Distant full Moon tonight

By Phil Plait | October 11, 2011 6:18 pm

I almost missed this, but an email from astrophotographer Anthony Ayiomamitis (whose photo I feature below) reminded me: tonight’s full Moon occurs at apogee, the point in the Moon’s orbit where it is most distant from Earth. I actually wrote quite a bit about this last year, so I’ll repost the article below. Full Moon occurs officially tonight at 02:06 UTC (10:06 p.m. Eastern US time), so in a couple of hours as I write this. Apogee occurs about 9 hours later (October 12 at 11:44 UTC), when the Moon will be 406,176 km (252,286 miles) from the Earth. It was at perigee on September 28, when it was a mere 357,555 km (222,174 miles) from us… but make sure you read the footnote below!

And I’ll note: the difference in size between the Moon at closest and farthest approach isn’t something you’d probably never notice it by eye, especially since you can’t compare the two at the same time. The change is gradual, and the Moon is actually pretty small in the sky. But it’s still neat when you take a picture and compare them…




I’ve been posting a lot of extreme close-ups of the Moon, but sometimes you can learn something by taking a step back.

For example, I imagine if I went out in the street and asked people what shape the Moon’s orbit was, they’d say it was a circle (or, given recent poll results, they’d say it was Muslim). In fact, however, the Moon’s orbit is decidedly elliptical. When it’s closest to Earth — the point called perigee — it’s roughly 360,000 kilometers (223,000 miles) away*, and when it’s at its farthest point — apogee — it’s at a distance of about 405,000 km (251,000 miles).

That’s a difference of about 10% — not enough to tell by eye, but certainly enough to see in a picture… like this one, by the Greek amateur astronomer Anthony Ayiomamitis:

lunar-apogee-perigee-2010

[Click to emperigeenate.]

Amazing, isn’t it? The Moon is noticeably different! He took those images at full Moon, but seven months apart, when the Moon was at perigee (last January) and apogee (just a few days ago as I write this). It’s part of a project he does every year, and it’s pretty cool. He was able to get these images within a few moments of the exact times of apogee and perigee.

You might wonder how the Moon can be at apogee when it’s full one time, and perigee at another time it’s full. That’s a good question, and it’s because the phase of the Moon doesn’t depend on the shape of its orbit, it depends on the angle between the Sun, the Moon, and the Earth.

If the Sun is behind the Moon from our viewpoint, we see only the dark side, and the Moon is new. If the Sun is behind us, and shining straight down on the Moon, we see it as full. The crescent and gibbous phases happen in between those times. So while the Moon’s phase depends on where it is in its orbit relative to the Sun and Earth, the orbit shape — the fact that it’s a bit of an ellipse and not a circle — isn’t all that important.

Not only that, the time it takes to go from full Moon to full Moon (called the synodic month) is not the same amount of time it takes to go from perigee, around the Earth, and back to perigee (called the anomalistic month). The first is about 29.5 days, the second about 27.6 days. That difference means that every time the Moon gets to perigee, it takes an extra 2.2 days or so for the phase to catch up.

Or, a better way to think about it is like this: say at some date the Moon is both full and at perigee. 29.5 days later, it’s full again, but it’s had an extra 2.2 days around the Earth. It’s a little bit past perigee when it’s full (or you could say it hit perigee before it was full again). Wait until the next full Moon and now it’s 4.4 days past perigee (or, it was at perigee again 4.4 days before it was full a third time). Keep doing that; after about 6 cycles of its phases, that extra time will add up to about half of the anomalistic cycle.

In other words, full Moon will happen at apogee!

It’s not an exact match, so you don’t really get a perfect full Moon at perigee and another at apogee in one year. But as Anthony showed, you can get pretty close.

And if you’re wondering why you’ve never noticed the 10% difference in Moon size, it’s because when you look at it, you’re not comparing it side-by-side with itself like in the picture. You don’t have a good gauge of exactly how big it is from month to month, so you never notice. You need to photograph it, or observe it very carefully through a telescope.

I’ll note that the Earth’s orbit around the Sun is also an ellipse, so the Sun appears bigger and smaller throughout the year; the change isn’t as big as for the Moon, but you can see for yourself because Anthony has images of that as well.

And if you’re curious about on what dates the Moon reaches perigee and apogee, head over to Fourmilab’s Perigee and Apogee calculator.

Amazing, isn’t it, that something that seems this obvious can be hidden in plain view. It makes you wonder what else you’re missing, doesn’t it?



* That distance is measured between the center of the Earth and the center of the Moon. Subtract the radii of each [(1737 + 6360) ≈ 8100 km (5020 miles)] to get the rough distance between the surfaces of the two objects.

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (24)

Links to this Post

  1. Bad Astronomy review: Terra Nova | Alkaon Network | October 17, 2011
  1. Thameron

    Anthony should put all four of them (sun and moon at apogee and perigee) into the same picture in order of size to show why you sometimes get annular eclipses and sometimes get full eclipses.

  2. Pete Jackson

    If the full moon at perigee is 10% larger in size than when we have full moon at apogee, then it will be about 20% brighter. Now, the Sun is 3.4% larger at perihelion (in January) than it is at aphelion (in July), then it will be 6.8% brighter, say 7%.

    So a perigee full moon in January will be 28% brighter than an apogee full moon in July. Getting to be quite a difference!

    And, in the northern hemisphere, the full moon will be much higher in the sky in January than in July, making a big effect on how much the landscape is illuminated. So, watch for a perigee full moon in at midnight in January with snow on the ground, and you’ll see the brightest lunar-lit landscape possible!

  3. minusRusty

    the difference in size between the Moon at closest and farthest approach isn’t something you’d probably never notice it by eye

    “Isn’t something you’d never notice”??? *blech*

    -Rusty

  4. Steve

    Since the moon is very slowly receding from the earth, does that mean that each perigee is as close as the moon will be ever again? I know that the rate of recession is exceptionally small (at least on our timescales), but are there other factors that come into play? Proximity of NEO’s or comets? I would believe that the effect of other planets would be negligible, but we’re talking about very small distances in the yearly recession of the moon from the earth…

    Thanks,
    Steve

  5. Ali

    Who really cares?
    It’s the most wonderful thing!

  6. I was watching the premier of Terra Nova the other night, and they claimed the huge moon in the sky 85 million years ago was due to the fact that the moon was so much closer at that time.

    I did a few calculations, and at about 1.27 cm/year, or 0.5 inches, it would have been about 670 miles closer 85 million years ago. That would be a little more than 0.2% of the current distance.

    Can anyone confirm that? If correct, I don’t think it would look much different.

  7. jjmcgaffey

    I agree with minusRusty – there’s something seriously wrong with your “And I’ll note”. Not exactly a double negative, but… Oh, and the random ‘it’ later in the sentence. It needs fixing.

    That aside – cool. I think I read this the first time you posted it, but it remains cool – thanks for the reminder!

  8. Pete Jackson

    @5Steve: The details of the Moon’s orbit are very complex because of the large size of the Earth and Moon compared to the distance between them, as well as because of perturbations from the Sun and planets. A consequence is that the eccentricity of the Moon’s orbit varies with time in a complicated way and as part of this, the Moon occasionally enjoys perigees which are closer than any others for many years (but only by a few percent).

    @7Dizzy: The moon’s mean distance is increasing by about 3.7 cm a year, but even so, would not have been appreciably closer 85 million years ago. Make it 2-3 billion years, and, yes, the moon would have appeared significantly larger in the sky back then.

    @6Ali: you’re sure right about that!

  9. harry tuttle

    @ “or, given recent poll results, they’d say it was Muslim”

    Well, you did give us the distances in ARABIC NUMERALS!
    Coincidence, I think not. I bet you even used Muhammad al-Khwārizmī’s heretical algebra to work them out.

    Admit it BA, you are actually the harbinger of a New American Caliphate and NASA is a plot to bring about Islamic codes of dress by getting women to wear space suits at all times. I read all about it on the interwebs, so it must be true.

    P.S. – Can you do a report on the Olson public timezone database being forcibly shut down by a US court order on behalf of the astrology company Astrolabe, due to them claiming copyright on the history of global time/date changes after they bought the rights to an atlas. Seems just your kinda thing. ;)
    linky – http://www.theregister.co.uk/2011/10/07/unix_time_zone_database_destroyed/

  10. Messier Tidy Upper

    @7. Dizzy :

    I was watching the premier of Terra Nova the other night, and they claimed the huge moon in the sky 85 million years ago was due to the fact that the moon was so much closer at that time. I did a few calculations, and at about 1.27 cm/year, or 0.5 inches, it would have been about 670 miles closer 85 million years ago. That would be a little more than 0.2% of the current distance. Can anyone confirm that? If correct, I don’t think it would look much different.

    From what I gather – saw the first two episodes – Terra Nova is set in an alternate time line so I guess that’d be their cop-out assuming that’s accurate. (My maths, alas, is weak.)

  11. Messier Tidy Upper

    Continued @ #7 Dizzy :

    I suspect the real reason for Terra Nova’s large Moon is the desire for a cool special effect but I’m not complaining. Now if they got the Mesozoic skies right by omitting stars younger than 85 million years, including stars that have since gone supernova such as the one in Orion that created the Geminga pulsar and Local Bubble and correcting positions for the proper motions (gradual movement) of nearby stars like Sirius, Alpha Centauri, procyon etc .. *then* I’d be really impressed! ;-)

    @5. Steve asks :

    Since the moon is very slowly receding from the earth, does that mean that each perigee is as close as the moon will be ever again? I know that the rate of recession is exceptionally small (at least on our timescales), but are there other factors that come into play? Proximity of NEO’s or comets? I would believe that the effect of other planets would be negligible, but we’re talking about very small distances in the yearly recession of the moon from the earth…

    I may be mistaken but I very much doubt that comets and Near Earth Asteroids have enough mass to influence the Moon much at all – especially since they spend very little time close enough to make any difference and are inconsistent in their approach vectors and trajectories coming from all angles and directions rather than a constant stream.

    That noted, I have seen ideas about shifting the Earth’s orbit outwards* using directed asteroids making gravitational slingshot type moves and using gravity Tractors to deflect asteroids.

    Hmm … if the brief presence of very low mass comets and asteroids *can* shift our Moon, I wonder what effect the longer lived presence of satellites like the International Space Station, the Hubble Space Observatory and the rest of our “artificial moonlets” might have! ;-)

    (I’m guessing precious little to absolutely none esp. considering their mass such as it is origiated on Earth but still.)

    ——-

    * As a potential solution to our Sun growing hotter with age and evolving into a red giant – article via space-dot-com. Will post link. Could also prove one possible answer to HIRGO (Global Overheating) as Futurama suggested! ;-)

  12. Messier Tidy Upper

    I have seen ideas about shifting the Earth’s orbit outwards* using directed asteroids making gravitational slingshot type moves ..

    See :

    http://www.space.com/107-life-earth-escape-swelling-sun.html

    for my source and details that idea.

    ..and using gravity Tractors to deflect asteroids.

    This link :

    http://en.wikipedia.org/wiki/Gravity_tractor

    gives the wiki-basics on that.

    Whilst this item :

    http://www.space.com/3373-earth-moon-destined-disintegrate.html

    via space-dot-com tells us some interesting things about our Moons likely future billennia hence. :-)

  13. PeteW

    The moon’s orbit isn’t really very elliptical – the difference between major and minor axes is only about 600km out of 385000 0r 0.15%. What we are really seeing is that the Earth is not at the centre of the orbit but on one of the ellipse foci which is offset from the geometrical centre by 20400km

  14. #4 Eric:
    I assume you mean the next full Moon at perigee is in May 2012. The Moon is at its perigee every 27.5 days.

    #2 Pete Jackson:
    “So a perigee full moon in January will be 28% brighter than an apogee full moon in July. Getting to be quite a difference!”

    Yes, but since our eyes response to light level is logarithmic, the difference still isn’t all that apparent.

  15. If a new moon occurs when the sun is behind the moon from our viewpoint, do new moons ever occur during the night-time? It seems for the sun to be behind the moon from our viewpoint, that new moon would only be visible to us during the daytime. I’ve wondered about this for so long, please help!!!

  16. DigitalAxis

    @16 Lee Bishop:

    Technically, yes. The moment of New Moon is when the Moon passes between the Earth and the Sun (not exactly, though; so it’s rarely a solar eclipse, of course). For half the planet, it will be night-time at that moment, and neither the Moon nor the Sun will be visible because they’re both on the other side of the planet. New Moons are indeed only visible during the day.

    The United States Naval Observatory has a page on those here:
    http://aa.usno.navy.mil/cgi-bin/aa_moonphases.pl?year=2011&ZZZ=END
    Notice that the New Moon on November 25th 2011 will happen at 06:10 UT, which means if you’re on the East Coast of the US (EST = UT-5), it’ll happen at 1:10 AM. Or maybe EST is UT-4, in which case it’ll happen at 2:10 AM; I always forget. Either way, it happens at night.

    Of course, if you’re not as neurotically precise as USNO, we consider the entire DAY that includes the New Moon to be “New Moon”, so you can see it later in the day on the 25th. There won’t be much to see though. Phil has occasionally posted pictures taken by talented “amateurs” of the extremely thin crescents just after New Moon, but those shots are extremely difficult to take.

  17. @ ^ DigitalAxis :

    Phil has occasionally posted pictures taken by talented “amateurs” of the extremely thin crescents just after New Moon, but those shots are extremely difficult to take.

    You mean like this post here? :

    http://blogs.discovermagazine.com/badastronomy/2008/05/05/newest-of-new-moons/

    Which is updated here :

    http://blogs.discovermagazine.com/badastronomy/2008/05/07/update-on-record-crescent-moon-sighting/

    (The link added to that first story doesn’t seem to be working, btw.)

    Plus this one :

    http://blogs.discovermagazine.com/badastronomy/2010/04/21/record-breaker-newest-new-moon-spotted/

    for instance? :-)

    Guess we could call that a “Cheshire Cat Moon” – our Moon when its nothing but a faint smile line – to go with the “Lemon Slice Moon” phase.

    *****

    PS. Click on my name for a “Lemon Slice Moon” photo – glad I’m not the only one to think of our Moon like that in that phase & that someone else is good enough to capture that & put it on flickr. Thanks Owen Christopher Wolter. :-) )

  18. Well, a crescent moon IS part of the symbol of Islam….

  19. @ Neil – Yes, sorry! That wasn’t a very good statement, but I missed the edit window. The next Full Moon perigee is May 2012. My previous comment is proof of why you should never comment from a smart phone – too easy to take a shortcut! :)

    @Steve – If you follow Phil’s link to the moon calculator, you will see the apogee and perigee vary each month. As Pete Jackson said, it has to do with the fact that the sun does interact (weakly) with the moon. There is also the fact that the earth is not stationary relative to the moon either. The earth has to orbit the sun so when the moon is “behind” the earth it has to than “catch up” and when it is “in front” of the earth it gains back that distance it lost. Also the earth does not orbit the sun at a constant velocity because its orbit is also an ellipse, so Kepler’s laws dictate here. (I know, some physicists are going to yell at me because this isn’t a technically robust explanation, but I’m just trying to get the principle there).

  20. The BA has now got a whole thread up reviewing Terra Nova esp. its “close Moon” aspect which is now linked to my name here.

    Or cut & paste :

    Bad Astronomy review: Terra Nova

    into your search box if that link fails.

    It was posted on October 17th, 2011 at 6:30 AM.

  21. @17 Digital Axis Thank you so much!!

  22. Matt B.

    That’s not just a 10% difference, it’s 13.5%, making a 27% difference in brightness. Note: I’m using a logarithmic percent difference here [Log%Δ = 100*ln(x/y)].

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