The moon IS the same size at moon rise!

[OneOfOne (Trevor)]

After reading several articles on the phenomenon of the moon appearing larger at moon rise than when it is on the meridian I had a chance last night to perform a very precise scientific measurement.

Around 8:30 or so the clouds cleared briefly as I stuck my head outside for a look. The Moon had just risen earlier and was quite close to the horizon and it appeared, well, BIG. I took careful measurement of the size of the disc by closing one eye and "eclipsing" it with the tip of my little pinky, arm fully extended. My finger was just a little larger than the disk, but completely covered it, with only a little to spare.

This morning around 7:00 I checked the same thing with my pinky, this time the moon was much higher in the sky. It covered the same amount of the moon. At a guess, I would say that it was at least "twice" as big when it was close to the horizon.

The best description I have had so far was in a recent article in S&T a couple of months ago? Unfortunately, I forgot to try viewing it between my legs as this is supposed to confuse our sense of judgement and will make the Moon appear to be its "normal" size (according to the article).

Of course, my daughter still insists that it is bigger because the Moon is closer when it rises...but then again so do most people, until they have given the Moon the finger test! If anything, I would expect it to be slightly further (something related to the radius of the earth) as it is at 90 degrees to the centre line between you and the earth...if you get what I mean, it's a Pythagoras thing!

Of course this raises the question, if you were on the Moon, would the Earth appear "bigger"? Possibly not, as the clues we use on earth don't exist on the Moon (the Appollo astronaughts could not judge distance with any great accuracy and almost walked over a cliff). I expect, however, the Earth would require something about twice the size of an outstretched thumb to block it out as it is about 4 times the diameter! I am saving my money to try this out for myself some time

[Starkler (Geoff)]

Originally Posted by OneOfOne

Of course this raises the question, if you were on the Moon, would the Earth appear "bigger"? Possibly not, as the clues we use on earth don't exist on the Moon

The simplest explanation I have heard for the moon illusion is tied to how we perceive the shape of the sky in spacial terms. Instead of perceiving it as a hemisphere, we see it more as a shallow upside down dish.

This gives the perception that objects overhead are closer than those toward the horizon and accounts for the apparent size differences that we perceive.
The same should apply I would think viewing Earth standing on the moon.

[vespine]

That's a decent explanation. I always thought of it in terms of perspective: When the moon is next to the horizon you can see it next to trees and houses and whatever you are looking at, it gives you a reference point, but when the moon is far from the horizon it's just in the wide open empty sky. That's why putting your thumb next to the moon breaks the illusion, because it's a common reference point.

[ving (David)]

thats it... an astrometric EP is what i am getting. I have been wondering what to get for a while now

[astrogeek (Leon)]

Infact the moon is slightly smaller when it is on the horizon because the distance from you to it is slightly longer than when it is at it's highest. This is because you are adding the radius of the Earth to the distance to the moon (~6000km). The difference is unnoticable though.

[sheeny (Al)]

All good stuff, and I think there could be an element of each of the points raised at play at the same time. The one that hasn't been mentioned, just for the fullness of the discussion , is the effect of the refraction of the light passing through the atmosphere. In theory it should slightly magnify the size of the image of the moon, but with a similarly technical experiment some years ago I convinced myself it wasn't significant.

May the atmospheric magnification is just about cancelled by the increased distance (earth's radius)??? Trust Pythagorus to be involved... he probably knew more than we give him credit for! ... and we all thought he was a bit of a square...

The interesting part of this discussion for me is the one of perception. I had previously linked the "it looks big near the horizon because it's close to things we are familiar with, but high in the sky it isn't" case, but I hadn't thought in terms of relative position to the observer.

As part of teaching map reading and navigation, we also teach judging distance. Part of that is understanding perception. Things look further away when:

• looking up at them;
• when laying down (and looking along the ground is the usual scenario);
• when looking across a gap (say a valley).

They look closer when:

• looking down at them;
• when all the ground between the observer and the object is visible.

Hmmm... My advice for a really impressive moon rise? Picture this: ocean cruise, deck chair aligned with the position of moon rise, supine position on the chair, drink of your choice in each hand (balance is important!)

Cool thread guys! I'll have to try looking at the moon thought my legs too! Sounds like a worthy experiment.

I hope your pinky was calibrated?

Al.

[OneOfOne (Trevor)]

Unfortunately, when the moon is rising at my house I can only see it when it has risen several degrees so I can't see it actually touching the horizon, where the effect would be at it's maximum. To try the "between legs" test I would have to go outside and stand on the side of a main road and look at the moon between my legs. I think shortly after doing the experiment, they may come to take me away! I think refraction may play some part in making it appear to be larger.

I guess it seems to be one of those things that the wisdom says is not true, and you measure it yourself and agree that it is not true....but somehow deep down you know it is true!

I recall when I used to work at the Telstra Research Labs that they performed a series of carefully controlled experiments on the "memory effect in NiCd batteries". The conclusion was that it does not exist, but from my own experience I KNOW that it does!

[circumpolar (Matt)]

Don't forget APOGEE & PERIGEE.

If you view the moon at apogee (furthest away from the earth) and then about 16 days later at perigee (closest to the earth) you will notice an apparent increase in size.

May 7th apogee - 404,572km distant = 29.8'
May 23rd perigee - 368,608km distant = 32.4'
ref. astronomy 2006