The spring equinox is Tuesday: 6 things to know about the first day of spring

The vernal equinox is upon us: On Tuesday, March 20, both the Northern and Southern Hemispheres will experience an equal amount of daylight. For those of us here in the Northern Hemisphere, it marks the beginning of spring, with daylight hours continuing to lengthen until the summer solstice in June. For those south of the equator, it’s the beginning of autumn.

Technically speaking, the equinox occurs when the sun is directly in line with the equator. This will happen at 12:15 pm Eastern time on Tuesday.

Below is a short scientific guide to the most equal night of the year.

1) Why do we have an equinox?

The equinox, the seasons, and the changing length of daylight hours throughout the year are all due to one fact: The Earth spins on a tilted axis.

The tilt — possibly caused by a massive object hitting Earth billions of years ago — means that for half the year, the North Pole is pointed toward the sun (as in the picture below). For the other half of the year, the South Pole gets more light. It’s what gives us seasons.

 

Here’s a time-lapse demonstration of the phenomenon shot over the course of a whole year from space. In the video, you can see how the line separating day from night swings back and forth from the poles during the year