Subject: The real truth about the moon
Newsgroups: sci.astro

A poster in sci.astro asked why the moon appears larger on the horizon than at the zenith. The following responses appeared [excerpts edited for grammar]:

From: brad@looking.UUCP (Brad Templeton)
Date: 16 Aug 87

Actually the moon appears so much larger because it is almost twice as close to you when on the horizon as it is when it is overhead.

When the moon is on the horizon, it is attracted by all the mass of the Earth you see running from where you are standing to the point on the horizon where you see the moon. Up above you, there is no mass of Earth between you and the moon, so the force is less.

The massive force brings the moon much closer when it rises and sets. As it gets higher, the force is less, and it moves further away. Then it comes back in again.

The moon is actually closest to the Earth when below the horizon, only you can't see it then. Those of us on the other side of the Earth actually get a really good view. Apollo reached the moon by leaving from the other side of the Earth when the moon was close.

From: dhesi@bsu-cs.UUCP (Rahul Dhesi)
Date: 18 Aug 87

> Actually the moon appears so much larger because it is almost
> twice as close to you when on the horizon as it is when it is
> overhead.

This is an exaggeration; it is only slightly closer, certainly much less than 10%.

A more important effect is that of gravity on light rays. When the moon is directly overhead, the effect of gravity is to speed up the light rays as they come from the moon to you, causing the image to appear to be contracted in all directions.

When the moon is just above the horizon, gravity bends the light rays, having the effect of a convex lens, but has little effect on their speed.

The net effect is as if you were looking at the moon through a magnifying glass when it is near the horizon, but looking at it through the wrong end of a telescope when it is directly overhead.

From: (David Palmer)
Date: 20 Aug 87

The real truth of the moon illusion is that it is all due to the same effect as "the aberration of starlight". When you are moving, distant objects appear to move forwards, towards the direction you're travelling. If you were going at .9999c, almost the whole sky would appear to be in front of you, only points almost directly behind you are in your rearward hemisphere, and so they appear distended, as a small patch of sky has to cover the entire hemisphere.

When you see the full moon setting, you are travelling directly away from it due to the rotation of Earth. This explains why the setting full moon appears larger than the moon at the zenith, and larger still than the rising full moon.

I hope this clears up all your questions.

From: (0732)
Date: 20 Aug 87

Of course, no one who lacks even a cursory knowledge of elementary physics can doubt that the above explanations have merit, but the primary effect is a physiological one. As one tilts his head back to observe the moon at the zenith, the lens of the eye is flattened by gravity and its magnifying power is decreased.

[ Back in 87, the Arpanet was used for serious scientific and technical dialog, rather than the frivolous recreational purposes so common today. (And Y2K was not yet a TLA.) ]