Lots of Moons: What is a “Mock Moon”?

5 March 2015

The Short Answer (TSA)

Mock moons are rare. Chances are, few have ever seen one of these first hand. So, to understand what a mock moon is, you have to, first, understand what a moon ring is.

Moon Ring

A ring of soft light or luminescent ring sometimes forms around the Moon on dark nights. This “halo” is caused by the Moon’s light passing through millions of ice crystals high in the atmosphere.

Traditionally, a ring around the Moon meant that storms were on the way. And, in fact, a ring often appears when the Moon can be seen through a thin layer of clouds covering the sky. The thin layer is made up of cirrus clouds, which often do come a few days before a storm front.

Because of the shape of the ice crystals, rings form a circle around the Moon that is always about 22 degrees away from the Moon itself. How far is 22 degrees? Well, looking up in the sky, extend your arm toward the Moon and look at the size of your hand. That’s about 22 degrees.

How does a “mock moon” fit into all of this?

When there is a ring around the moon, very rarely, a large bright spot appears somewhere on the ring. The spot is circular and appears as a slightly fainter visual version -- a kind of visual echo -- of the nearby moon. The moon must be bright, more than half full, to produce enough light to form any mock moon at all.

Moon & "Mock Moon"

Could someone mistake a mock moon for the real thing? Well, not if both the real moon and mock moon were in the sky together. The mock moon is much fainter than the actual moon. So much fainter, that it would never stand up to a side-by-side comparison with the real thing.

Maybe, if trees, mountains or other objects just happened to be blocking your view of the moon, you might mistake the mock moon for the real thing. Even, then, the real moon would be so close-by in the sky that you would probably soon discover the mistake.