Astronomy

Why don't we build an interferometer between Earth and the Moon?

Why don't we build an interferometer between Earth and the Moon?


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The Event Horizon Telescope (EHT) does interferometry from an array of telescopes spread all across Earth. The data is locally stored on a hard drive and shipped to a central location, so the telescopes in the array don't need to be connected to each other in real time. Would it be possible to make an EHT-like interferometer with telescopes on the Earth and on the Moon? It seems like such a telescope would have a much higher resolution than the EHT, and would make it possible to observe in wavelengths shorter than radio.

Of course, money is likely an issue here, but is there any technical reason why this wouldn't be feasible?


Put simply: controlling the phasing would be impossible (not just difficult). Not only do you have to deal with a constantly changing physical distance but also with the dynamics of the atmosphere. It's difficult enough to maintain transverse phasing across an earth-based receiver (think adaptive optics); I can't imagine a reliable way to track the radial variation. Without that, unless you are sticking with multi-meter wavelength radio systems, you cannot hope to phase up the receivers.


The Moon is an Ideal Spot for a Gravitational Wave Observatory

In the coming years, multiple space agencies will be sending missions (including astronauts) to the Moon’s southern polar region to conduct vital research. In addition to scouting resources in the area (in preparation for the construction of a lunar base) these missions will also investigate the possibility of conducting various scientific investigations on the far side of the Moon.

However, two prominent scientists (Dr. Karan Jani and Prof. Abraham Loeb) recently published a paper where they argue that another kind of astronomy could be conducted on the far side of the Moon – Gravitational Wave astronomy! As part of NASA’s Project Artemis, they explain how a Gravitational-wave Lunar Observatory for Cosmology (GLOC) would be ideal for exploring GW in the richest and most challenging frequencies.

The paper, titled “Gravitational-Wave Lunar Observatory for Cosmology,” recently appeared online and is being considered for publication. Whereas Dr. Karan Jani is an astrophysicist from Vanderbilt University and a member of the LIGO Scientific Collaboration, Prof. Abraham Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University and the director of the Harvard-Smithsonian Center for Astrophysics (CfA).

Originally predicted by Einstein’s Theory of General Relativity, GWs are essentially ripples in space-time caused by the merger of massive objects like black holes or neutron stars. The first confirmed GW event happened in 2016, where scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) announced GWs coming from Markarian 231 – a binary system of black holes over 1.3 billion light-years away.

Since then, with more detectors coming online, collaborations between observatories around the world, and improvements in the technology and methodology, astronomers have detected a total of 56 candidate events. Using these events, astrophysicists have been able to conduct tests of General Relativity which have helped

The time is ripe to explore what science programs could be best pursued from the lunar surface.” -Professor Abraham Loeb

What’s more, astronomers have found many instances where GW astronomy could succeed where conventional methods fall short. As Prof. Loeb told Universe Today via email:

“Traditionally astronomy was all about detecting light by telescopes. Some environments, like the nuclei of galaxies or star-forming regions, are obscured behind opaque clouds of gas and dust. Others, like black holes with no matter around them, do not emit any light. Gravitational waves offer a glimpse at these environments that we could have never probed before. Their strongest sources are mergers of black holes, which provide a new testbed for Albert Einstein’s theory of gravity because they are the most extreme structures of spacetime that the theory predicts.”

The concept at work here is similar to what’s involved with space telescopes, or what astronomers hope to accomplish with lunar radio astronomy – i.e. operating beyond interference. For space telescopes – like Hubble, TESS, Gaia, and others – operating outside of Earth’s atmosphere means being able to gather light that is not subject to wavelength distortion without the need for adaptive optics.

The situation is similar when it comes to interferometers and gravitational waves. Basically, an interferometer relies on two or more merging sources of light in order to create an interference pattern, which is then analyzed by photodetectors to note any sudden changes. When an interferometer intercepts gravitational waves, the ripples cause measurable distortions that scientists use to determine the nature and distance of the source.

Unfortunately, interferometers have to be extremely sensitive since gravitational waves are very difficult to detect, which makes them vulnerable to interference. For one, the arms of a detector need to be kept in a state of vacuum in order to eliminate possible interference for air molecules and seismic events (aka. earthquakes) will also result in false positives.

But on the Moon, which is geologically inactive and there is no atmosphere to speak of, interference would be virtually non-existent. As Prof. Loeb explained:

“The Moon has an extremely low seismic noise, since it has no geological activity. This allows it to probe a frequency range of gravitational waves that is two orders of magnitude smaller than can be accessed from Earth. The situation is analogous to building a radio telescope instead of an optical telescope. The Moon also has no atmosphere, so its surface already has levels of vacuum far lower than the vacuum tubes of the LIGO and Virgo instruments on Earth.”

As for what a Gravitational-Wave Lunar Observatory for Cosmology (GLOC) on the far side of the Moon could reveal, that’s where things get really interesting. On Earth, scientists are limited when it comes to what kinds of mergers they can detect. On the Moon, says Loeb, an observatory could access domains that GW astronomers currently have no insight into:

“The new frequency range allows us to detect intermediate-mass black holes (between stellar-mass objects formed from the collapse of stars – currently probed by LIGO-Virgo and supermassive objects formed at the centers of galaxies – to be probed by the space observatory LISA) through most of the volume of the observable universe.”

Already, scientists have proposed using gravitational waves to study the interiors of black holes, supernovae, locate dark matter, and measure the expansion of the cosmos (aka. the Hubble Constant). This last possibility is especially tantalizing since scientists have been gradually reducing the level of uncertainty they have with their measurements for over a century.

At the same time, scientists have had to deal with a discrepancy (known as the “Hubble tension”) where the reduction of uncertainties with cosmic expansion has not been paralled by a reduction between different measurements. “The orbit of the Moon allows GLOC to pinpoint the host galaxies of merging black holes and neutron stars,” added Dr. Jani. “This is crucial to solving the Hubble tension.”

Illustration of Artemis astronauts on the Moon. Credits: NASA

Another compelling reason why Dr. Jani and Prof. Loeb recommend the creation of GLOC is because of NASA’s (and other space agencies) plan for lunar exploration in the coming years. In addition to sending astronauts back to the Moon for the first time since the Apollo Era (by 2024), NASA also hopes to create a program of “sustainable lunar exploration” beyond that.

This will include an orbiting space habitat that will allow for regular trips to the lunar surface (the Lunar Gateway) and infrastructure on the surface that will facilitate long-term exploration missions (the Artemis Base Camp). For this reason, says Loeb, now is the perfect time to contemplate the kind of infrastructure we want to build there based on what would offer the best scientific returns:

“The time is ripe to explore what science programs could be best pursued from the lunar surface. In the past, scientists contemplated radio, UV and X-ray telescopes because of the lack of an atmosphere. We are suggesting an exciting new possibility for a large scale science project, which we hope the scientific community will endorse.

This raises another exciting aspect about plans for space exploration in this decade and the next. In addition to going back to the Moon to stay in the 2020s and building the infrastructure that will take to Mars by the 2030s (and beyond), future missions will enable the types of scientific experiments that are challenging here on Earth. In this respect, exploring more of our Solar System will allow to explore more of the Universe!


Could We Put A Telescope On The Far Side Of The Moon?

LROC WAC color (689 nm, 415 nm, 321 nm) overlain on WAC sunset BW image. Note the proximity of the . [+] landing site to a contact between red and blue maria.

Many times! There is currently a small UV-sensitive telescope on the Moon, which landed there in 2013 as part of the Chinese lander Chang’e 3, and it has taken some interesting images from the Moon to relay back to us, but it’s been placed on the near side of the moon, along with the rover, for easier communication with Earth.

In general, the Moon has been considered an interesting place to put telescopes, because it’s a stable patch of ground, with no atmosphere around to interfere with the telescope. There are a lot of wavelengths of light that can currently only be observed from orbiting space telescopes - ultraviolet is mostly blocked by our atmosphere, as is gamma radiation, and infrared. So, if you can place a UV telescope on the surface of the Moon (as the Chinese lander did), you have a setup that doesn’t need gyroscopes to stabilize the telescope, and it can observe freely without the interference from the Earth's atmosphere.

The Moon isn’t an ideal place to put all telescopes though - your telescopes have to be pretty durable to survive the temperature extremes on the Moon between daylight and shadow. With temperatures swinging from -298F to 224F, this is not a particularly kind place for electronics. A temperature swing of more than four hundred degrees, once every two weeks, is not for the fainthearted.

An Apollo 11 oblique view of the lunar farside in the area of International Astronomical Union . [+] crater No. 312, which is about 30 statute miles in diameter.

Optical telescopes on the Moon are similarly tricky - for two weeks out of every month, the telescope would be in daylight, no matter where you put it. The telescope would have to survive two weeks of extremely hot temperatures, cool back down, and then it could observe for a maximum of two weeks. Infrared cameras are an even worse fit the thermal cameras attached to an infrared telescope are extremely sensitive to heat (by design) and without being able to fully shield the camera from the Sun, as we do for space telescopes, the detectors are likely to be unhappy after a few lunar temperature cycles.

There’s an additional complication when it comes to the far side of the Moon in particular - it’s very hard to communicate with anything on the far side of the Moon. Radio waves can’t reach it, and so orbiting crafts have habitually just had a communications blackout while they’re behind the Moon (from an Earthbound perspective). There’s only one type of observatory for which this is an explicit benefit - the radio.

On Earth, almost everything interferes with the clear observation of an astrophysical source in radio wavelengths. Everything from radar, to cell phones, to microwaves, to GPS satellite communications with the ground, to digital cameras, will interfere with the incoming signal from space. In general, the only solution is to put the radio telescopes very far away from as many of these things as we can, and hope for the best. But the far side of the Moon is shielded from all of this by the entire bulk of the Moon, and would truly be an interference-free area to put a radio telescope.

It isn’t problem free, or we might already have a lunar radio telescope. The temperature stresses are still significant for a radio telescope. One option would be to construct a dish, like Arecibo, inside a lunar crater, but that would be a technical challenge without a more significant human presence on the Moon. However, some radio telescopes can function extremely well with simpler electronics. The LOFAR telescope, for instance, is scattered across Europe, and is mostly made up of a large number of very simple antennas, instead of the complex machine that is the coordination of the dish-style antennas that comprise the Very Large Array in New Mexico. In principle, we could scatter a similar set of simple antennas all over the lunar far side, and create a very large radio telescope.

Photo showing a low-band antenna (LBA) of the Low-Frequency Array (LOFAR), an interferometric radio . [+] telescope build in Europe. In the right back of the antenna, a LOFAR cabin is visible that contains electronics. The full array consists of thousands of such antennas.

A. R. Offringa, CC BY-SA 3.0, via Wikimedia Commons

On the other hand, we still want to get the data back. And transmitting from the far side of the Moon directly, as orbiters can attest, is impossible. So how could we submit the data back home to Earth? Your two options are a heavy-duty cable which extends far enough around the Moon that the Earth would be visible, and if you can feel engineers around the world cringing, don’t worry - that’s not the preferred solution. The better solution is to put a communication satellite in orbit around the Moon, whose primary role would be to communicate between the telescope on the lunar ground, and to Earth, when Earth is again visible from orbit. While these communications may be a source of interference to the telescope, it wouln’t be an issue as long as the telescope stopped trying to see distant objects while its communication tether to the Earth was overhead.

The main challenge now, other than developing a way to deploy a large number of antennas on the Moon, is simply cost. While we have no rocket which could transport humans to the Moon at the moment, we have certainly sent orbiters to the Moon in recent years. It’s expensive, though, and with a NASA budget that’s increasingly constrained, pulling together the funds for a telescope on the far side of the Moon will be the major constraint.


Why Do High Tides Happen Both on the Side of the Earth Facing Toward and Away from the Moon?

Question: During new moon, moon is between earth and sun. The spring tide then occurs. The height of high tide is higher. But on right side of image why is water having high tide. As there is no force pulling water from right side of image. As sun and moon are on left. — Priyanshu

Answer: You are right in that high tide occurs on the sides of the Earth which face toward and away from the Moon. This is due to the fact that around the Moon is pulling on the Earth, and the ocean, on the sides facing the Moon. The Earth compensates for this pulling by bulging out both toward and away from the Moon. This results in more water being displaced in these directions, resulting in high tide. One gets the higher tides, called spring tides, when the gravitational force of the Sun is added to that of the Moon, which makes the bulge on the Earth’s surface a bit larger than that caused by the Moon alone. These spring tides happen around New and Full Moon.


Elevator debate

Some experts say a classical space elevator might make more sense than a lunar space elevator, at least initially, because it could help facilitate exploration. For example, a classical space elevator might be used to assemble a huge spacecraft in Earth orbit and then launch it from there. Experts say that would likely be easier than launching the spacecraft from Earth, where gravity is so intense.

But the classical space elevator would require a much stronger cable to withstand the higher forces exerted on the cable — and it’s unclear when materials strong enough and of sufficient length to make a suitable cable will be available.

"The classical space elevator is a really tough problem, because the Earth's gravity field is so great that you need such strong materials that we don’t have right now," says Jerome Pearson, an aerospace engineer and president of Star, Inc., an aerospace company in Mount Pleasant, South Carolina, and the author of several papers on space elevators. "On the other hand, you could build a lunar space elevator with existing materials right now."

Given that concern, Pearson endorses the idea of moving forward with a lunar space elevator as a precursor to building a classical space elevator. "There are a lot of advantages with a lunar space elevator," Pearson says. "And with this new NASA program to return to the moon, there may be additional interest."


In an expanding universe, is Earth getting farther from the sun?

No. While astronomers do believe that the universe has been expanding since the Big Bang, this expansion works on the largest of scales, the scale of the galaxies. In other words, our solar system – our sun and its family of nine planets – is not expanding.

Earth is located 150 million kilometers – about 93 million miles – or 8 light-minutes from the sun. It’s thought to have been located at this distance from the sun since our solar system was born, some four-and-a-half billion years ago. So the sun isn’t getting farther from Earth. And, likewise, our sun isn’t getting farther from other stars in our own galaxy.

Why don’t the solar system and galaxy expand, while the universe as a whole does? The solar system and galaxy are held together gravitationally. Our Milky Way galaxy is a collection of hundreds of billions of stars. It’s thought to be one of billions of galaxies in the universe.

Now we’re at the scale that astronomers talk about when they speak of the “expanding universe.” Our galaxy is getting farther from other galaxies – every galaxy is. There are billions of galaxies, and they are all moving away from each other. In that sense, the universe is thought to be expanding.


Question Why can’t we go to Pluto?

In principle, we could. Trouble is (for human passengers) it would take a terribly long time.

Catastrophe

Approaching asteroid? Is this THE one?

"New Horizons launched on January 19, 2006, and it'll reach Pluto on July 14, 2015. Do a little math and you'll find that it has taken 9 years, 5 months and 25 days. The Voyager spacecraft did the distance between Earth and Pluto in about 12.5 years, although, neither spacecraft actually flew past Pluto."

That's 19 years round trip.

BUT, if you want to stop there, you have to factor a lot of time slowing down (as opposed to just flying by at top speed). Likewise speeding up and slowing down on return.

You are talking about a lifetime - and that is assuming humans could travel at the same speed.

Thomas Likes Space

This is Thomas

So in general, we could go out and go to Pluto, For a lifetime.
Because i am interested in space, if we decide, it’s a lifetime choice with no way of regreting.

My only question is, can we?

Catastrophe

Approaching asteroid? Is this THE one?

Well, we have sent a non-manned vessel.

Catastrophe

Approaching asteroid? Is this THE one?

When you said "go to", did you mean stop, or even turn around and come back?

Catastrophe

Approaching asteroid? Is this THE one?

BTW, when sending a human you need life support for all those years.
Food, water, etcetera.

Wolfshadw

No. We can't. We do not have the proven technology for a human crew to survive in space for such a journey. Solve the problems of radiation, extended periods of weightlessness, and the human psychological factor and then we might have a chance.

Catastrophe

Approaching asteroid? Is this THE one?

XXXXXUSERXXXXX

Catastrophe

Approaching asteroid? Is this THE one?

WE did not. We just sent an object there.

Rj.rivero

BTW, when sending a human you need life support for all those years.
Food, water, etcetera.

Grinning Bandicoot

Helmer

Helmer

Robots do have problems with these things.
Too hot, they overheat or even melt.
Too cold and their mechanical joints freeze up - oil in the hydraulics gets too thick.
Radiation affects circuitry, solar radiation even affects computers here on earth - that's why we have Error Correction Code (ECC) memory in them where needed.

Some planets are even highly acidic, and corrosion have to be factored in.

Rothauserc

Historically survival and wealth creation are stronger drivers of exploration and settlement than curiosity. Traditionally, science(knowledge gathering) was a tool in the long process of exploration, which included surveys, mining, infrastructure creation and settlement(all advanced and protected with military assistance). This was the model of national exploration prior to the twentieth century.

The Lagrange point between the Earth and Moon is a convenient and practical location to store supplies for space faring. We need an incremental, cumulative program that could be paid for as we established these incremental capabilities. The Moon is a destination, but more importantly, it is an enabling asset. By using lunar resources including the ice at the poles, we would learn to "cut the cord" with Earth of space logistics.

What is needed is the incremental, cumulative build-up of space faring infrastructure that is both extensible and maintainable, a growing system whose aim is to transport us anywhere we want to go, for whatever reasons we can imagine, with whatever capabilities we may need.


We Just Had the Super Flower Blood Moon and an Eclipse – Here’s What’s Next!

A perigee full moon, or supermoon, is seen during a total lunar eclipse on Sunday, September 27, 2015, in Washington, DC. The combination of a supermoon and total lunar eclipse last occurred in 1982 and will not happen again until 2033. Credit: NASA/Aubrey Gemignani

This Week’s Full Moon was the Flower, Corn Planting, or Milk Moon, Vesak or Buddha Purnima, a Lunar Eclipse, and a Supermoon.

The full Moon on Wednesday morning, May 26, 2021, appeared opposite the Sun in Earth-based longitude at 7:14 a.m. EDT. The Moon appeared full for about three days around this time, from Monday night through Thursday morning.

While this time was on Wednesday for most of the Earth, for Baker Island and the Pacific Ocean in the timezone just west of the International Date Line this will was before midnight on Tuesday morning. On the other side of the International Date Line, for the Pacific Ocean and islands that fall under Phoenix Island Time, West Samoa Time, and Line Islands Time, this was after midnight on Thursday morning.

A perigee full moon, or supermoon, is seen next to the Empire State Building, Sunday, Sept. 27, 2015 in New York City. The combination of a supermoon and total lunar eclipse last occurred in 1982 and will not happen again until 2033. Credit: NASA/Joel Kowsky

The Moon was so close to opposite the Sun that it passed through the northern part of the shadow of the Earth for a total lunar eclipse. From the Washington, D.C. area, we could only see the barely-detectable start of the eclipse as the Moon sets. Viewing was higher in the sky across the U.S., Pacific Ocean, and Australia, occurring around moonrise from the eastern part of Asia.

One Moon, Many Names

In the 1930s the Maine Farmer’s Almanac began publishing American Indian Moon names for each of the full moons of the year. According to this Almanac, the Algonquin tribes of what is now the northeastern United States called this the Flower Moon for the flowers that are abundant this time of year. Other names include the Corn Planting Moon and the Milk Moon.

“This full Moon is a supermoon and is the closest full Moon of the year.” — Gordon Johnston

This full Moon corresponds to Vesak, also known as Buddha Purnima, and by other regional names. Vesak is a Buddhist holiday that commemorates the birth, enlightenment, and death of Gautama Buddha. The actual date of Vesak varies depending upon the lunar calendar in use in the particular country or region, but this year for most areas it falls on or near the day of this full Moon.

This full Moon is a supermoon and is the closest full Moon of the year, slightly closer than the prior full Moon on April 26, but only by about 0.04% of the distance from the Earth to the Moon at perigee.

Full Moons, New Moons, and Calendars

In many traditional lunisolar calendars, full Moons fall in the middle of the lunar months. This full Moon is in the middle of the fourth month of the Chinese calendar and Sivan in the Hebrew calendar. In the Islamic calendar, the months start with the first sighting of the waxing crescent Moon shortly after the New Moon. This full Moon is near the middle of Shawwal.

As usual, the wearing of suitably celebratory celestial attire is encouraged in honor of the full Moon.

Key Upcoming Celestial Events

Summer Solstice

As spring ends and summer begins, the daily periods of sunlight reach their longest on the solstice, then begin to shorten again. On Wednesday, May 26, 2021 (the day of the full Moon), morning twilight will begin at 4:38 a.m. EDT, sunrise will be at 5:47 a.m., solar noon will be at 1:05:10 p.m. when the Sun will reach its maximum altitude of 72.37 degrees, sunset will be at 8:23 p.m., and evening twilight will end at 9:33 p.m. The earliest sunrise of the year will occur at 5:42:11 a.m. on Sunday, June 13, with twilight starting at 4:30 a.m.

The summer solstice will be on Sunday night, June 20, at 11:32 p.m. June 20 will be the day with the longest period of sunlight, 14 hours, 53 minutes, and 41.6 seconds. On the day of the solstice, morning twilight will begin at 4:30 a.m., sunrise will be at 5:43 a.m., solar noon will be at 1:09:45 p.m. when the Sun reaches its maximum altitude of 74.56 degrees (its highest for the year), sunset will be at 8:37 p.m., and evening twilight will end at 9:49 p.m. By Thursday, June 24, (the day of the full Moon after next), morning twilight will begin at 4:31 a.m., sunrise will be at 5:44 a.m, solar noon will be at 1:10:37 p.m. when the Sun will reach its maximum altitude of 74.51 degrees, sunset will be at 8:37 p.m., and evening twilight will end at 9:50 p.m.

Solar Eclipse

There will be an eclipse of the Sun in the middle of this lunar cycle. On Thursday morning, June 10, 2021, the silhouette of the Moon will block part of the Sun. For parts of northeastern North America (including the Washington, D.C. area), Northern Europe, and Northern Asia, this new Moon will cause a partial eclipse of the Sun. From the Washington, D.C. area, the Moon will be blocking about 80% of the left side of the Sun as the Sun rises in the east-northeast at 5:42 a.m. EDT, causing the Sun to appear as a crescent. As the Sun and Moon rise together, the silhouette of the Moon will gradually shift off the Sun to the lower left, allowing more of the Sun to show until the eclipse ends around 6:29 a.m., when the Sun will appear about 7 degrees above the horizon in the east-northeast.

Please don’t try looking directly at the Sun to see this partial eclipse, you can damage your eyes! One safe way to observe a solar eclipse is to take two sheets of white paper, poke a hole in one, and let the sunlight pass through the hole to cast an image of the Sun on the second sheet of paper. The larger the hole, the brighter but less well-focused the image. When the Moon is blocking part of the Sun, causing the Sun to appear as a crescent, anything with small holes in it will cast shadows that look like crescents. Normally the dapples within the mottled shade cast by a tree will appear as circles, but during an eclipse, these dapples can appear as crescents.

Unlike other planets in our solar system, when we look at our Moon from Earth it appears to be almost the same size as the Sun. When the Moon is nearer to Earth and passes directly in front of the Sun it blocks the Sun completely, called a total solar eclipse. The only time it is safe to look directly at a solar eclipse is for the short period when the Moon is completely blocking the Sun. When the Moon is farther from Earth and passes directly in front of the Sun, it does not block the Sun completely. As the Moon and Sun line up, the Sun appears as a ring of fire around the silhouette of the Moon, called an annular solar eclipse. Since this eclipse occurs less than 3 days after the Moon was at its farthest from the Earth of this orbit, for an area running from a part of Canada, across the Northwestern part of Greenland, the North Pole, and onto part of Siberia, this will be one of these annular solar eclipses.

Summary of Evening Sky Events

On the evening of Wednesday, May 26, 2021 – the day of the full Moon – as evening twilight ends at 9:33 p.m. EDT, the brightest planet visible will be Venus, appearing only 1 degree above the horizon in the west-northwest. To the upper left of Venus will be the planet Mercury, appearing 3 degrees above the horizon. The planet Mars, slightly brighter than Mercury, will appear 23 degrees above the west-northwestern horizon. The constellation Ursa Major, also known as the Big Dipper, will appear in the north close to nearly overhead.

None of the 20 brightest stars will appear close to overhead. The highest bright star will be Arcturus, the 4th brightest star in our night sky, appearing 62 degrees above the southeastern horizon. Arcturus is about 37 light-years from Earth and nearly the same mass as our Sun, but older. Arcturus has used up its core hydrogen and become a red giant, swelling to about 25 times its previous size and shining about 170 times brighter than the Sun. Our Sun is about halfway through this lifecycle and is expected to become a red giant in about 5 billion years.

As the lunar cycle progresses, the planet Mars and the background of stars will appear to shift toward the west (although it is actually the Earth that is moving around the Sun toward the east). Mars will appear to shift more slowly than the stars (since Mars is moving the same direction we are). Low on the horizon on the west-northwest, the planet Mercury will also appear to shift slowly toward the horizon each night, while the brighter planet Venus (appearing as the Evening Star) will appear to move in the opposite direction, shifting higher above the horizon.

Mercury and Venus will appear closest to each other on the evening of May 28, 2021, less than half a degree apart and only 1 degree above the horizon at the time evening twilight ends, setting about 9 minutes later.

May 29, will be the last evening Mercury will appear above the horizon at the time evening twilight ends, although Mercury should continue to be visible about 30 minutes after sunset low on the west-northwestern horizon for the next 5 or 6 evenings (until about June 4).

The planet Mars will appear to pass near the bright star Pollux as May ends and June begins, with the pair appearing at their closest (a little over 5 degrees apart) on the evening of May 31.

On the evening of June 11, the thin, waxing crescent Moon will appear near the bright planet Venus low on the west-northwestern horizon, setting about 12 minutes after evening twilight ends. The next evening (June 12), the crescent Moon will have shifted to appear near the bright star Pollux, and the evening after that (June 13) near the planet Mars.

On the evening of June 15, the bright star Regulus will appear to the lower left of the waxing crescent Moon. On the evening of June 19, the bright star Spica will appear below the waxing gibbous Moon. The bright planet Venus as the Evening Star will appear to pass near the bright star Pollux in the latter part of June, appearing at their closest (a little over 5 degrees apart) on the evening of June 21. On the evening of June 22, the bright star Antares will appear below the waxing gibbous Moon.

By the evening of Thursday, June 24, 2021, (the day of the full Moon after next), as evening twilight ends (at 9:50 p.m. EDT), the brightest planet visible will be Venus, appearing as the Evening Star 4 degrees above the horizon in the west-northwest. To the upper left of Venus will be the planet Mars, appearing 10 degrees above the horizon. The bright star closest to overhead still will be Arcturus at 68 degrees above the horizon in the south-southwest.

Summary of Morning Sky Events

On the morning of May 26, 2021, (the day of the full Moon), as morning twilight begins at 4:38 a.m. EDT, the bright planet Jupiter will appear 29 degrees above the southeastern horizon with the fainter planet Saturn in the south-southeast at 31 degrees above the southeastern horizon. Two stars from the “Summer Triangle,” Deneb and Vega, will appear close to overhead. Closest to overhead will Deneb at about 78 degrees above the northeastern horizon. Deneb is about 2,600 light-years from Earth and is the 19th brightest star in our night sky. A close second with be Vega at just under 77 degrees above the western horizon. Vega is about 25 light-years from Earth and is the 5th brightest star in our night sky.

As the lunar cycle progresses, the background of stars and planets will appear to shift toward the west each morning. On the morning of May 30, 2021, Jupiter, Saturn, and the waning gibbous Moon will appear to form a line from Jupiter in the southeast to the Moon in the south.

By the morning of May 31, the waning gibbous Moon will have shifted to appear to the lower left of Saturn in the south-southeast. By the morning of June 1, the Moon will have shifted to appear below Jupiter in the southeast.

By the morning of June 2, the waning half-moon will have shifted to appear in a rough line with Jupiter and Saturn, from the Moon in the southeast to Saturn in the south-southeast. Beginning the morning of June 20, the planet Mercury starts appearing above the horizon about 30 minutes before sunrise (an approximation of when it may start being visible in the glow of dawn). Mercury will not start appearing above the horizon at the time morning twilight begins until July 1.

By the morning of June 24, 2021, (the day of the full Moon after next), as morning twilight begins at 4:31 a.m. EDT, the bright planet Jupiter will appear 39 degrees above the southeastern horizon with the fainter planet Saturn 33 degrees above the southern horizon. The bright star appearing closest to overhead will still be Deneb from the Summer Triangle, appearing about 78 degrees above the northwestern horizon. Mercury will not have risen yet but will rise in the east-northeast about 23 minutes later at 4:54 a.m. and may be visible for about 20 minutes before it is masked by the glow of dawn.

Daily Guide

Here is a more detailed, day-by-day listing of celestial events between now and the full Moon after next. Times are based on the location of NASA Headquarters in Washington, D.C.:

Even though they are not usually visible, I include in these Moon missives information about Near Earth Objects (mostly asteroids) that may pass the Earth within 5 lunar distances, because I find it interesting that we have discovered so many. In late May or early June 2021 (2021-May-25 09:26 UTC with 7 days, 17 hours, 11 minutes uncertainty), Near-Earth Object (2013 VO11), between 19 to 43 feet (6 and 13 meters) across, will pass the Earth at between 3.1 and 43.4 lunar distances (nominally 3.4), traveling at 22,800 miles per hour (10.18 kilometers per second).

May 25

Tuesday night, May 25, 2021, at 9:51 p.m. EDT, the Moon will be at perigee, its closest to the Earth for this orbit.

May 26

The next full Moon will be on Wednesday morning, May 26, 2021, at 7:14 a.m. EDT. There will be a total eclipse of the Moon, but in the Washington, D.C. area, only the barely detectable start of the eclipse will be visible as the Moon sets. Locations farther west will have better views. The Moon will appear full for about 3 days around this time, from Monday evening through Thursday morning.

Wednesday morning at 10:25 a.m. EDT (2021-May-26 14:25 UTC with 2 minutes uncertainty), Near-Earth Object (2021 JG1), between 96 to 214 feet (29 and 65 meters) across, will pass the Earth at 2.2 lunar distances traveling at 20,500 miles per hour (9.16 kilometers per second).

Wednesday evening at about 9 p.m. EDT (2021-May-27 00:59 UTC with 51 minutes uncertainty), Near-Earth Object (2021 KP), between 55 to 123 feet (17 and 38 meters) across, will pass the Earth at 1.6 lunar distances traveling at 25,000 miles per hour (11.17 kilometers per second).

May 28

On Friday evening, May 28, 2021, the planets Mercury and Venus will appear closest to each other, less than half a degree apart and only 1 degree above the horizon at the time evening twilight ends at 9:35 p.m. EDT, setting about 9 minutes later.

May 29

Saturday evening, May 29, 2021, will be the last evening Mercury will appear above the horizon at the time evening twilight ends. Mercury should continue to be visible about 30 minutes after sunset low on the west-northwestern horizon until about June 4.

Sometime Saturday night or Sunday morning, May 29 or 30, 2021 (2021-May-30 05:14 UTC with 11 hours, 37 minutes uncertainty), Near-Earth Object (2021 KM1), between 46 to 102 feet (14 and 31 meters) across, will pass the Earth at between 4.4 and 4.9 lunar distances (nominally 4.6) traveling at 8,200 miles per hour (8.11 kilometers per second).

May 30

On Sunday morning, May 30, 2021, the planets Jupiter and Saturn and the waning gibbous Moon will appear to form a line from Jupiter on the left in the southeast to the Moon on the right in the south.

May 31

On Monday morning, May 31, 2021, the waning gibbous Moon will have shifted to appear to the lower left of the planet Saturn in the south-southeast.

Monday evening, the planet Mars and the bright star Pollux will appear at their closest (a little over 5 degrees apart). The pair will appear near each other as May ends and June begins.

June 1

On Tuesday morning, June 1, 2021, the waning gibbous Moon will have shifted to appear below the bright planet Jupiter in the southeast.

Sometime around June 1, 2021 (2021-Jun-01 17:05 UTC with 19 hours, 12 minutes uncertainty), Near-Earth Object (2018 LB), between 55 to 123 feet (17 and 38 meters) across, will pass the Earth at between 0.9 and 7.2 lunar distances (nominally 2.9) traveling at 17,200 miles per hour (7.70 kilometers per second).

June 2

On Wednesday morning, June 2, 2021, the waning Moon will appear half-full as it reaches its last quarter at 3:24 a.m. EDT. The Moon will have shifted to appear in a rough line with the planets Jupiter and Saturn, from the Moon on the left in the southeast to Saturn on the right in the south-southeast.

June 7

Monday night, June 7, 2021, at 10:28 p.m. EDT, the Moon will be at apogee, its farthest from the Earth for this orbit.

June 10: Solar Eclipse

Thursday morning, June 10, 2021, at 6:53 a.m. EDT, will be the new Moon, when the Moon passes between the Earth and the Sun. As described above, the Moon will eclipse the Sun. Remember that it is unsafe to look directly at the Sun (unless you have special eclipse glasses to protect your eyes). Parts of Canada, Greenland, the Arctic Ocean, and Siberia will see an annular eclipse.

For much of the rest of northeastern North America, Greenland, Northern Europe, and northern Asia, this will be a partial eclipse. From the Washington, D.C. area, the Moon will be blocking about 80% of the left side of the Sun as they rise together in the east-northeast at 5:42 a.m., causing the Sun to appear as a crescent. As the pair rises higher in the sky, the silhouette of the Moon will gradually shift off the Sun to the lower left, allowing more of the Sun to show until the eclipse ends at around 6:29 a.m., with the Sun about 7 degrees above the horizon in the east-northeast.

On Thursday evening, the planet Mercury will be passing between Earth and Sun as seen from Earth – this is called inferior conjunction. Mercury will be shifting from the evening sky to the morning sky and will begin emerging from the glow of dawn on the eastern horizon after about June 20.

The day of – or the day after the new Moon – marks the start of the new month for most lunisolar calendars. The fifth month of the Chinese calendar starts on June 10, 2021 (at midnight in China’s time zone, which is 12 hours ahead of EDT). Sundown on June 10, marks the start of Tammuz in the Hebrew calendar. In the Islamic calendar, the months traditionally start with the first sighting of the waxing crescent Moon. Many Muslim communities now follow the Umm al-Qura Calendar of Saudi Arabia, which uses astronomical calculations to start months in a more predictable way. Using this calendar the eleventh month of the year, Dhu al-Qadah will begin at sunset on Thursday, June 10.

June 11

On Friday evening, June 11, 2021, the thin, waxing crescent Moon will appear to the lower right of the bright planet Venus on the west-northwestern horizon, setting about 12 minutes after evening twilight ends. Venus will likely be easier to spot in the glow of dusk than the thin crescent of the Moon.

June 12

On Saturday evening, June 12, 2021, the bright star Pollux will appear about 5 degrees above the waxing crescent Moon, with the bright planet Venus appearing about 8 degrees to the lower right of the Moon.

June 13

For the Washington, D.C. area (and similar latitudes, at least), the earliest sunrise of the year will occur on Sunday, June 13, 2021, at 5:42:11 a.m. EDT with twilight starting at 4:30 a.m..

On Sunday evening, the planet Mars will appear about 3 degrees below the waxing crescent Moon.

June 14

Monday, June 14, 2021, is the fifth day of the fifth month of the traditional Chinese calendar, the day of the Dragon Boat Festival.

June 15

Tuesday evening, June 15, 2021, the bright star Regulus will appear about 4 degrees to the lower left of the waxing crescent Moon.

June 17

On Thursday night, June 17, 2021, the Moon will appear half-full as it reaches its first quarter at 11:54 p.m. EDT.

June 19

Saturday evening, June 19, 2021, the bright star Spica will appear about 5 degrees below the waxing gibbous Moon.

June 20: Summer Solstice

Beginning the morning of Sunday, June 20, 2021, the planet Mercury will begin appearing above the horizon about 30 minutes before sunrise (approximately when it may start being visible in the glow of dawn). Mercury will not start appearing above the horizon at the time morning twilight begins until July 1.

Sunday night, at 11:32 p.m. EDT, will be the summer solstice, the astronomical end of spring and the beginning of summer. This will be the day with the longest period of daylight.

June 21

On Monday evening, June 21, 2021, the bright planet Venus (as the Evening Star) and the bright star Pollux will appear at their closest to each other, a little over 5 degrees apart. The pair will appear near each other during the latter part of June.

June 22

Tuesday evening, June 22, 2021, the bright star Antares will appear about 5 degrees below the waxing gibbous Moon.


Earth, Sun and Moon

From our perspective, the three objects that have the greatest impact on our lives are the Earth, Sun, and Moon. The Earth, of course, is the planet beneath our feet. Without it, well, we wouldn’t have anything at all. The Sun warms our planet, and with the Moon, creates the tides.

The Moon orbits the Earth and in turn, the Earth orbits the Sun. We see the Universe from a platform that is both rotating on its axis, and traveling in an elliptical orbit around the Sun. The Earth’s rotation on its axis makes the Sun rise in the east and set in the west, and is a big part of why the Moon rises and sets too although the Moon takes 29 days to complete an orbit around the Earth as well.

The average distance from the Earth to the Moon is 384,403 km. And the average distance from the Earth to the Sun is 149,597,887 km. If you divide these two numbers, you get approximately 389. Now, if you divide the diameter of the Sun (1.4 million km) by the diameter of the Moon (3,474 km), you get 403. Those two numbers are pretty close. This is why the Moon and the Sun appear to be the same size in the sky it’s a total coincidence.

Because they appear to be the same size in the sky, the Sun, Earth and Moon work together to create eclipses. When the Moon is directly in between the Earth and Sun, we see a solar eclipse. The Moon appears to pass in front of the Sun and darken it completely. And in the opposite situation, when the Earth is in between the Sun and the Moon, the Earth’s shadow darkens the Moon. This is a lunar eclipse. We don’t see eclipses every month because the Moon’s orbit it tilted slightly away from the Earth’s orbit around the Sun. Sometimes the Moon is above this orbit and sometimes it’s below, so it doesn’t block the light from the Sun, or get caught in the Earth’s shadow.

The Sun and the Moon work together to create the tides we experience here on Earth. Most of the rise of the tides comes from the gravitational pull of the Moon, but a small amount comes from the Sun. When the two objects are on the same side of the Earth, we get the highest and lowest tides, and when they’re on opposite sides of the Earth, the tides are less extreme.

The brightest object in the Sky is the Sun. Astronomers measure its apparent magnitude as -26.73. This makes it 449,000 times brighter than the full Moon. The brightness of the Moon is only -12.6. Of course all of the Moon’s brightness is just reflected light from the Sun.

We have written many articles about the Earth for Universe Today. Here’s a more detailed article about the Sun and the Moon.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.


Why Eclipses Don't Happen Every Month

Why don't solar eclipses happen every month? After all, you only need three things to make a solar eclipse: the Sun, the Earth and the Moon.

Saad Amer : A total solar eclipse will cross the US, and people are really excited. That’s because many Americans have never seen a total solar eclipse. But, why is that? After all, you only need three things to make a solar eclipse: the Sun, the Earth and the Moon in between.

The Âoon passes between the Earth and the Sun once every 27 days. So, why don’t we get an eclipse every month?

Well, solar eclipses only happen when the Moon’s shadow falls on Earth. And most months, that shadow misses us. It’s all due to the science of orbital mechanics.

See, the Earth revolves around the sun in this plane. The Moon orbits the Earth in this plane, which is 5 degrees tilted. The Moon’s shadow only lands on us when the Moon and the Earth’s planes line up, making the Sun, Earth and Moon all align.

Eclipses happen every year, but you still might not get a total solar eclipse. Most commonly, a partial solar eclipse occurs, where the Moon covers only part of the Sun.

About one third of solar eclipses are “annular.” They happen when the Moon’s orbit takes it slightly farther away from the Earth – the outer edges of the Sun are exposed, producing a glowing ring around the Moon.

Total solar eclipses only happen when the moon is close enough to Earth to block out the entire Sun. That means around every 18 months there’s a total solar eclipse somewhere on Earth.

Luckily for us, a solar eclipse is happening on August 21st from Oregon to South Carolina along this path. It’s the first one in 99 years to go all the way across America!

So hope for clear skies and don’t forget your ISO approved eclipse glasses!