What causes the moon to appear larger on some nights than others?

Question

When the moon is low on the horizon it often appears considerably larger than when it's high in the sky - what's the reason for this?

Also, even from one night to the next the moon can appear to be a different size even when it's in approximately the same part of the sky regardless as to whether it's close to the horizon.

Answer 1

Good question, and good observation. You are correct. The moon does look larger or smaller at different times. There are three reasons for this.

1. The Moon looks larger depending on the orbital position. That is because it is about 5% closer when the Perigee occurs in alignment with the Earth and Sun. This can be at new or full phase. the effect is greatest when the Earth is nearest the Sun in January.

2. Atmospheric refraction. The rising full moon looks slightly larger because of atmospheric effects. But that is not the most important reason:

3. The human eye's potential for changing the eye's iris in response to light levels. The iris it the part of the eye behind the cornea. It has a hole in it that gets larger or smaller depending on how bright the light is. When the moon is bright the iris contracts - gets smaller - and the image becomes sharper and is perceived as being larger. When the moon is at the horizon, there are many times other lights as well, all contributing to the effect. When the moon is high in the sky, there is less overall light. There is no atmospheric effect, the sky is darker, and the light of the moon is no longer able to cause the iris to contract and the iris gets larger. The perception is that the moon is not as large as the rising moon.

When you have a nice big moon on the horizon, measure the moon at arm's length with a compass, (The drafting instrument used for drawing circles, not the one that points North!) closing the compass until the points just hold the moon between them. Later, go outside and compare the measurement with the moon high in the sky. What an interesting experiment to discover the perceptual differences of the moon!

;-D The moon is the soul of romance. Don't measure it unless you have your sweetheart with you!

The link is a very interesting article about the moon!

Answer 2

Indeed, when the moon is closer to the horizon, it APPEARS larger. If measured, however, it is exactly the same size. So, these are all illusions for the very most part. This is because it is difficult to judge size when it is in the middle of a huge sky.

I doubt the eye can detect the tiny variation in orbit because the orbit is quite round rather than elliptical, so the size should remain fairly constant.

Note that the same thing happens with the Sun! Also note that the size of the sun and moon are almost exactly the same apparent size (which is a coincidence) as they are totally different sizes and distances.

As for the moon appearing different sizes while in about this same position? This should not happen. It might be that your memory is a bit off. I could also be that you see some kind of "comparitive" in some cases and not in others. For example, you might see the big dipper behind it one night and no stars behind it another. This might make it seems a bit smaller when there is nothing to compare it to, just like the horizon does but to a smaller extent.

Answer 3

Sometimes, in its elliptical orbit, the moon is farther from earth than other times, but this is not the reason.

Atmospheric refraction, when the moon is nearly touching the horizon, changes the moon's shape, but this is not the reason either. It has the effect of making the moon look flatter, but not bigger.

What makes the moon appear larger is the comparison with distant surface objects like buildings and trees. The moon looks much bigger than them. When it's higher in the sky, it seems dwarfed by the vastness.

Each evening, the moon appears larger or smaller than the night before, depending on its phase. The full moon is coming up August 8 in the Western Hemisphere (August 9 in the Eastern) and that's when you'll notice the size phenomenon best.

Answer 4

Its called lensing. It's what happens when you look at the moon through more atmosphere. You see, the atmosphere is curved..... but you knew that right? Anyhoo, it always appears to be a similar size when straight above you, but larger when on or just above the horizon.

But you could also be viewing something called the halo effect. This comes from viewing the light reflected from the moon through ice crystals high in the atmosphere. If the halo effect is large enough, you can see that it is a halo around the moon. But sometimes, it is very small, and only makes the moon look larger, not like it has a halo.

Answer 5

It is mostly an illusion.

However, the diameter of the moon is 2160 mi (3476 km) and the distance to the moon can vary by about 31,250 miles (50300 km) at different places in its orbit, so depending on how good your eyes are, it may make a slight difference over some time.

But, from the time the moon rises until it reaches its highest point in the sky, its distance does not change enough to notice any significant change in apparent size. The effect is actually more an illusion than anything else.

Answer 6

When the moon is above you the light travels directly through the atmospher (which circulates the earth).

If it lower in the horizon it hits the atmosphere at a lower angle. The atmospher causes the light to refract (bend) making the moon appear larger even though the distance from the earth itself is the same.

Answer 7

It is an optical effect from the earths atmosphere. As the earth is round, so is the atmosphere. As the moon is lower in the sky you see it though more of a curved part of the atmosphere which effectively works like a giant magnifying glass.

When the moon is high in the sky you get almost nothing of the magnifying effect.

Answer 8

The distance between the earth and moon. The shorter the distance, larger it will appear.

Answer 9

If the moon is near houses or trees it seems larger, but it isn't. Try measuring it's apparent width with a ruler.

Answer 10

The position of the earth and the sun.