Rotating Question Curious About Astronomy? Ask an Astronomer

Why does the Moon look big on the horizon?

Please answer this question--we're arguing about it at work. Why does the moon appear to be different sizes at different times? Sometimes when the moon is rising, it seems huge, and then seems to get smaller as it gets higher in the sky. Is this an illusion because of the moon's orbit, the gasses in the atmosphere and the curvature of the earth magnifying the moon, or is it all relative to having a comparison/perspective as it rises--such as having the mountains/trees to compare it's size?? Or is it something entirely different? We'll be anxiously waiting for your explanation..............

No, it's not caused by either the orbit of the moon and the atmosphere of the Earth, or by comparison to other objects as the moon rises, although these are both commonly given as reasons.

The Moon-Horizon Illusion is a cognitive illusion, meaning that it is an illusion due to the processing of information in our brains. No one really knows why the brain always interprets the Moon this way, but one thing is for sure, though: it is not due to comparison with other objects. Prove this to yourself in two different ways: first, by looking at the Moon on the horizon while standing on your head: it looks normal-sized! Second, look at the Moon rising over the ocean where there are no objects for comparison. It still looks big.

So it's something entirely different. It's a psychological effect that we still do not fully understand.

May 2002 addition by Karen: Take a look at The Bad Astronomy Page on the Moon Illusion. I encourage you to follow the link to this page which offers a very thorough discussion of why the Moon and Sun appear bigger on the horizon.

March 1999, Karen Masters (more by Karen Masters) (Like this Answer), Dave Kornreich (more by Dave Kornreich) (Like this Answer)

Still Curious?

Get More 'Curious?' with Our New PODCAST:

Related questions:

More questions about Stargazing: Previous | Next

More questions about The Moon: Previous | Next

How to ask a question:

If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't exist

This page has been accessed 51686 times since May 23, 2002.
Last modified: November 2, 2002 6:34:11 PM

Legal questions? See our copyright, disclaimer and privacy policy.
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.

Warning: Your browser is misbehaving! This page might look ugly. (Details)