optical illusion moon

[icytailmark (Mark)]

Tonight i watched the moon rise as the sun was setting. The moon looked really big. Then i just went outside now and the moon is directly above and i thought to myself has the moon shrunk?

[astrospotter (Mark)]

Our solar eclipse is rather special because by freak chance the apparent size of the sun and moon come out mighty close to each other from our viewpoint.

It is 'unlikely' that either will shrink anytime soon although it is predicted that the sun will get extremely large as it ages. I should think that even SPF1000 sunscreen may not be quite enough at that point ...

It is what we see them close to as they are near earth that cause our mind to think they are bigger or smaller.

It is well beyond unlikely that any of us or perhaps even humans as we know today as a race will be around in the timescale for the sun to grow big enough to impact Earth to that scale.

It is because of this that I don't spend a lot of time searching the web for SPF 1000 sunscreen.

[OneOfOne (Trevor)]

In actual fact, the Moon is slightly larger when it is directly overhead (~1.5% as it is closer by the radius of the Earth).

When you see a large Moon rising, put your hand out and you should be able to blot out the whole thing with one finger (close one eye). Come out several hours later when the Moon appears smaller and do the same thing. You will still only cover the Moon with the same finger, showing that it is the same size. I haven't tried this, but they say if you bend over and look at the rising Moon between your legs, it removes this size perception and the Moon no longer looks "large"...just don't do this where you can be seen by other people .

[Robh (Rob)]

This article in wikipedia might shed some light on the matter. What causes the illusion seems to be a bit a matter of debate.
http://en.wikipedia.org/wiki/Moon_illusion

Regards, Rob.

[bmitchell82 (Brendan)]

i cant remember if its a 20 or 10 cent piece. same thing hold it at arms length and it is the same as a eclipse! do it at any time of the night and you will find its the same size! bloody optical illusions got everybody trippin out!

[astrospotter (Mark)]

Good point on the 1.5 % difference of around 1 earth radius from rise till above. So on any given night it would be actually larger overhead by a tad. Wikipedia is such a great thing for all sorts of info. Thanks for link.

We might as well talk about the full moon cause nobody's going out to observe mag 15 galaxies for a week or more. That was a great picture one of the IceInSpace guys posted on the color moon a while back.

Also related to the moon I caught a spot on the web yesterday talking about if (when) Eta Carinae goes supernova it might be about the same mag as the moon but still small. Now that would be amazing (I will miss it from +36 dec). That is such an amazing little object to view live a few times. You guys get a lot of kool objects from down there.

[Waxing_Gibbous (Peter)]

Lucky you. We haven't seen the moon (or anything else) for over a month.
This is one one of the oddest illusions around. There is no, one, accepted explanation. I believe the current leader is how we perceive the 'shape' of the sky: like the flattened dome of a stadium as opposed to its 'real' shape of a half-sphere.
Apparently the effect disappears if you look at the moon upside-down.
For those of you without a "prosperous" waistline, bend over with your head between your legs and look at the rising moon. It (is supposed to) appear much smaller.
But as Trevor notes - not something you want to do with a lot of people around!

Addendum:19:04 EST. Miraculously, the month-long cloud has disappeared and I just tried the experiment. For a second it looked smaller, but then I got dizzy, fell over and had to sit down!!

[multiweb (Marc)]

Originally Posted by icytailmark

Tonight i watched the moon rise as the sun was setting. The moon looked really big. Then i just went outside now and the moon is directly above and i thought to myself has the moon shrunk?

Because when it's up ahead you have no point of reference. When it's on the horizon you see trees, buildings, etc... which give you an indication of scale and your brain does the rest. It's only perception.

... having said that it tends to get bigger in apparent diameter and fuzzier at the zenith too after a couple of

[Barrykgerdes]

The effect of the moons apparent size on the horizon has been a topic for discussion for 70 years that I know of. When we noticed this as children we were told that if you bend over and look at the moon between your legs it would look normal.

Barry

[Robh (Rob)]

Originally Posted by Barrykgerdes

The effect of the moons apparent size on the horizon has been a topic for discussion for 70 years that I know of. When we noticed this as children we were told that if you bend over and look at the moon between your legs it would look normal.

Barry

I don't think this would look normal at all!

A few others have suggested this experiment.

Originally Posted by Waxing_Gibbous

Addendum:19:04 EST. Miraculously, the month-long cloud has disappeared and I just tried the experiment. For a second it looked smaller, but then I got dizzy, fell over and had to sit down!!

Regards, Rob.

[Peter Ward]

Originally Posted by Barrykgerdes

if you bend over and look at the moon between your legs it would look normal.

Barry

Mooning the Moon??

[JD2439975 (Justin)]

Now that would make a great picture at a star...er...moon party, lots of people mooning the moon.

[rupert32001]

I cycle to work early in the morning and the last two mornings I have noticed that there is a definite slight glow on the dark side of the moon across most of the face. It seems to only be visible until about 06:30 (Perth time) and I guess that's because the sun has started to overwhelm it seeing as the sun is so close behind.
Anyone know what causes the glow?

[mithrandir (Andrew)]

Originally Posted by rupert32001

I cycle to work early in the morning and the last two mornings I have noticed that there is a definite slight glow on the dark side of the moon across most of the face. It seems to only be visible until about 06:30 (Perth time) and I guess that's because the sun has started to overwhelm it seeing as the sun is so close behind.
Anyone know what causes the glow?

It's called earthshine. Light bouncing off the earth and illuminating the moon. Once the sun rises the sky is so bright it overwhelms the small amount of light as you surmised.