Why does the Moon look bigger near the horizon? The Moon illusion and other sky questions
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You step outside and a huge golden Moon is sitting just above the rooftops. A few hours later, high in the sky, it looks ordinary. Did it shrink? No. This is the Moon illusion, one of the oldest puzzles in perception, and it fools almost everybody.
The Moon does not really change size
The Moon is on average about 384,000 kilometres away. Across one night, that distance barely changes, so its size in the sky stays at roughly half a degree, about the width of a pencil held at arm's length. In fact, when the Moon is on the horizon you are slightly further from it than when it is overhead, by roughly the radius of the Earth, so it is very slightly smaller, not bigger.
You can check this yourself:
- The coin test. Hold a small coin or a pencil end at arm's length and compare it with the rising Moon. Do the same later when the Moon is high. It covers the same amount.
- The tube test. Look at the horizon Moon through a rolled-up piece of paper so you cannot see the landscape. It suddenly looks normal.
- Photograph it. A photo taken with the same zoom at moonrise and later shows the same size disc.
So why does it look huge?
The honest answer is that scientists still debate it. The leading explanations are about how the brain judges size and distance:
- The flattened sky. We tend to see the sky not as a perfect dome but as a flattened bowl, so the horizon feels further away than the point overhead. If the brain thinks something is further away but it covers the same space in our view, it concludes the object must be bigger.
- Comparison with objects. Near the horizon, the Moon sits beside trees, buildings and hills, which give the brain clues about distance. Overhead, it floats in empty sky with nothing to compare.
Whatever the exact mechanism, it is a trick of perception, not of the atmosphere or the Moon itself.
Why is the rising Moon orange?
That part is real. When the Moon is low, its light passes through much more of the atmosphere to reach you. Air scatters blue light more than red, so more of the blue is lost along the way and the Moon looks yellow, orange or even reddish. It is the same reason sunsets are warm in colour.
Other sky questions people ask
- What is a supermoon? The Moon's orbit is an oval, so its distance varies. A full Moon near its closest point is popularly called a supermoon. NASA says such a Moon can look up to about 14% larger and 30% brighter than one at its furthest point, but without a side-by-side comparison most people struggle to notice.
- Why do we always see the same side of the Moon? The Moon spins once on its axis in the same time it takes to orbit the Earth. This is called tidal locking. There is a far side, but it is not permanently dark; it gets sunlight too.
- Why can you sometimes see the Moon in daytime? The Moon is bright enough to show through a blue sky, and for much of each month it is above the horizon during part of the day.
- Why do stars twinkle but planets mostly do not? Stars are so far away that they are effectively single points of light, and moving pockets of air bend that light this way and that. Planets show a tiny disc, so the wobbles average out and their light looks steadier.
- What is a blue moon? Usually the second full Moon in a calendar month. It is not actually blue.
Getting more out of moon-watching
- Watch moonrise around the time of a full Moon for the most dramatic illusion.
- Binoculars show craters and dark plains, especially along the line between light and shadow when the Moon is not full.
- Check rise times for your location with a weather service or astronomy app, since they change from night to night.
Have you ever caught the Moon illusion at its most dramatic? Tell us about it in the comments.