If the full moon appears especially large when it rises over the horizon around sunset on Monday and Tuesday, Feb. 18 and 19, that’s because … well … it is.
Regular readers of this column know about the optical illusion that causes the rising moon to appear immense. It’s called the moon illusion, and it occurs because the landscape behind which the moon rises tricks the brain into thinking the moon is larger than it really is.
Look at the rising moon through a loose fist or cardboard tube that blocks that landscape from view and the moon seems to shrink back to a normal size.
For an even more remarkable demonstration, blink your eyes back and forth — looking through, and then around, your loose fist. The moon will appear to shrink, grow, shrink, grow. … Pretty weird!
But this month, the rising full moon will not only appear larger because of the moon illusion but actually be larger. Now, wait a minute. You’re thinking, “How can this be? The moon is a solid hunk of rock; its actual size can’t change.” This is true, but it can change its apparent size in our sky.
This effect occurs because the moon doesn’t orbit the Earth along a circular path but rather along an ellipse that carries it closer to us at times (perigee) and farther from us at times (apogee) during its monthly cycle.
This particular full moon occurs near its perigee — when the moon lies 221,681 miles (356,761 kilometers) from the Earth — thus making it the largest full moon of 2019.
It’s just such a full moon that the media have come to call it a “supermoon” in recent years. In fact, they’ve even begun calling full moons occurring the month before and after perigee a supermoon. Where does it all end?
So, will this particular perigee full moon actually appear larger to the eye? At its closest, the moon will appear only about 7.2 percent larger than its average apparent size. Let’s put it another way. Suppose you were to order a 14-inch pizza from your favorite restaurant but were given a 15-inch pizza instead. Would you excitedly call it a superpizza? Probably not. In fact, you’d probably not even notice the 7.2 percent size difference without measuring it.
Same is true for the moon. First of all, our memory of such things is not terribly reliable from month to month, and second, there’s that pesky moon illusion thing that confuses the issue even more when the moon appears low in the sky.
Experienced lunar watchers might legitimately notice its larger size this month, but most folk will not see a difference — though many will be convinced they do. Now if you were to compare two full moon images side by side, one shot when the moon lies at perigee and another when it’s at its average distance, or even at apogee, well, the size difference would become quite obvious.
So, will the supermoon be super this month? There’s only one way to find out. Go outside to check it out. It may be a while until you see a supermoon again … well, not until next month, anyway!
— Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at mammana@skyscapes.com and follow him on Twitter: @dennismammana. The opinions expressed are his own.