It’s official, spring has arrived. This year the official moment that the Earth is tilted neither toward nor away from the sun happened at 11:06 p.m. on March 19. That’s the earliest Equinox since 1896.

The arrival of spring-like weather may not be here to stay, but the official calendar season is earlier than it’s been in anyone’s lifetime. Typically, the Equinox falls on March 20 or 21, but this year it’s on the 19 because it’s a Leap Year.

The calendar runs a little ahead of real-time, and that’s why every four years we add a Leap Day to slow the calendar time down and sync it more closely to reality. If we didn’t celebrate Leap Year, the first day of summer would happen in December in about 700 years.

Leap years typically skip every century, but the year 2000 (and all centuries divisible by the number 400) was an exception. That’s why every Leap Year for the rest of the century will have the earliest equinox in our lifetime. The Equinox will never arrive earlier than March 19.

Although both hemispheres are bathed with equal amounts of light on the Equinox, the actual length of daylight is longer than night for both hemispheres.

The sunrise was 6:59 a.m. and the sunset was 7:08 p.m. The actual date that day length is equal happens a few days before the Spring Equinox. It’s called the Equilux. This year it was on March 16.

There are two reasons for the phenomenon. Sunrise and sunset are defined as the moment that the top of the sun’s disk touches the horizon. It’s just semantics, but it adds extra time to our “hours of daylight.”

The moment the sun sets and rises actually appears earlier than it would if the planet didn’t have an atmosphere. The sun’s rays refract upward when they hit the atmosphere. Light’s behavior changes when it travels into a denser object. Next time you look at a glass of water, you’ll see the science behind this on a smaller scale!