 The 2018 vernal equinox occurs on Tuesday, March 20th at 12.15pm Eastern Time. So if you're in the northern hemisphere, that means you're celebrating the first day of spring. If you're in the southern hemisphere, you're celebrating the first day of autumn. But what is the equinox? The equinox has everything to do with the earth, the sun, and axial tilt. Our earth's north-south axis, the axis we rotate around, is tilted 23.5 degrees relative to our orbit around the sun. The tilt is fixed, always pointing in the same direction. Actually, our north pole points at a star called Polaris, also known as the North Star, a star that's always fixed in the same location on our sky. Now, since our rotational axis is tilted, and it's always pointing in the same direction, how sunlight falls on the surface of the earth changes as we orbit the sun. So, for example, in December, the earth's axis is tilted away from the sun. It means it spreads the light out over more area, and it gives us a colder temperature. But six months later, in June, the north pole is tilted towards the sun, which concentrates the sunlight, gives longer days and hotter temperatures. Now, between those two extremes, there's a midway point, a point where the north-south axis of earth is not tilted away or towards the sun. It's a point where the shadow of earth is perfectly parallel to the axis of rotation, and that exact moment is called the equinox. Roughly speaking, the amount of sunlight that earth gets on an equinox is about 12 hours. In fact, the word equinox originates from the term equal night. We get two equinoxes a year, the vernal equinox in spring and the autumnal equinox in the fall. And yes, we tend to think of them as the start of spring or the start of fall, but it's also a really good chance to think of how a simple axial tilt can change the weather of an entire planet.