Big Moon tonight?

By Phil Plait | June 30, 2007 10:34 am

A lot of websites are saying that the Full Moon will look huge tonight (Saturday), and urging people to go out and look. This is true, but I want to make sure people understand what’s going on. It’s not that the Moon will look any bigger tonight than it does at any other time; it’s that it stays near the horizon longer.

Any time you see the Moon rising, especially when it’s full, you can get the Moon Illusion: the appearance that the Moon is huge, looming over the Earth. I won’t go into the details here; go to that link where I wrote up a brief intro to the phenomenon.

You only get the illusion when the Moon is near the horizon; when it’s overhead the illusion vanished. Because this month is the summer solstice, tonight the Moon stays close to the horizon for northern hemisphere observers. When the Moon is full, it’s opposite the Sun in the sky. And because the Sun is high in the sky during the day during the summer, the full Moon will be low in
the sky at night. For me, in Boulder Colorado (latitude almost exactly 40 degree north), the Moon only gets to an altitude of about 23 degrees above the horizon tonight — about the width of two fists outstretched.

The angle at which the Moon rises is also small, so it takes a while to rise, prolonging the illusion.

What’s funny is that physically, the Moon should look smaller tonight than usual! It orbits the Earth in an ellipse, so sometimes it is farther from the Earth, and sometimes closer. I checked the good ol’ Naval Observatory website, and looked up the distance to the Moon for a couple of weeks centered on tonight. Here are the results:

Date   Distance (km)  
Jun 24   404320  
Jun 25   404445  
Jun 26   403418  
Jun 27   401396  
Jun 28   398592  
Jun 29   395246  
Jun 30   391604 <–tonight
Jul 01   387893  
Jul 02   384299  
Jul 03   380954  
Jul 04   377939  
Jul 05   375291  
Jul 06   373024  

The Moon was farthest from the Earth about a week ago (June 25), and will be closest around July 10. Right now, the Moon is a little bit farther then average, so if you were to measure its apparent size very carefully, you’d see it’s smaller than usual!

Anyway, whatever reason works for you is good enough for me tonight. Go out and take a look. And when you do, take a look to the "upper right" of the Moon — the bright "star" there is the mighty planet Jupiter, fully 40 times the diameter of the Moon, but well over 1600 times as distant. That’s where distance really matters!


Comments (23)

Links to this Post

  1. The Rising Moon « SpaceWatch | June 30, 2007
  2. The moon-size illusion at Science ID Australia | June 30, 2007
  1. Justin

    Cool beans I just used stellarium app on my computer to check exactly when it will rise and how close it will be to the horizon throughout the night, then I fast forwarded time to compare where it would be down the road. Unfortunitly my view will be blocked by trees but good info none the less!

  2. As usual around my neck of the woods, the local media (TV station – the local version of the National Enquirer) made a big deal about it. I fortunately wrote on our astronomy club’s weblog the real reason behind the “Moon Illusion” and provided a link to the recent SCience @ NASA article about it. In fact, once I”m done posting this I am going back and amending my article there to link to this post of yours BA.

  3. OneHotJupiter

    I guess it a great night for the old scope to get some use!

  4. Chip

    As a rough experiment, hold an aspirin tablet* at arm’s length next to the moon and close one eye. The two will appear to be very close to the same size.

    Later, when the moon appears a bit higher (and smaller,) hold the tablet next to it again. Same relative sizes.

    * That’s the traditional white flat aspirin tablet still common today; any brand will do, even generic. ūüėČ

  5. Gary Schumacher

    cool tip Chip

  6. Chris

    I grew up on The Hill in Boulder and we were always taught in school that Baseline Road gets its name from being exactly 40 degrees north latitude.

    I know that it snakes around a bit, especially once you start to go east of Broadway, but it’s something people from there are quietly proud of. :)

    Care to check it out to see where the story comes from?

  7. I remember a grade school textbook talking about being able to cover the Moon with the rubber eraser on a standard pencil held at arm’s length. It’s fun to try to actually sketch out the apparent size of the Moon for yourself. Minnaert’s “Light and Color in the Outdoors” covers this and other illusions of size extensively.

    I hope a few people going out to see this fairly ordinary (which is to say, absolutely, breathtakingly beautiful and amazing) full Moon got around to looking to the West to see the amazing spectacle of Saturn dangling just one degree above Venus. I was in a car driving West with a clear horizon about 10:40 ET last night, and these two planets were lined up over the vanishing point of the road like an image from 2001: A Space Odyssey. I wonder how many people chased these “UFO”s down the road, frustrated that they never seemed to catch up with them?

  8. Cloudy here last night. Oh well. But this doesn’t depend on the Moon being exactly full, so maybe I’ll try tonight.

  9. Gary Ansorge

    Is the elliptical orbit part of the evidence that Luna was once a part of the earth? I was under the impression that debris clouds around a planet should condense into a moon with a nearly perfect circular orbit.

    GAry 7

  10. JB of Brisbane

    I think Harold means, like a scene from Close Encounters Of The Third Kind.

    Can’t do italics, dammit!

  11. No, there was a scene in 2001 where several objects aligned in a sort of stack. I don’t remember if this is in the “monolith on the Moon” scene, or in the “wow things just got weird near Jupiter” scene – maybe both. I don’t actually remember a “stacked alignment” in Close Encounters, but there was a lot of stuff with the vanishing point of the road in that movie. (This being Pennsylvania, even with a relatively low horizon the vanishing point of the road was only about 100 feet away.)

    Italics are started by using (without the spaces) and closed by using .

  12. Sorry. Italics are opened using (pointy bracket pointing left)i(pointy bracket pointing right) and closed using (pointy bracket pointing left)(forward slash)i(pointy bracket pointing right)

  13. JB of Brisbane

    {}Thank you, Harold{/}.

  14. JB of Brisbane

    Or is this what Harold means by pointy brackets?

  15. Bill

    I forgot to do any moon watching, but I read the Science @ NASA article about the illusion — very good; it points out that we don’t really know why the brain interprets the size of the moon the way it does. Thanks Harold for the Venus/Saturn comment. I did see the object near Venus and wondered what it was. I usually keep up with planets but it’s easy to lose track of Saturn! P.S. to JB: pointy left is less than, that is <, and pointy right is greater than. And, yes, the 2001 alignment came in several places, including the scene when the sun rose over the lip of the lunar monolith excavation.

  16. Here’s a fairly poor image of Venus and Saturn I took Sunday night (7/1/07 at 9:50 EDT), taken with a Nikon L4 4 megapixel digital camera mounted on a plastic tabletop tripod with foam rubber feet taped on, all perched on the roof of my car:

    The night before Saturn was directly above Venus, but now it’s at the 2:00 position! Seeing that sort of apparent change in Saturn’s position gives you a sense of how quickly Earth and Venus are moving in their orbits!

  17. MichaelS

    Last night, my friend asked me “isn’t the moon supposed to look really big tonight?” I hadn’t heard anything about it, but I immediately answered, “No, not noticably.” He asked where it was and I pointed through the Earth’s limb and said “About there,” then pointed at a landmark and said it would rise about there.

    When it started rising, he said “Oh it is much bigger!” and I told him it was just the moon illusion and it always does that near the horizon. I thought it was odd that he’d never noticed that before–I’ve noticed it since I was 4 or 5.

    A bit off-topic, I’ve had some beefs with your explanation of the moon illusion for a while now, but now I have a good excuse to write about it:

    Something I think is funny: the WWdN site you linked to quotes you as saying it’s the Ponzo effect combined with the sky-dome illusion causing the illusion, but the link on your moon illusion page dismisses both as not true (possible contributors, but not The Cause). I can’t get to the page to see what they say; I get the error “You are not authorized to view this page”.

    Different people see different thing, so the causes are likely different for different people, or at least in different proportions (a fact oddly not mentioned in most discussions of the moon illusion). But it’s definately not a Ponzo illusion for me: the Ponzo illusion explanation predicts that the moon would appear to be further away, physically larger, and about the same angular size; what I actually see is that the moon appears to be about the same physical size, closer and a larger angular size. Predictions fail horribly; hypothesis is wrong. Also, in a Ponzo illusion, if I look back and forth at the two objects, I can see through the illusion; it’s only if I look at the entire image that the illusion shows. With the Moon illusion, I can look at the sun on the horizon and the moon in the sky (or vice versa), and the object on the horizon consistently looks closer and angularly larger, even though they are nearly the same angular size.

    I’m not sure exactly what does cause the illusion, but I know the biggest contributor is, by far, the presence of foreground objects. In a place with lots of foreground objects, the moon illusion can persist even at 70 or 80 degrees above the horizon. The moon seems the biggest when there are mountains or such surrounding it as it rises. On flat, barren terrain like west Texas, the moon illusion is very slight.

    I also have a couple beefs with the average description of the sky-dome illusion that accompanies moon illusion “explanations”: first, the sky-dome is where clouds and planes fly–not where the stars and moon sit. So any illusions related to the sky-dome are not necessarily related to the moon illusion. Second, the average description of the sky-dome illusion states that clouds on the horizon are farther away than clouds straight up, and leaves the reader to guess what effect that has on anything. In reality, this is not an illusion; the clouds on the horizon really are much further away than the clouds directly overhead. The illusion is that the brain perceives the clouds on the horizon to be closer than they really are, and the clouds above you to be further than they really are, because the brain can’t truly comprehend the vast distances involved, nor the angles represented. The reality is that the visible portion of the skydome is very nearly flat (if my math is correct, the clouds 50 miles away are about 35 feet lower than the clouds directly overhead, due to the Earth’s curvature), while the illusion is that the visible portion of the sky-dome is very rounded. If anything, the sky-dome illusion reduces any effect a Ponzo illusion might have, by making the apparent difference between the sky above and the horizon much smaller than it actually is.

    The space-dome (the one on which the stars, moon and sun appear to be fixed) appears to my eyes to be somewhat farther away straight up than at the horizon. This seems to correlate with the moon illusion, and the sky-dome illusion, but the effect is not directly proportionate in magnitude. So it’s possible that all three illusions have a common root cause, or at least a common contributor, but I can’t see how any of the 3 would cause each other. Furthermore, none of them seem to be related to the Ponzo illusion in any way.

  18. Buzz Parsec

    When I heard about the sky-dome illusion, I thought “That’s it, it makes perfect sense to me!”, but now I’m totally confused!?! Maybe the reason it doesn’t work is different people see it different ways, like some people initially see the dancing lady spinning clockwise and others counterclockwise, or some people see an angel and I saw some kind of small brown dog or maybe a koala? Maybe some people don’t see any Moon illusion at all and just think the rest of us are nuts?

  19. Buzz Parsec

    OOPS… need to read before posting… Re: the reason *it* doesn’t work…
    I meant to say “The reason no one explanation seems to work…”

  20. MichaelS

    I have met one person who claimed to not see the moon illusion, but he was also a chronic liar who did anything to attract attention, so that information isn’t very reliable. (He has also seen aliens, been bitten by a vampire, occasionally turns into a werewolf, eats salt because it tastes like sugar, had 3 – 17 “best friends” die within 2 days – 2 months [depending on what day he was telling the story], among other things.)

    From what I’ve read, my perception is consistent with a large percent of the population, but I’ve never seen a poll or study to determine exactly how much of the population sees what. I am reasonably certain that different people see different things but don’t have a reference for that.

    Don McCready has a website ( that explores the idea of macropsia being the root of the illusion. This idea seems like the best one I’ve heard so far, but simple experiments with fingers and pictures tell me it’s not the only cause, because the effect just isn’t big enough. By the way, I emailed him a while back, and I guess he gave all that to somebody else, so he doesn’t have any new info on it. Unfortunately, I can’t find the email he sent me, so I’m not sure who is doing the research.

  21. I have been out my front door several times to see that MOON since it did not look that large to me. Perhaps I should have gotten new glasses. Or maybe it is because I am on a second floor, I saw the one in 1993 – remembered as brighter. So glad I could express my confusion :>)


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