It’s the hysteria that’s been sweeping the headlines in the last few days: a spectacular “supermoon” that is here to destroy us all and everything we hold dear. But if you were disappointed in what you saw when you looked up at the sky Monday night – a totally ordinary looking full moon – you probably weren’t alone.

Why does the supermoon not look that impressive? To understand why, you have to realize that the moon is on an elliptical orbit around the Earth, not a completely circular one, which means that there are times of the year when it is closer to the Earth than usual, and farther from Earth than usual. When it reaches its closest point, this is referred to as the moon’s perigee. At the farthest point, the moon has reached its apogee. The moon reaches both every 27.55 days, which is how long it takes to orbit Earth.

The supermoon simply refers to a fairly common occurrence when the moon reaches its perigee at around the same time as it becomes “full,” or fully illuminated by the sun. The alternative, when it reaches “full” during its apogee, is called a minimoon.

It’s called a supermoon because it is illuminated in its full glory and it as close to the Earth as it can possibly, thus appearing larger in the sky. But the reality is, it’s not going to appear much larger to the naked eye: at perigee, it is only 10 percent larger than at apogee. That’s not a very big difference, and certainly it’s not one you’re going to notice casually observing the moon.