How Long is a Day on the Moon?

A photo of the full moon, taken from Apollo 11 on its way home to Earth, from about 18,520 km (10,000 nm) away. Credit: NASA

The Moon has been around since the earliest days of the Solar System. To human beings, there has never been a time when we couldn’t look up in the night sky and either see the Moon hanging there, or know that it would be back the very next night (i.e. a New Moon). And thanks to the development of modern astronomy and space exploration, our understanding of the Moon has grown immensely.

For instance, we know that the Moon formed early in Earth’s history, and that it may have played an important role in the development of life here on Earth. We’ve also learned that Moon is tidally-locked with Earth, which means that one side is constantly facing towards it. But how long is a day on the Moon? With one side facing the Earth and the other side facing out, what constitutes a single day on the lunar surface?

To break it down simply, a day on the Moon lasts as long as 29.5 Earth days. In other words, if you were standing on the surface of the Moon, it would take 29.5 days for the Sun to move all the way across the sky and return to its original position again. However, as with all bodies in the Solar System, distinguishing between different types of days (based on different types of periods) is necessary.

Orbit and Rotation:

Since ancient times, lunar calendars have been based on thirteen months of 28 days each, reflecting the lunar cycle. But as astronomers have discovered from centuries of studying the Moon’s behavior, the Moon’s orbital period (i.e. the time it takes for the Moon to complete a single orbit around the Earth) is actually the equivalent of about 27.3 Earth days – or 27 days 7 hours 43 minutes and 11.5 seconds, to be precise.

And while the Moon rotates on its own axis, the speed at which it rotates (aka. it’s sidereal rotation) is very slow. In fact, it takes the Moon the equivalent of 27.3 Earth days to complete a single rotation on its axis, the same amount of time it takes to complete a single orbit around Earth.  What this means is that the Moon is tidally-locked with Earth.

In other words, the Moon always points the same face towards the Earth, which is why human beings are so familiar with the “face” of the Moon, and refer to the side that faces away from us as the “the dark side”. Therefore, if you were standing on the surface of the Moon, you would always see the Earth in exactly the same position, while the stars and the Sun would continue to move around in the sky.

Sidereal vs. Synodic Day:

However, the Moon’s sidereal rotation is not where we get a the value of a single lunar day from. While it takes 27.3 days for it to orbit the Earth, we have to keep in mind that the Earth is also orbiting the Sun. The Earth returns to its same position in orbit every 365 days. So in order for the Sun to catch up to its same position in the sky from the perspective of the Moon, it has to turn a little more.

The extra 2.2 days is the time for the Moon to catch up in its rotation. And while the amount of time the Moon takes to complete one turn on its axis with respect to the stars is 27.3 days (a sidereal day), the amount of time it takes for the Sun to return to the same position in the sky is called a synodic day, and that’s what takes 29.5 days.

Ergo, a single day on the Moon, with respect to the Sun returning to the same position in the sky, is actually about as long as an average month here on Earth. So if people are planning on living there someday, and aren’t living in the permanently shadowed craters that exist in the southern and norther polar regions, that’s something they might have to get used to.

As with all the bodies of the Solar System, it all comes down to a matter of perspective. And if you’re living on the Moon, your perspective on what constitutes a day will be vastly different from that of a person who was born on Earth.

We have written many interesting articles about how long a day is on the planets of the Solar System. Here’s How Long is a Day on the Other Planets of the Solar System?, How Long is a Day on Mercury?, How Long is a Day on Venus?, How Long is a Day on the Earth?, How Long is a Day on Mars?, How Long is a Day on Jupiter?, How Long is a Day on Saturn?, How Long is a Day on Uranus?, How Long is a Day on Neptune?, and How Long is a Day on Pluto?

For more information, check out NASA’s Lunar and Planetary Science page. And here’s NASA’s Solar System Exploration Guide.

Astronomy Cast also has a good episode on the subject. Listen here: Episode 17: Where Did the Moon Come From?


What is a Super Moon?

It’s a bird, it’s a plane it’s…

OK, it’s a bad gag, I know. But the movie Man of Steel isn’t the only thing that’s “super” about June this year. The closest full Moon of 2013 occurs on June 23, when it will be 356,991 kilometres from Earth, within 600 kilometres of its closest possible approach. When the Moon is closest to Earth in its orbit, it also appears just a bit larger in the sky. But that’s if you’re really paying attention, however!

Some claims circulating on the Internet tend to exaggerate how large the Moon will actually appear. And as for the assertions that the Moon will look bright purple or blue on June 23, that’s just not true. As seems to happen every year, the term “supermoon” has once again reared its (ugly?) head across ye ole Internet. Hey, it’s a teachable moment, a good time to look at where the term came from, and examine the wonderful and wacky motion of our Moon.

I’ll let you in on a small secret. Most astronomers, both of the professional and backyard variety, dislike the informal term “supermoon”. It arose in astrology circles over the past few decades, and like the term “Blue Moon” seems to have found new life on the Internet.  A better term from the annuals of astronomy for the near-coincidence of the closest approach of the Full Moon would be Perigee Full Moon. And if you really want to be archaic, Proxigean Moon is also acceptable.

On June 23, 2013, the Moon will be full at 7:32 AM EDT/ 11:32 UT, only 20 minutes after it reaches perigee, or its closest point to Earth in its orbit.

You can see the change in apparent size of the Moon (along with a rocking motion of the Moon known as nutation and libration) in this video from the Goddard Space Flight Center’s Scientific Visualization Studio. You can also see full animations for Moon phases and libration for 2013 from the northern hemisphere and southern hemisphere.

And all perigees are not created equal, either. Remember, a Full Moon is an instant in time when the Moon’s longitude along the ecliptic is equal to 180 degrees. Thus, the Full Moon rises (unless you’re reading this from high polar latitudes!) opposite as the Sun sets. Perigee also oscillates over a value of just over 2 Earth radii (14,000 km) from 356,400 to 370,400 km. And while that seems like a lot, remember that the average distance to the Moon is about 60 earth radii, or 385,000 km distant.

Astronomers yearn for kryptonite for the supermoon. The Moon passes nearly as close every 27.55 days, which is the time that it takes to go from one perigee to another, known as an anomalistic month. This is not quite two days shorter than the more familiar synodic month of 29.53 days, the amount of time it takes the Moon to return to similar phase (i.e. New to New, Full to Full, etc).

This offset may not sound like much, but 2 days can add up. Thus, in six months time, we’ll have perigee near New phase and the smallest apogee Full Moon of the year, falling in 2013 on December 19th. Think of the synodic and anomalistic periods like a set of interlocking waves, cycling and syncing every 6-7 months.

You can even see this effect looking a table of supermoons for the next decade;

Super Moons for the Remainder of the Decade 2013-2020.



Perigee Time

Perigee Distance

Time from Full



June 23



< 1 hour


July 21



-21 hours


July 13



+21 hours


August 10



< 1 hour


September 8



-22 hours


August 30



         +20 hours


September 28



-1 hour



October 26



-23 hours


October 16



+19 hours



November 14



-2 hours



December 4



+16 hours


January 1



-4 hours


January 21



+14 hours



February 19



-6 hours


March 10



+12 hours


April 7



-8 hours

Sources: The fourmilab Lunar Perigee & Apogee Calculator & NASA’s Eclipse Website 2011-2020.Note: For the sake of this discussion, a supermoon is defined here as a Full Moon occurring within 24 hours of perigee. Other (often arbitrary) definitions exist!

Note that the supermoon slowly slides through our modern Gregorian calendar by roughly a month a year.

In fact, the line of apsides (an imaginary line drawn bisecting the Moon’s orbit from perigee to apogee) completes one revolution every 8.85 years. Thus, in 2022, the supermoon will once again occur in the June-July timeframe.

To understand why this is, we have to look at another unique feature of the Moon’s orbit. Unlike most satellites, the Moon’s orbit isn’t fixed in relation to its primaries’ (in this case the Earth’s) equator. Earth rotational pole is tilted 23.4 degrees in relation to the plane of its orbit (known as the ecliptic), and the Moon’s orbit is set at an inclination of 5.1 degrees relative to the ecliptic. In this sense, the Earth-Moon system behaves like a binary planet, revolving around a fixed barycenter.

The two points where the Moon’s path intersects the ecliptic are known as the ascending and descending nodes. These move around the ecliptic as well, lining up (known as a syzygy) during two seasons a year to cause lunar and solar eclipses.

The complex motion of the Moon, depicting the precession of the nodes versus the average movement of the line of apsides. (Credit: Geologician, Homunculus 2. Wikimedia Commons graphic  under a Creative Common Attribution 3.0 Unported license).
The complex motion of the Moon, depicting the movement of the nodes versus the average movement of the line of apsides. (Credit: Geologician, Homunculus 2. Wikimedia Commons graphic under a Creative Common Attribution 3.0 Unported license).

But our friend the line of apsides is being dragged backwards relative to the motion of the nodes, largely by the influence of our Sun. Not only does this cause the supermoons to shift through the calendar, but the Moon can also ride ‘high’ with a declination of around +/-28 degrees relative to the celestial equator once every 19 years, as happened in 2006 and will occur again in 2025.

Falling only two days after the solstice, this month’s supermoon is also near where the Sun will be in December and thus will also be the most southerly Full Moon of 2013. Visually, the Full Moon only varies 14% in apparent diameter from 34.1’ (perigee) to 29.3’ (apogee).

Can you see the difference? A side by side comparison of the perigee and apogee Moon. (Credit: Inconstant Moon).
Can you see the difference? A side by side comparison of the perigee and apogee Moon. (Credit: Inconstant Moon).

A fun experiment is to photograph the perigee Moon this month and then take an image with the same setup six months later when the Full Moon is near apogee. Another feat of visual athletics would be to attempt to visually judge the Full Moons throughout a given year. Which one do you think is largest & smallest? Can you discern the difference with the naked eye? Of course, you’d also have to somehow manage to insulate yourself from all the supermoon hype!

A comparison of the rising Moon (left) & the Full Moon high in the sky... as you can see, atmospheric refraction actually tends to "shrink" the apparent size of a rising Moon! (Credit:
A comparison of the rising Moon (left) & the Full Moon high in the sky… as you can see, atmospheric refraction actually tends to “shrink” the apparent size of a rising Moon! (Credit & Copyright: Richard Fleet (@dewbow) The Moon Illusion). 

Many folks also fall prey to the rising “Moon Illusion.” The Moon isn’t visually any bigger on the horizon than overhead. In fact, you’re about one Earth radii closer to the Moon when it’s at the zenith than on the horizon. This phenomenon is a psychological variant of the Ponzo illusion.

The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Note the size difference. Image Credit: Marco Langbroek, the Netherlands, via Wikimedia Commons.
The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Note the size difference. Image Credit: Marco Langbroek, the Netherlands, via Wikimedia Commons.

Here are some of the things that even a supermoon can’t do, but we’ve actually heard claims for:

–      Be physically larger. You’re just seeing the regular-sized Moon, a tiny bit closer.

–      Cause Earthquakes. Yes, we can expect higher-than-normal Proxigean ocean tides, and there are measurable land tides that are influenced by the Moon, but no discernible link between the Moon and earthquakes exists. And yes, we know of the 2003 Taiwanese study that suggested a weak statistical correlation. And predicting an Earthquake after it has occurred, (as happened after the 2011 New Zealand quake) isn’t really forecasting, but a skeptical fallacy known as retrofitting.

–      Influence human behavior. Well, maybe the 2013 Full Moon will make some deep sky imagers pack it in on Sunday night. Lunar lore is full of such anecdotes as more babies are born on Full Moon nights, crime increases, etc. This is an example the gambler’s fallacy, a matter of counting the hits but not the misses. There’s even an old wives tale that pregnancy can be induced by sleeping in the light of a Full Moon. Yes, we too can think of more likely explanations…

–      Spark a zombie apocalypse. Any would-be zombies sighted (Rob Zombie included) during the supermoon are merely coincidental.

Do get out and enjoy the extra illumination provided by this and any other Full Moon, super or otherwise. Also, be thankful that we’ve got a large nearby satellite to give our species a great lesson in celestial mechanics 101!

The Birth of a Saros – This Weekend’s Hidden Eclipse

As the first eclipse season of 2013 comes to an end this weekend, an extremely subtle lunar eclipse occurs on the night of Friday, May 24th going into the morning of Saturday, May 25th. And we do mean subtle, as in invisible to the naked eye… this eclipse only lasts 34 minutes in duration and less than 2% of the disk of the Moon enters the bright outer penumbra of the Earth’s shadow!

So, why talk about such a non-event at all?

Great things come from such humble beginnings. And while this weekend’s eclipse is one mostly for the almanacs and astronomical tables rather than a true observational event, it also marks the start of a new lunar saros cycle.

This weekend’s eclipse is one of five for 2013, a year which contains two solars and three lunars. This eclipse marks the end of the first “eclipse season” of the year, a time when the intersection of the Moon’s orbit (known as nodes) and the ecliptic nearly coincide with the position of the Sun (for a solar eclipse at New Moon) and the Earth’s shadow (for a lunar eclipse at Full Moon).

The current season began with a very slight partial eclipse on April 25th, followed by an annular eclipse on May 10th. It will last only 33 minutes and 45 seconds in duration starting at 03:53:11 UTC on May 25th. The Moon will be high over the Americas at the time, but again, shading on the southern limb of the Moon will be too slight to be seen.

Curiously, SLOOH will be providing live coverage of the eclipse, although again, it will be too slight to see.

Starry Night
The Full Moon just nicks the Earth’s penumbra in the early morning hours of May 25th. (Created by the author in Starry Night).

What is a saros? A saros is a period of 18 years 11 days and 8 hours after which an eclipse cycle lines up, producing a similar eclipse to the one that preceded it 18 years before. Note that due to its 8 hour offset, the Earth will have rotated 120° and the visibility region will have shifted westward.

In said period, three lunar cycles very nearly line up;

The Anomalistic month (the period the Moon takes to go from one perigee to another) = 27.555 days.

The Draconic month (the period the Moon takes to return to the same node) = 27.212 days.

The Synodic month (the most familiar one, the period between similar phases) = 29.531 days.

Note that:

239 Anomalistic months = 239×27.555= 6585.645 days.

242 Draconic months = 242×27.212=6585.304 days.

223 Synodic months = 223×29.531=6585.413 days.

There’s that mis-alignment of a third of a day again (8 hours) for every 18 years and 11 days. This also causes the node of each eclipse in the cycle to drift eastward by 0.5° along the ecliptic. Thus, each eclipse isn’t exactly the same. A lunar saros series starts with a very brief penumbral like this weekend’s, becomes deeper and deeper every 18+ year period until partial and total eclipses begin centuries down the road. Thereafter, the cycle reverses, until a final faint penumbral marks the end of the lunar saros.

The progression of selected eclipses of the same saros cycle. (Credit: Matthew Zimmerman. Wikimedia Commons graphic in the Public Domain).

After this weekend’s eclipse, the next start of a lunar saros won’t occur until November 8th 2060 with the start of saros 156. The last new saros series (number 149) began on June 13th, 1984.

There are numbered saros series for both lunar and solar eclipses. There are currently 41 saroses (the plural of saros) active with the inclusion of this weekend’s start of lunar saros 150.

Saros 150, of which this eclipse is the 1st of 71, will last for just over 1,262 years. It will begin to produce partial eclipses on August 20th, 2157 and produce its 1st total on its 32nd lunar eclipse on April 29th, 2572.

It amazes me that ancient cultures such as the Chaldeans new of saros cycles and could predict eclipses. Being geographically isolated, lunar eclipse cycles would have been easier to decipher than solar ones, as you only have to be on the Moonward facing hemisphere of the Earth to witness the eclipse. They may well have stumbled upon the saros while attempting to calculate a slightly longer 19 year period known as a Metonic cycle to align ancient luni-solar calendars.

And yes, that 8 hour offset also means that after a triple saros period, lunar and solar eclipses of the same saros series do return to roughly the same longitude every 54 years & 34 days. This is known as an exeligmos, and if you get this on a triple-word score in Scrabble, you can safely retire from the game.

The theoretical visibility circumstances for this week’s penumbral eclipse. (Credit: F. Espenak/NASA/GSFC).

And while this eclipse is more of academic than observational interest, you can always enjoy the light of a brilliant Full Moon. The May Full Moon is referred to as the Flower, Milk, and Corn Planting Moon by the Algonquian Indians of North America, alluding the latent season of Spring.

Also, keep an eye out for several conjunctions and occultations this week by the Moon with bright stars and planets.

The first up is the bright star Spica (Alpha Virginis) which gets occulted by the waxing gibbous Moon around ~11:00 UT on Wednesday, May 22nd for viewers across northern Australia, southern Asia and the South Pacific. Spica is one of four stars brighter than magnitude +1.5 that the Moon can occult, the others being Antares, Aldebaran and Regulus. This is the 6th occultation in a cycle of 13 of Spica by the Moon spanning 2013.

The planet Saturn will lie about 4° north of the waxing gibbous Moon on the following evening of May 23rd.

Also, watch for an occultation of the +2.6th magnitude star Beta Scorpii on the evening of May 24th around the time of the lunar eclipse. This will be a difficult one, as the Moon will be near 100% illumination. Conjunction of the Moon and Beta Scorpii in right ascension occurs at 3:04 UT on May 25th, about 2.5 hours after Full. The occultation will span the southeastern US, Caribbean, northern South America and western Africa.

Created by Author
Visibility path of the occultation of Beta Scorpii by the Moon. (Credit: Occult

2013 isn’t a grand year for eclipses. We’ve got two more in the late season of the year, another slightly deeper penumbral on October 18th and a hybrid solar eclipse on November 3rd. And when, may you ask, will we FINALLY have another total lunar eclipse? Stick around ‘til U.S. Tax Day next year (April 15th 2014) for a total lunar eclipse spanning the Americas!