Is a low full moon really bigger?

NASA, the US space agency, suggests photographing a newly risen moon and a moon high in the sky with the same camera at the same zoom, then comparing them. Their horizontal diameters measure the same in both pictures. In fact, air bends the light from a low moon, making it look like an oval slightly flattened at the top and bottom. Its light also travels through more air, giving it an orange tint.

The distance is the opposite of what you might expect, too. A moon on the horizon is about 1.5% farther from us than a moon overhead. People often say the air magnifies the moon like a lens, but NASA explains that the air actually squashes it a little.

One familiar explanation compares it to the Ponzo illusion. Place two bars of equal length across railway tracks that narrow into the distance, and the farther bar looks longer. We perceive the horizon as farther away, but the moon there does not look smaller, so the brain interprets it as a very large object.

NASA does not consider this a complete explanation, though. Astronauts in orbit also see the illusion, even without buildings or trees to give them clues. People have watched it for thousands of years, and there is still no agreed answer.

You can easily check this from your doorstep. Stretch out your arm and hold your index finger next to the moon, and your fingernail and the moon are about the same size. It's the same whether the moon is low or high. Look at the moon through a rolled-up paper tube, or bend over and look at it upside down between your legs, and it no longer looks as big as before.

Constellations near the horizon look bigger, too. It isn't only the moon; it means our brain perceives size differently when we look at the low sky.

Then why does the horizon moon look so enormous in some photos? NASA explains that such photos are usually taken with a telephoto lens, zooming in on the moon behind distant buildings or mountains. The moon and the background are magnified together, so the moon dominates the scene.

If you've ever been disappointed when a huge moon came out as a tiny dot in your phone photo, it was your eyes and brain, not your phone, that made the moon bigger. Next time, zoom in and include distant buildings or mountains in the frame.