Elizabeth Jeffery, Ph.D., is an astronomer and researcher at the Space Telescope Science Institute in Baltimore City.
Pop Quiz: During a full moon, if you were to stretch your hand out to arm's length and measure the size of the moon between your thumb and index finger, would it be about the same size as (a) an aspirin, (b) a dime or (c) a quarter?
You won't have to wait long to discover the answer, as tonight is this month's full moon, although it may surprise you. (Please write your answer to the pop quiz in the comment section.)
Some have dubbed tonight's full moon a "super moon." There are two reasons for this: It will be bigger and brighter than usual. So, why is that?
The explanation for both is the same. Most of us understand that our lunar companion orbits around the earth, a trip that takes roughly a month to complete. (This is, in fact, the origin of the length of a month.) What some people may not realize is that the path it takes around the earth is not a perfect circle, but rather more like an oval. (The technical term is "ellipse.") As such, there are times when it is closer to the earth. When perigee (that is, when the moon is at its closest monthly approach) coincides with the full moon phase (when the moon is at its brightest), the so-called super moon will appear to be bigger and brighter than normal. Such an event happens about every 18 years.
But just how much bigger will it be? Hold your hand out at arm's length again. Compared to the average full moon, the super moon will be bigger by about the thickness of your fingernail. It will be brighter too, although, because our eyes are poor detectors (that's why astronomers use cameras these days), we may not be able to tell unless the average moon is standing by for comparison.
What about when the moon is rising and close to the horizon? It often looks much bigger than it does when it is high overhead. This phenomenon is actually an optical illusion; your eyes are playing tricks on you. When the moon is close to the horizon we have things to compare its size with, such as trees and buildings. Yes, compared to these, the moon is big! But measure it between your fingers again, and you'll find it has the same size as it does overhead. The moon illusion has fascinated mankind since ancient times.
A full moon is always a spectacular sight, one that we often take for granted. Fortunately, there are times when things align just right for our nearest celestial neighbor to grab our attention and remind us to keep looking up.