Why Does the Moon Sometimes Look Huge?


Have you ever seen the moon floating above the horizon of your city, and noticed that it looked oddly huge? I sure have. In fact, I've seen the effect in lots of popular media, including that one iconic shot from E.T. and other "supermoon" photos. But aside from movie magic, why does this happen in real life? If the moon gets bigger in the sky, it would have to get much closer to the Earth -- and while the moon's orbit does bring it a bit closer at times, it doesn't come close enough to account for the massively visible change in size. So how do we explain this effect?

There are several things going on here. First up, let's stipulate that our moon is really big, relative to other moons we see in our solar system -- our moon is roughly a quarter of the diameter of our planet. That's huge, and what that means from our perspective on the ground is that the moon is always quite large -- even sometimes large enough to block out the sun (in the case of solar eclipses), though the sun is of course much larger and farther away. The other major factor appears to be psychological. When the moon appears to be near other objects (as it tends to be when near the horizon), our brains register its relative hugeness and effectively inflate its size because we finally see its size relative to other objects. When the moon is all alone up in the sky, with tiny points of light around it, we have no frame of reference -- though it's still the same size it would be if by the horizon. (A related effect occurs when you can see the big ol' moon in the sky on a sunny day -- it looks weirdly big, perhaps because the sun is also up there.) Finally, there is one more odd effect relative to items approaching the horizon...but I'll leave that last one for this most excellent ASAP Science video to explain:

How you experienced the extra-huge moon? Have you tried what they suggest in this video -- looking at it upside down? I have not, though I encourage you all to try the next time the moon is near the horizon.