Right Ascension and Declination

In order to identify the position of an object on the celestial sphere, we need a coordinate system that can cope with the position of an object on the sphere at a given time from a given location. Imagine that you are stood on the North Pole and when you look directly up you will see the Moon. Now imagine another observer stood on the Equator, where would they see the Moon? It certainly won't be overhead instead, it will be on the horizon. In the same manner, the position of an object will appear to change if one observer was in Europe and another in America, however, over time the object will move into the position observed as the Earth rotates.

For these reasons, the celestial coordinate system is based on the latitude and longitude system used on the surface of Earth. The coordinate system consists of two figures, Right Ascension (RA) and Declination (Dec). Right Ascension is a measure of the position of an object from the First Point of Aries and can be thought of as the celestial sphere equivalent of longitude, while Declination is a measure of the position relative to the celestial equator and is similar to latitude. The celestial equator is a projection of the Earth's equator onto the celestial sphere.

In the diagram above, declination is marked on the blue lines and is analogous to latitude as it measures the angular distance in degrees from the equator, from 0° at the equator to +90° at the North Pole and -90° at the South Pole.


What is the right ascension overhead at Greenwich at noon on 21 march - Astronomy

Right ascension (RA) is like longitude. It locates where a star is along the celestial equator. The zero point of longitude has been chosen to be where the line straight down from the Greenwich Observatory in England meets the equator. The zero point for right ascension is the vernal equinox. To find the right ascension of a star follow an hour circle "straight down" from the star to the celestial equator. The angle from the vernal equinox eastward to the foot of that hour circle is the star's right ascension.

There is one oddity in right ascension: the unit used to report the angle. Right ascensions are always recorded in terms of hours, minutes, and seconds. One hour of right ascension (1 h ) is 15°. Since 24x15°=360°, there are 24 h of right ascension around the celestial equator. The reason for this oddity is that the celestial sphere makes one full rotation (24 h of RA) in one day (24 hours of time). Thus the celestial sphere advances about 1 h of RA in an hour of time.


What is the right ascension overhead at Greenwich at noon on 21 march - Astronomy

Visualizing the Celestial Sphere

To understand the movements of the Moon, the planets, the Sun, the stars, and other objects through the sky, we'll need to define a coordinate system and become comfortable with a few terms describing various positions. Let's begin by identifying the primary causes for the apparent movement of objects across the sky.

Next let's consider the distance scales involved. Which ones matter most?

When considering the movements of the stars, the distance between two positions on the surface of the Earth, and the distance between two positions on the Earth's orbital path, are negligible. Why then do the stars appear different at the equator and the poles, or from winter to summer? The angle of Earth's rotational axis, and the direction you are looking at as you shift around the curved surface of the Earth, present views of different portions of the sky.

Imagine that you are standing on the equator, looking up at the night sky above your head. If you shift 10,000 kilometers northward, you'll end up at the north pole. The diorama of the stars overhead will have shifted considerably &ndash Polaris, the North Star, used to lie on the northern horizon, but now it is directly overhead. If instead you shift 10,000 kilometers to one side or the other along a flat plane passing under your feet, however, you'll end up observing the same set of stars above. What is the difference? In both cases, you have moved by 10,000 kilometers. In the first case, however, you have also rotated your point of view by 90 degrees, while in the second case, you are still looking in the same direction. The linear shift in position of 10,000 kilometers is irrelevant when compared to the distance between stars (10,000 billion kilometers), but the angular shift plays a huge role in defining your view.

Let's now consider the movement of the stars, and define a celestial sphere, a transparent sphere with infinite radius which is centered at the center of the Earth. Like the Florentine poet Dante Alighieri with his crystalline spheres, we place, or project, the celestial objects upon this sphere. The sphere is fixed in place, so as the Earth rotates daily in a complete circle the celestial sphere appears to rotate once a day in the opposite direction.

We extend the rotational axis of the Earth far above the north pole and far below the South Pole, and define the points at which it intersects our celestial sphere as the north and south celestial poles (NCP and SCP). We then define the portion of the celestial sphere which lies in the same plane as our equator as the celestial equator (CE). We define coordinates on the surface of the Earth according to northern and southern latitudes (starting at zero at the equator and rising to 90 degrees at the poles) and longitudes east and west of the position of the Royal Observatory in Greenwich, England ranging around the Earth. On our celestial sphere, we similarly use declination (angles north and south of the celestial equator) and right ascension (the angle around the celestial equator, with the zero point corresponding to the constellation where the Sun is found at noon on the first day of spring).

On the surface of the Earth, one feels as though one is standing on a giant, flat plane stretching away toward the horizon in all directions, with half of the sky (and the celestial sphere) appearing above the horizon and half hidden below. If you drew a line from the center of the Earth through your upright body, and then extended it upwards all the way to the celestial sphere, this would mark the zenith (the point directly above your head in the heavens). At the other end of the line, hidden far below the horizon, lies the nadir (literally, the lowest point).

The figure shown above identifies landmarks used when observing, both in an observer's frame of reference (their local horizon, stretching off to the north and south, and east and west, and the sky above them), and along the celestial sphere that surrounds the Earth. The red line indicates the path of the Sun through the sky over the course of a day on the Spring Equinox (March 21, on the left), where the Earth is tipped neither toward nor away from the Sun, and three months later on the Summer Solstice (June 21, on the right), where the Earth's North Pole is tipped over by 23 degrees toward the Sun and the Sun thus appears to rise and set in the north, and to pass overhead and culminate (transit, or intersect the observer's meridian) higher in the sky.

At the north pole, the zenith aligns with the north celestial pole and the celestial equator lies on the horizon. As you shift southward toward the equator, what happens to the sky? The zenith tilts toward the celestial equator, as it rises overhead, and the north celestial pole descends toward the horizon.

If you stand at the north pole you can extend a line upward to the celestial sphere to Polaris, the North Star. You can draw a similar line northward from any other point in the northern hemisphere. You might think that this line would be tilted relative to the first line, because you have shifted your base point out from directly underneath the North Star. However, recall that shifting your position from one side of the Earth to the other is a shift of 0.000000001% of the distance to the nearest star (0.00000000001% of the distance to Polaris) &ndash far too small a difference to matter. Because the Earth is so tiny compared to the distances to the stars, all lines pointing north from all over the surface are essentially parallel.

The Earth rotates from west to east on its own rotational axis (or counterclockwise, when viewed from above the north pole). Because of this, when we stand still on the Earth and observe the stars above we perceive them to be rising in the east and setting in the west, taking 24 hours to travel once around the entire Earth.

Our position with respect to the Earth's rotational axis controls the apparent movements of the stars during a night. If you stand at the north pole, then the entire northern sky appears to be rotating in a huge circle around the North Star above your head. (This is because you are turning around in a tiny circle in the opposite direction.) Over the course of one night the stars neither rise nor set instead, the star in view travel in arcs (taking 24 hours to complete an entire circle).

If you stand at the equator, however, you are now aligned perpendicular to the Earth's rotational axis. From your point of view, a steady stream of stars are rising in the east, traveling across the sky overhead, and then setting in the west hours later. The difference is due to the fact that rather than standing in one place and spinning, you are now tracing out a giant circle around the Earth along the equator. Rather than looking up along the rotational axis toward Polaris, you are now looking up 90 degrees away, and finding the celestial equator overhead.

Star that move in a circle which never drops below the horizon are called circumpolar, and the closer you are to the poles, the more of them you will observe. They have the largest declination values, and appear closest to the north and south celestial poles on the celestial sphere. In contrast, stars which lie near to the celestial equator rise in the east, transit overhead, and then set in the west. If at one month of the year they lie overhead at midnight, then six months later they will lie unseen behind the Sun at noon.

Your location on Earth determines how much of the sky will contain circumpolar stars.


This Week's Planet Roundup

Mercury (brighter than usual at about magnitude –0.7) is emerging into a nice evening apparition low in the fading twilight. Look for it low in the west-southwest about 45 minutes after sunset.

Venus (magnitude –3.9) is very low in the southeast as dawn grows bright. Look for it about 20 or 30 minutes before sunrise.

Mars (about magnitude +0.2, in Aries) shines at its highest in the south in late twilight. It's still high in the southwest as late as 8 or 9 p.m.

Mars continues to fade and shrink into the distance. It's now 9 arcseconds wide in a telescope, maybe still large enough to show some very large-scale surface markings during steady seeing. It's gibbous, 89% sunlit from Earth's point of view. (To get a map of the side of Mars facing you at the date and time you observe, you can use our Mars Profiler. The map there is square, so remember to mentally wrap it onto the side of a globe features near the map's edges become very foreshortened.)

Jupiter (magnitude –1.9) is finally vanishing from sight in the glare of sunset. See the scene at the top of this page. The earlier in the week you look the better your chance, and bring binoculars. On Friday the 15th Jupiter is still only 6° lower right of Mercury, but the gap between them widens by 1° per day.

Saturn is lost from sight in the glare of the Sun it's lower right of Jupiter.

Uranus (magnitude 5.7, in Aries) is highest in the south right after dark, just a couple degrees or so from eastward-flying Mars. They're closest together at conjunction, 1.5° apart, on the 20th see January 20 above. Mars will be within 2° of Uranus from the 18th through the 22nd.

In binoculars Uranus is a little pinpoint "star." But with an apparent diameter of 3.6 arcseconds, it's a tiny, fuzzy ball at high power in even a smallish telescope with sharp optics — during spells of good seeing.

Neptune (magnitude 7.9, in Aquarius) is getting low in the southwest right after dark. Neptune is 2.3 arcseconds wide. Finder charts for Uranus and Neptune.

All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Standard Time, EST, is Universal Time minus 5 hours. (Universal Time is also known as UT, UTC, GMT, or Z time.)

Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential magazine of astronomy.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6, and hundreds of telescopic galaxies, star clusters, and nebulae among them. Shown here is the Jumbo Edition, which is in hard covers and enlarged for easier reading outdoors at night. Sample charts. More about the current editions.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5 nearly three times as many. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to magnitude 9.75). And be sure to read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger (and illustrated) Night Sky Observer's Guide by Kepple and Sanner.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the heavens above. It's free.

"The dangers of not thinking clearly are much greater now than ever before. It's not that there's something new in our way of thinking, it's that credulous and confused thinking can be much more lethal in ways it was never before."
— Carl Sagan, 1996


Pyephem - does the right ascension calculation for the sun account for the Equation of Time

I am looking to calculate the highest precision lat lon for the subsolar point, in a particular datetime moment, as is reasonably possible using pyephem, with the help of some other library(s) if they are needed.

Relevant context: Anyone who has used pyephem, already knows that for certain calculations it requires certain setup values before computing body positions, those values including the datetime (epoch of the observation), the location of the observer, and of course, the body being investigated. Solutions for the subsolar point through the use of pyephem, that I have found online, show the time in utc as the time needed for the pyephem setup.

Remembering way back to my first exposure to astronomy, and to celestial navigation, utc is a variant of a mean day, compared to an actual solar day, where an actual solar day's duration throughout the year varies due to several factors of the nature of the earth's orbit. Because the length of an actual solar day varies throughout the year, for certain types of astronomical calculations, this requires the Equation of Time to more precisely map the actual solar day measurements to a mean and fixed 24 hour day system such as utc. Before the advent of sufficiently accurate 'pendulum movement' clock mechanisms, and now crystal controlled clock mechanisms, going back to when sundials were the accurate timepiece, the more sophisticated sundials included markings to apply a yearly approximation of this important Equation of Time, soon after it had been observed and definitively documented. Therefore, relevant to my question, since utc is a variant of mean day, and not the true solar day, but normalized to 24 hours exactly, there is this question now of how or if pyephem incorporates the Equation of Time in its right ascension solutions for the sun. At present, I imagine the EoT is required for accuracy, as I try to visualize the sun's position against the background of stars, as seen from the earth, as the earth revolves around the sun, with historically observed variations that are made available and useful and essential with the Equation of Time.

Summary then of my question:

If it is not necessary to explicitly enter an EoT value in pyephem, because it is not relevant for computing the most accurate subsolar point, please explain why. If it is relevant, as I presently think it is, please tell me if pyephem, in its right acension calculation of the sun (and other bodies), as a body, does in fact, apply the Equation of Time as appropriate. Does it do so transparently? Is there a way to input an explicit value for it, if such is known, an EoT value that might be more accurate or more up to date compared to what pyephem is using transparently?

Some initial research results that formed the question: Upon doing a search through various search engines, I found several posts in topical forums that give what seems a very simple answer for finding the subsolar point. Finding the lattitude apears to be the less complicated part of the solution, being simply the computed declination. Finding the longitude is where the question arose in my thinking, and now I wonder if it is applicable for the declination as well, since using the properly precise time is essential for the most precise result of both declination(lat) and longitude of the subsolar point. I always applied the EoT from the Nautical Almanac, back when I was involved with celestial navigation.

Two links, specific to pyephem, present the same approach to the subsolar point solution. When the question(s) was first asked, Brandon Rhodes quickly presented the single line formula using pyephem's computing of the sun's right ascension. His was specifically the code for the longitude calculation in a more theoretical tone, without all the pyephem contextual details. Liam Kennedy presented a more complete context of python code, showing those additional pyephem details, so that one could 'copy and paste' the entire block of code, (needing only to add the import ephem and import datetime), and modify it as appropriate, which I also found to be a useful review. The code is from these links.

subsolar point:

Nowhere in these two posts is there a mention of the Equation of Time, and yet a variant of mean day is being used as an input value here

utc as a variant of mean day is EoT unaware, by its construction definition as a mean day, which then normally makes it a requirement to adjust it with the EoT for certain astronomical usefulness.

To further clarify this requirement, there are many navigation and astronomical references that go into considerable detail discussing it. But I will stick to refering to some forum posts such as the following:

specifically the post by grant hutchison 2007-mar-20, 04:33 pm

note: the NOAA calculator, as of this writing, 2019-12-19, does have an input box where one is to enter the Equation of Time in minutes. That page has a link to a more updated calculator.

The more up to date page also calculates and displays the Equation of Time, clarifying its relevance. Now, continuing to quote Grant's post.

First, use the calculator to derive the Equation of Time and Solar Declination for the date and time you're interested in, at the location zero latitude and zero longitude, with no UTC offset.

The 2007 March equinox is at 21 March 00:08:30 UTC. Type that time and date into the calculator and, sure enough, you find the solar declination is zero: the sun is over the equator at that moment. For any other date and time, the solar declination will convert directly to the latitude of the subsolar point.

Now we need the longitude. First, work out the true solar time using the Equation of Time figure: it's -7.42 minutes in this case. That's the offset between the position of the mean sun and the real sun. Adding that figure to our UTC time tells us that the real sun is just 1.03 minutes past midnight (8.5-7.42) at the time of interest. Divide that figure by 60*24 (to get the fraction of a day) and multiply by 360 (to get degrees): that gives us 0.2575 degrees past midnight. So the sun will be on the noon meridian at 180-0.2575 degrees east = 179.7425 E. That's our longitude.

Combine the two, and the subsolar point is 0.0000N 179.7425E.

We can check that I haven't mixed my pluses and minuses by typing the derived coordinates of the subsolar point into the solar calculator (Lat 00:00:00, Lon -179:44:33), keeping the UTC offset at zero and the date and time at your time of interest, 21 March 00:08:30. That comes up with an Azimuth of zero and an Altitude of 89.98 degrees, confirming that we have the sun crossing the meridian within a couple of hundredths of a degree of directly overhead. Phew. It works, but it's a bit of a pain. Maybe someone can offer a calculator that will do more of the work for you.

And a followup post of his dated about an hour and a half later.

Some notes to the above, FWIW:

The difference between Dynamical Time and UTC this year is 65 seconds, so working from the Dynamical Time of the solstice we get the UTC time (to the nearest second) to be 00:07:25 UTC, which fits with G O R T's nearest-minute value, above.

The reason G O R T and I come up with a different subsolar longitude for the same time (00:07:00 UTC) is because of that pesky -7.42 minutes in the equation of time: although that time is after midnight at Greenwich, the real sun is still 42 seconds short of crossing the midnight line. That shifts the calculated subsolar point from the eastern to the western hemisphere. 7.42 minutes is equivalent to 1.855 degrees, which is exactly the difference between my calculated longitude of 179:53:42W and G O R T's of 178:15:00E.

My question is therefore based on this research, and based on my past experience with celestial navigation. I imagine that as vital as the Equation of Time might be to the problem, it would be incorporated into pyephem's calculation(s), since a mean day is input into pyephem's API. Seeing nowhere in these snippet solution postings where EoT is to be specified in the pyephem API, my assumption is that it would be internally and transparently implemented? I am not comfortable with this assumption, and so I have posted this question. Clarification would benefit the confidence of users, particularly newbies such as myself.

Update 12-20-2019:

I suspect the answer is yes, pyephem accounts for EoT, but it does not call it that? The way ephem, libastro, takes into account some other effect or relationship probably answers my question(s). I am reviewing:

Needing to read it very slowly, while drawing some pictures, and waiting for an astronomy book so I can catch up on a very much misplaced education on this matter. I'm thinking that perhaps the term Equation of Time only has meaning in a narrow context of reconciling the solar day to a mean day metric, as experienced on earth, while pyephem solves in a broader context and uses more broadly applicable terminology, of which I need to be re-educated, which includes such resulting effects as the Equation of Time? Or I am only displaying my ignorance? Until I can more competently write my own answer, please do contribute any helpful comments or answers that can steer my study.


Path of the sun in the sky

The sun does not rise exactly in the east and set exactly in the west on all days during the year. The point where the sun rises and sets on the horizon changes depending on the day of the year, that is, where the earth is in its orbit around the sun (or equivalently, where the sun is in its ecliptic).

Fig. 12 – Paths of the sun on different days of the year

There are two motions of the sun from the point of view of the observer on the earth.
1)The motion of the sun along the ecliptic (traversing the ecliptic in a year) due to the revolution of the earth around the sun. (indicated in red)
2)The apparent rising and setting motion of the sun in the sky (traversing the sky in a day) due to the rotation of the earth on its axis. (indicated in yellow and orange)

For a person on the northern hemisphere,
Summer solstice (June 21) – On the summer solstice, the sun reaches its most northerly declination of +23.5 °.
The longest day of the year is indicated by the greater section of the yellow line remaining above the horizon

Autumn equinox (March 21) – On the autumn equinox, the sun has a declination of 0°, since the ecliptic intersects the celestial equator.
The day and night being equal is indicated by equal sections of the orange line remaining above and below the horizon.

Winter solstice (December 21) – On the winter solstice, the sun reaches its most southerly declination of -23.5 °. The shortest day of the year is indicated by the smaller section of the yellow line remaining above the horizon.

Spring Equinox (September 22) – During the spring equinox, the sun has a declination of 0°, since the ecliptic intersects the celestial equator.
The day and night being equal is indicated by equal sections of the orange line remaining above and below the horizon.


The astronomical basis of timekeeping

The purpose of this page is to provide background information about the different ways in which time has been determined since the Observatory&rsquos founding in 1675. It covers basic astronomical principles, together with some of the many definitions of time that scientists use. It also refers to the specific instruments that were used at the Royal Observatory, together with some of the principles of their operation.

Besides the sundial, methods for determing the local time from observations of the sun or stars have been known since ancient times. Timekeepers are instruments that measures the passage of time. They need to be adjusted so that they go at the correct rate and reset periodically to show the correct time. Until the introduction of the pendulum clock in the latter half of the seventeenth century, timekeepers were unable to keep time to better than about 15 minutes a day. By contrast, the first pendulum clocks were able to keep time to about 10 seconds a day. By the mid twentieth century, this had been improved by a factor of 10,000, with the best clocks being able to keep time to around a few seconds a year. Today&rsquos atomic clocks are capable of keeping time to better than one second in 1,400,000 years.

At Greenwich, time determinations were made from observations of the Sun until Edmond Halley obtained the Observatory&rsquos first transit telescope in 1721. After that date, all time determinations were made from observations of the stars.

Timekeeper Earth

When the Royal Observatory was founded at Greenwich in 1675, it was generally believed that the Earth was spinning at a steady rate (or in technical speak, isochronous). Our day is based on the length of time it takes for the Earth to spin around once on its axis. Historically this was measured either by reference to the position of the Sun, or by reference to the positions of the stars. Each day is divided in 24 hours, each hour into 60 minutes and each minute into 60 seconds.

The Observatory was established with the specific and practical purpose of &lsquorectifying the Tables of the Motions of the Heavens, and the places of the fixed Stars, so as to find out the so much desired Longitude of Places for perfecting the art of Navigation&rsquo. Since it was proposed to measure longitude differences by measuring time differences, it was important to establish at the outset if the Earth was indeed isochronous. To this end, Flamsteed set up the so-called Sirius Telescope in the Great Room (Octagon Room) of Flamsteed House. His observations confirmed not only the isochronal nature of the Earth, but also enabled him to determine how the length of the solar day varied with the seasons (more about this in the next section).

The development of new types of clock in the mid twentieth century led to the discovery of small variations in the rate at which the Earth is turning. The subsequent development of atomic clocks which were more accurate still, led to a fundamental change in the way that the second is defined. More about this later.

Different sorts of time

One of the oldest ways of finding the time is with a sundial. The further west you are, the later the Sun rises and the later it sets. When a sundial in Greenwich is showing 9.00 a.m., one to its west in Cardiff will show 8.47 a.m. The time indicated by a sundial is called the &lsquolocal apparent time&rsquo.

The local meridian is an imaginary line connecting the north and south poles which passes though the observer&rsquos position. The time when the Sun transits (crosses over) the local meridian is called the &lsquolocal noon&rsquo.

The interval between successive transits of the Sun is about four minutes longer than the interval between similar transits of other stars. This is because at the same time that the Earth is spinning on its axis, it is also orbiting the Sun, progressing about 1° around its orbit with each complete turn. Between one transit of the Sun and the next, the Earth therefore has to turn through an angle of about 361° rather than the 360° required for the other stars &ndash thereby accounting for the extra 4 minutes. Time measured by the stars is called sidereal time. Time measured by the Sun is called solar time.

In practice, the length of each day measured from one local noon to the next varies in a periodic manner throughout the year. The longest is about 51 seconds longer than the shortest. The difference arises partly as a result of the Earth&rsquos orbit being elliptical rather than circular and partly as a result of it being tilted on its axis. Each day measured by a clock has the same length and is equal to the average or mean length of a solar day. This is where the word &lsquomean&rsquo in &lsquomean time&rsquo comes from. When solar days are shorter than average, a clock will seem to loose time compared to a sundial. When they are longer, it will appear to gain.

Greenwich Mean Time (GMT) is the local Mean Time at Greenwich. Today it is reckoned from one midnight to the next, but until 1925 was also reckoned for astronomical purposes from one midday to the next (the astronomical day), giving an ambiguity to its meaning. Greenwich Mean Time is 13 minutes ahead of Cardiff Mean Time (the local mean time in Cardiff) and 10 minutes ahead of Bristol Mean Time. Until the coming of the railways, clocks in most towns and cities were set to show local mean time. In order to make their timetables less confusing, railway companies began introducing a single standard time across their networks. In mainland Britain, it was Greenwich Mean Time that was normally adopted. By 1855, 98% of the public clocks in Great Britain were set to show Greenwich Mean Time. Greenwich Mean Time became the legal Time of Great Britain in 1880. Nowadays, everybody within a country or time zone sets their clocks and watches to the same time for civil purposes.

Like solar days, sidereal days also very slighly in length from one day to the next. The variation in the length of the sidereal day was too small to be sensibly measured until the introduction of the Shortt free-pendulum Clocks in the 1920s which set a new standard in precison timekeeping. As a result, Astronomers began to refer to a new unit of time &ndash the mean sidereal day.

Determining time from the Sun by the Double Altitude Method (used at Greenwich from 1675&ndash1721)

The Octagon Room at the Royal Observatory in the 1670s. The Tompion clocks can be seen in the centre. The quadrant on the left is shown looking northwards. It could be wheeled from window to window and was probably the one used by Flamsteed for his equal altitude measurements. Engraving by Francis Place after Robert Thacker c.1676, republished in The Old Royal Observatory (HMSO, 1960)

Determining time from the stars with a transit instrument (used at the Observatory from 1721&ndash1955)

The Troughton 10-foot Transit Instrument. Drawn by J Farey and engraved by T Bradley. Plate 16 (adapted detail) from Pearson's An introduction to practical astronomy (London, 1829). Image courtesy of Robert B. Ariail Collection of Historical Astronomy, Irvin Department of Rare Books and Special Collections, University of South Carolina Libraries

The relative right ascensions of the 67 stars used as clock stars at Greenwich in 1851

During the Second World War, the Greenwich Time Service operated mainly from Abinger in Surrey. To allow for the fact that Abinger is to the west of Greenwich, all time determinations made there were adjusted to allow for its difference in longitude. Similar adjustments were made following the Observatory&rsquos move to Herstmonceux.

The list below details the various transit instruments used for time determination at the Observatory together with their date of use and the site on which they were operated:

    (Greenwich, 1721&ndash1750)
    (Greenwich, 1750&ndash1816)
    (Greenwich, 1816&ndash1850)
    (Greenwich, 1850&ndash1927)
    (1927&ndash1957)
    (Abinger, 1940s & 50s)
Aligning a transit instrument to the meridian

The Greenwich instruments like those in other observatories around the world, were aligned to the meridian by making observations of the circumpolar stars &ndash stars that never rise nor set.

These stars are always present in the sky and transit the meridian twice rather than once each day. When the telescope is correctly aligned, the measured interval between successive transits of any particular circumpolar star is constant.

If a good catalogue was to hand, the right ascensions of a high and a low star could be used instead. Once a transit telescope had been adjusted to the meridian, it was possible to create a mark on the horizon for use as a quick alignment check.

The standard way of checking the collimation (alignment of the optics) of a transit telescope prior to the introduction of collimators in the 19th century was to adjust the telescope to a distant point or mark on the horizon and then reverse it in its mountings. Other things being equal, if the optics were correctly aligned, the mark would still be seen in the centre of the field of view. Reversing the telescope was a time consuming and a potentially hazardous operation. Once a transit telescope had been initially collimated provided correctly placed meridian marks were available to both the north and the south, there was no further need to reverse the telescope unless, when aligned to one mark, it was out of alignment with the other. By then reversing the telescope, it was possible to determine if the problem was one of collimation or a shifting of one or other of the marks. A further way of checking the collimation independently of any mark (and one occasionally used by Maskelyne) was to reverse the telescope during the passage of Polaris.

There were a variety of factors relating to the setting up, use and maintenance of the marks at Greenwich which the Astronomers Royal had to consider and contend with. These included:

  • Local topography &ndash which affected the choice of locations, particularly to the south
  • Availability of suitable sites
  • Stability &ndash including subsidence or marks becoming loose
  • Accessibility &ndash especially if the mark was on the chimney or wall of a private building or some distance from Greenwich
  • Visibility &ndash which could be seriously impaired by moored boats on the Thames, trees and atmospheric pollution, to say nothing of the mists that rolled in over the Greenwich marshes
  • Errors emanating from the design or manufacture of the telescopes, or the observing procedures adopted.
Errors associated with Transit Instruments

The following discussion of the instrumental errors associated with Transit Instruments is taken from a lecture titled The Determination of Precise Time, which was given by the Astronomer Royal, Harold Spencer Jones in 1949:

&lsquoThe pivots rest in fixed bearings, adjusted so that the common axis of the pivots is as nearly as possible horizontal and pointing in an east-west direction. If the axis of the pivots were exactly horizontal and in the east-west direction and if the optical and mechanical axes of the telescope coincided, the axis of the telescope would be in the meridian plane, whatever direction the telescope was pointing to. This ideal condition is never achieved and there are always small errors of level, of azimuth, and of collimation. These adjustments are liable to continual change there are slow seasonal changes, associated with changes of temperature and possibly also with subsurface moisture there are also more rapid changes, which are correlated with changes of circumambient temperature and with the direction of the wind. To control these changes frequent observations of level, of azimuth, and of collimation are essential, which take up a disproportionate amount of the observing time. The error of collimation can, however, be eliminated if the telescope is reversed in its bearings in the middle of each transit, half the transit being observed before reversal and the other half after reversal. It is not possible to reverse large transit instruments sufficiently quickly and it has accordingly become customary to use small transit instruments, which can be rapidly reversed, for the determination of time as it is the brighter stars which are observed, a large aperture is not needed.

There are other factors which have also to be taken into consideration. The pivots will never be absolutely cylindrical their figures have to be determined with great accuracy and appropriate corrections made to the observations. Flexure of the axis can cause troublesome systematic errors. If the horizontal axis is not equally stiff in all directions, its flexure will vary according to the direction in which the telescope is pointed. If the two halves are not equally stiff, the telescope will be twisted from the meridian by a variable amount. Personal equations between different observers are somewhat troublesome, though they do not exceed a few hundredths of a second when the so-called impersonal micrometer is used. Before its introduction, the method of observing was for the observer to press a hand-tapper at the instant the star crossed each of a number of vertical spider wires in the focal plane of the telescope by so doing, he closed an electric circuit which sent a current to a recording chronograph, which recorded not only the signals from the telescope but also time signals, every second or alternate seconds, from the clock. The instants of the star crossing the wires could then be read off at leisure after the observations had been completed. With this method of observing, the times determined by different observers could differ by as much as half a second. The reason is easy to see one observer might wait until he saw the star actually bisected by the wire before he pressed the tapper, with the result that, because of the time required for the message to travel from his brain to his eye and to be converted into muscular action, his signal would inevitably be late another observer would, as it were, shoot the flying bird, gauging the rate of motion of the star so that his tap is made as nearly as possible at the instant at which the star is actually bisected. The personal equations can be determined by what are called personal equation machines the transit of an artificial star is observed, the times at which the star is at certain positions during the transit being compared with the observed times. Although an observer will unconsciously form a fixed habit in observing so that his personal equation remains substantially constant, small variations, depending upon the physical condition of the observer, do occur.

The method of observing now almost universally employed is to have a single movable wire in the micrometer eyepiece instead of a number of fixed wires. The wire can be traveled along by the observer, who adjusts its speed so as to keep the star continually bisected by the wire. As the wire moves along, contacts are automatically made in certain positions, sending signals which are recorded on the chronograph. In order to relieve the observer of some of the strain of maintaining a uniform motion of the wire, it is now common to drive the wire mechanically at the speed appropriate to the motion of the star, using an electric motor with some form of continuously variable gearing. With this method of observing, the personal equations of different observers are very small, usually not more than two or three hundredths of a second it is for this reason that this form of micrometer is called the &ldquoimpersonal&rdquo micrometer. Small though these residual personal equations are, they remain remarkably constant and can be determined by personal equation machines. They seem to arise from two causes: there is &ldquobisection error,&rdquo an observer systematically bisecting an image to the right or to the left of its center this error changes sign at the zenith with instruments in which the observer changes the direction in which he faces, according to whether he is observing a north or a south star there is also &ldquofollowing error,&rdquo an observer systematically setting the wire in front of or behind the center of a moving image. This error does not change sign at the zenith.

If the pivots are not exactly cylindrical, the telescope will be twisted out of the meridian by an amount varying with its position. The figures of the pivots must therefore be determined with great accuracy and appropriate corrections applied to the observed times of transit. The figures of the pivots must be determined at intervals, as they may change slowly in the course of use through wear. Other variable errors can be introduced through slight mechanical imperfections in the telescope if there is the slightest play in the eyepiece micrometer or in the objective, errors will be introduced which will vary with the position of the telescope.

When all the possible sources of error which can affect observations with a transit instrument are borne in mind, it is rather surprising that the observations are as accurate as they are. The probable error of a single time determination is usually about two-hundredths of a second.&rsquo

Determining time with a photographic zenith tube (used at Herstmonceux from 1955&ndash1984)

The Photographic Zenith Tube. From an RGO photo published in 1958. Image courtesy of Phillip Gething

Determining time with a Danjon Prismatic Astrolabe (used at Herstmonceux in 1962&ndash3)

The Danjon Prismatic Astrolabe is a highly specialist instrument developed in the 1950s by the French astronomer André-Louis Danjon as a replacement for the transit instrument which was considered to have reached its technological design limit. Danjon Astrolabe OPL. No. 9 was brought into service at the Royal Greenwich Observatory, Herstmonceux in July 1959, primarily to extend the study of its effectiveness in determining catalogue corrections which had been carried out elsewhere. It could however also be used for time determinations and between July 1962 and 1963, was used to provided data for the Greenwich Time and Latitude Service while the Photographic Zenith Tube was out of service for investigation and overhaul.

The International Meridian Conference of 1884 and the creation of Universal Time (UT)

Although latitude has always been measured from the Equator, there is no equivalent point from which to measure longitude. Over the years, it has been measured from many different places, including national observatories, the island of Hierro in the Canaries and St Paul&rsquos Cathedral in London &ndash each country having chosen for itself where to measure from.

The introduction of the Nautical Almanac in 1767 had required sailors to make use of astronomical time, where the day was reckoned from noon, beginning twelve hours after the start of the civil day. Until then, sailors had used the civil day along with the nautical day. The nautical day also began at noon, but started twelve hours earlier than the civil day. The potential for confusion because of the similarity between astronomical and nautical days was much reduced when on 11 October 1805 the British Admiralty issued an order to end the use of the nautical day. A similar change was made in America in 1848.

The start of the nineteenth saw calls for unification and the adoption of a single common meridian. But the problem was not one of geographical location alone it was also linked to the measurement of time. To rationalise one, would require the rationalisation of the other. After much preparation of the ground, an International Meridian conference took place in October 1884 in Washington DC. Attended by 41 delegates from 25 nations, if passed a total of seven resolutions:

    That it is the opinion of this Congress that it is desirable to adopt a single prime meridian for all nations, in place of the multiplicity of initial meridians which now exist.

The resolutions from the conference were only proposals &ndash it was up to the respective governments to show political will and implement them . and progress was slow . very slow. Resolution six was particularly problematic &ndash not because of the nautical day (which had been dealt with before and was easy to change), but because of the implications of changing the astronomical day. Changing it would take a great deal of coordination if confusion and misunderstandings were to be avoided as Samuel Franklin the man in charge of the Naval Observatory in the United States was soon to discover. The order he issued on 4 December 1884 for the changes to be introduced from 1 January had to be rapidly rescinded, following his failure to properly anticipate the practical implications of so sudden a change. William Christie his opposite number at Greenwich was rather more circumspect. He made only a symbolic gesture, which included altering the Observatory&rsquos public clock with its 24-hour dial. From 1 January 1885, instead of showing astronomical time as it had in the past, it was set to show civil time &ndash a change that involved moving the hour hand half way around the dial.

The end of the astronomical day was effectively sealed in 1918, when the Council of the Royal Astronomical Society reported in favour of change, recommending that Universal Time should be introduced into the Nautical Almanac with effect from 1 January 1925.

What the International Meridian Conference did not do was recommend a global system of time zones. Our system of time zones emerged largely by default as one by one different countries chose to adopt a standard time based not on their capital city or national observatory, but one that was generally a whole number of hours ahead or behind Universal Time.

Click here to read more about the build up to the International Meridian Confence and how its resolutions were enacted.

Redefining the second in the 1950s and 60s

Although long term and irregular variations in the speed of the Earth had been suspected from the eighteenth century onwards, it was not until the advent of better time keepers in the form of the Shortt free-pendulum clock together with the subsequent analysis of the observed motions of the Sun, Moon and planets which was published by Spencer Jones in 1939, that the necessity for a new astronomical timescale independent of the rotation of the Earth became evident.

Discussions about its introduction began in the late 1940s, with a new definition being adopted by the General Assembly of the International Astronomical Union (IAU) in Rome in 1952. Instead of being defined as 1/86,400 of a mean solar day as it had been the case in the past, the second was now defined as a fraction of a year. But because years, like days, also vary in length, the specific year of 1900 was chosen. Various names for the new timescale were considered including &lsquoNewtonian time&rsquo and &lsquouniform time&rsquo time but in the end, the name which ended up being adopted was &lsquoephemeris time&rsquo.

Following further discussion, it was deemed desirable to tweak the new definition of the second, and change the type of year referred to from the sidereal year (which is measured by reference to the stars) to the tropical year (which is measured with reference to the seasons and is around 6 minutes shorter). The change from sidereal year to tropical year was adopted by the IAU in 1955. In science speak the second was now defined as 1/31,556,925.975 of the tropical year for 1900.0. In 1956, this value was adjusted to 1/31,556,925.9747 of the tropical year for 1900 January 0 at12 hours ephemeris time. This definition was enshrined in the new system of International System of Units (SI) that formally came into being in 1960. As such, it became the definition used by everyone rather than just astronomers.

Although the new definition had the great advantage that the length of the second was no longer prone to vary slightly form one day to the next, it had the great disadvantage that it was rooted in the past and as a result was a very difficult standard with which to make comparisons.

Meanwhile, back at the National Physical Laboratory in Teddington in London, Louis Essen had developed a new type of clock that worked using microwave radiation and the properties of the caesium 133 atom. Referred to as atomic clock, it set a new standard in timekeeping proving more stable than the quartz clocks which had been developed in the 1930s and 40s. More importantly, it proved to be a more stable timekeeper than the Earth. Atomic time also had an important practical advantage over ephemeris time, in that it was easily obtainable for comparative purposes. As a result a new definition for the second was drawn up whose value was equal to the observationally determined value of the ephemeris second. The new definition was formally adopted as part of the SI system at the Thirteenth General Conference on Weights and Measures in October 1967. Since that date, the second has been defined as &lsquothe duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the atom of caesium133&rsquo. Although the atomic timescale is now the norm, ephemeris time is still of importance to astronomers.

Our slowing Earth and the introduction of Coordinated Universal Time (UTC)

With the Earth gradually slowing, but the new atomic timescale not slowing with it, the position of the Sun in the sky was set to get more and more out of step with the time it represented. The idea of 12 noon gradually shifting into the morning and then into the night was not something that most people wanted to contemplate &ndash even if it was going to take tens of thousands of years to become significant.

The problem was solved in 1972, with the introduction of an adjusted atomic time scale, Coordinated Universal Time (UTC). The rotation of the Earth is monitored by various organisations around the world, the data having been coordinated since 1987 by the the International Earth Rotation Service (IERS) in Paris and then the International Earth Rotation and Reference Systems Service (IERS) after it was renamed in 2005. Every now and again, (historically at the end of either June or December with an announcement being made by the IERS roughly six months earlier), an extra second known as a leap second, is introduced to keep UTC within 0.9 seconds of GMT. Although UTC has no legal status in the UK, it is this rather than Greenwich Mean Time which is used in practice.

In recent years, there has been a push by some scientists to have UTC abolished as it is said to create difficulties, especially in computing. Click here to read more.

Satellite Laser Ranging

A Lageos satellite. Image courtesy of NASA

The satellites which are tracked, orbit the Earth at heights between 400 and 20,000 km. They fall into four main groups: geodetic, gravitational, altimetry and navigation. The Lageos satellites are an example of a geodetic satellite &ndash the name being an abridgement of Laser Geodynamic. They look like giant golf balls. They are made of aluminium-covered brass and are fitted with numerous retro-reflectors. They have a diameter of 60 cm, a mass of 411 Kg and are in highly stable and predictable orbits at an altitude of 5,900 Km.

The Royal Observatory first became involved with Satellite Laser Ranging in the early 1970s. A facility for Herstmonceux was approved by the Science Research Council&rsquos Astronomy, Space and Radio (ASR) Board in July 1978, with funding being approved by the Science Reserch Council (SRC) on 27 November 1979 and subsequently by the Department of Education and Science (to whom the SRC were answerable). By September 1980, the general design had been completed and most of the equipment ordered. The planning and preparatory work was carried out in collaboration with the University of Hull, which was responsible for the design assembly and testing of the laser, detection and timing subsystems

The facility was located in the by then redundant Solar Building. A 50-cm Cassegrain receiving telescope was installed on 3 July 1981 and the laser in April 1982. The first return signals were detected at the end of March 1983, with the system becoming fully operational by October the same year. A contribution towards the running costs was made by the Department of Trade and Industry (DTI), the Ministry of Defence (MoD) and Natural Environment Research Council (NERC).

A military radar on loan from RSRE Malvern was mounted on the roof of the control room to detect any aircraft that might fly into the laser beam, necessitating a temporary shutdown of the laser in order to prevent anyone looking into the beam and being blinded. An additional visual watch was kept for low flying aircraft and gliders.

The Satellite Laser Ranger facility at Herstmonceux in the 1980s. The dome on the left contains the radar equipment and that on the right the receiving telescope. Mounted on the left side of the receiving telescope is a smaller telescope through which the laser beam is emitted. Photo courtesy of Patrick Moore

The Ordnance Survey placed a trigonometric pillar (solar) near the building and linked it trigonometrically to the pillar (Herstmonceux) near the Isaac Newton Telescope dome so that the UK network could be linked accurately to the international network.

When the Observatory vacated the site at Herstmonceux in 1990, this one facility was retained, a small hut being built alongside to provide office accommodation to replace that lost in the West Block, which was sold with the Castle. When the RGO was shut down completely in 1998, the facility was taken over by NERC (Natural Environment Research Council). In April 2013, it became part of Earth Hazards and Observatories research theme under the management of the British Geological Survey (BGS). Its purpose remains much the same as before, the data it gathers being used together with that from other institutions to inform the decisions made by IERS on the introduction of the next leap second. Click here to read about the current facility and find out about the satellites being tracked today.

Further reading

Lectures and articles by the Astronomer Royal. Harold Spencer Jones

The Earth As A Clock, being the Halley Lecture delivered on 5th June 1939. Published by Oxford University Press, 1939

The Determination of Precise Time. Sixteenth Arthur lecture, given under the auspices of the Smithsonian Institution April 14, 1949

Royal Greenwich Observatory. August 15, 1949, Proc. R. Soc. Lond. A 198:141-169. Click here to download from the British Geological Survey website (pdf file)

Modern Methods of Timekeeping. Record of a discussion held at the Royal Astronomical Society on 21 March 1947. Amongst those taking part were the Astronomer Royal and Louis Essen. The Observatory, Vol. 67, p. 132-136 (1947)

Lectures given by Donald Sadler, Superintendent of HM Nautical Almanac.

Astronomical Measure of Time, being the Presidential Address of the Royal Astronomical Society given at the anniversary meeting on 9 February 1968. Quarterly Journal of the Royal Astronomical Society, Vol. 9, p.281

Account in the Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac, (HMSO London, 1961)

This volume, which was reprinted with amendments in 1972, 1974, and 1977, has a chapter on Systems of Time Measurement. Click here for a history of the volume and its later editions.

Except where indicated, all text and images are the copyright of Graham Dolan


The Sun Physical Data

The following shows the known values of the most important physical parameters of The Sun. Source: JPL Small-Body Database

Physical Parameter Value Relative to Earth
Diameter 1391684 km 109.2202
Mass 1989000 x 10 24 kg 333,043.6574
Density 1.41 gr/cm 3 0.2557
Escape Velocity 618.02 km/s 55.2297
Sideral Rotation 587.2800 hours 24.5370
Absolute Magnitude -26.8