The official first day of fall, otherwise known as the autumnal equinox, is this weekend in the Northern Hemisphere. According to Encyclopedia Britannica, it is one of only two days, the other being the vernal equinox, when the sun is directly above the Equator, essentially dividing night and day into equal portions.

Today marks the autumnal equinox, or the first day of astronomical fall. Have you ever wondered why it occurs on a different day and at a different time each year? Let’s break it down! Let’s start with the basics.