Test This Supermoon Illusion Tomorrow

Posted on Tue, January 30, 2018

It’s a big month for the Moon—it will get closer to Earth not once, but twice in January! You can catch the second “supermoon” of this month on January 31, which will also be a lunar eclipse for viewers in other parts of the world. It’s being called a “super blue blood moon,” and it’s really rare: The Moon is not only closest to Earth in its orbit (that’s the “super” part), but it’s also a blue Moon, which is what we call the second full Moon of the month. The “blood” in its title comes from the red color the Moon will turn as it passes through the Earth’s shadow during that lunar eclipse.

The year’s first full moon is sometimes known as the Wolf Moon. This year’s Wolf Moon occurred on January 1, 2018. Photo by Geneviève de Messières.

When you look up to view the supermoon, how do you know that the Moon is actually bigger? In reality, a supermoon is not much bigger than a normal moon—this month it’s only about 7 percent bigger. During any full Moon, you might see the Moon looking much bigger than it normally does high in the sky, or in pictures. That happens when it is on the horizon.

A comparison of a typical full Moon and a supermoon. Credit: Geneviève de Messières.

Let’s get something straight—the Moon does not get closer or farther away in the course of an evening, though that does happen slightly over the course of a month (hence, the supermoon!). However, the full Moon appears bigger when it’s low on the horizon, even though it’s almost exactly the same size as when it’s high overhead. This phenomenon is dubbed the “Moon illusion.” Psychologists are still discussing the cause of this illusion, but it’s most likely because we perceive the sky overhead to be close and the sky on the horizon to be far away. When the Moon is on the horizon, we interpret it to be farther away.

An illustration of the Moon illusion. Credit: Geneviève de Messières.

Try at Home

You can try it out! Hold your finger up to your eye to block an object in the distance from view. What is bigger, your finger or the object? Your finger is taking up as much space in your view as the object, but you know the object is farther away, so the object seems bigger. In a similar way, when we perceive the Moon on the horizon as farther away, it seems bigger and more dramatic than normal. To enjoy the Moon illusion, turn your back on sunrise or sunset around the dates of the full Moon and look for the orange sphere of the Moon hovering near the opposite horizon.

Even though your brain might be playing a trick on you, the full Moon any month is always a sight to see. So head outside at sunset, look east, and enjoy! (And for our friends on the West Coast of the United States, you can spot the “blood” Moon tomorrow morning, between 4 a.m. and 7 a.m.!)

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