Monday March 21, 2011 Q&A

Why does the moon appear larger when it’s low on the horizon?


1 Because it’s closer

2 Because it’s magnified by the atmosphere

3 It’s an optical illusion

Question related image

Answer: it’s an optical illusion.

The moon’s orbit is elliptical, which means it may be closer to us or further away, depending on its position in the orbit. It’s just over 363,000 km away at its closest point, and around 405,000 km away at its furthest. However, this variation – which takes nearly 15 days to complete – results in an apparent difference in size of just 10%, which is difficult to see with the naked eye. This is why, when the celestial body made almost its closest possible approach to us during the recent full moon of 19th March, there was no significant or noticeable difference compared to previous full moons.
The atmosphere doesn’t make the moon look larger, since it isn’t shaped like a convex lens as in a magnifying glass. However, it does distort the moon by flattening it and making it redder in color, just like the sun when rising or setting.
Even though the enlargement isn’t caused by any physical phenomenon, the moon always appears bigger to us when it’s low on the horizon. This is the effect of an optical illusion linked to the way in which we perceive objects . The brain assesses the distance and scale of objects by comparing them with one another. When the moon is high in the sky, there’s nothing to compare it with and so it looks small to us. When it’s low down, a great many other things are present in our field of vision and these fool the brain into thinking that the moon is bigger.
Author: Laurent Laveder