This story was updated with details of the 2018 lunar eclipse.
Have you ever stared at a rising full moon and felt the crushing realization of how small and alone you are in the universe? Just wait a few hours, and you might feel a little less small.
If you’ve ever noticed that the moon seems bigger when it’s rising than when it’s high up in the sky, NASA has an easy way to prove that’s just an illusion. And with this week bringing the 21st century’s longest lunar eclipse, now’s a good time to try.
We can blame our brains for tricking us into seeing the moon as more round and full when it’s rising. NASA suggests a DIY test to demonstrate that it’s all just a game our minds are playing on us.
First, photograph the moon as it starts rising just above a distant horizon. Then, photograph it again a couple of hours later (NASA advises using a camera with a long telephoto lens). Although the lower moon might look a bit more squashed, when you measure the horizontal diameters of the two moons, they will be the same. Writes NASA:
The squashed-looking Moon will also have a much warmer tint than the high Moon. This is due to the low Moon’s light passing through more of Earth’s atmosphere than the high Moon’s light. That squashed look is due to Earth’s atmosphere behaving like a weak lens with moonlight being bent more near the horizon than it is slightly higher above the horizon.
Ideally, you can use a tripod and cable release to capture your pictures, and NASA has other tips for taking the best pictures, including suggestions on shutter speeds and exposure bracketing. You can try this nifty test out with any full moon, or for the next two supermoons happening in January.
If you don’t have a camera, you can do the same test with a rolled sheet of paper, following these instructions from Sky and Telescope.