If Thursdays's storm threat is any indication of the season, Mother Nature definitely thinks it's springtime.  But now we are officially in the spring season!  11:49 PM Eastern Thursday was the official start of Spring. Because 2020 is a leap year, this is the earliest spring equinox since 1896. Yes, the earliest spring (or vernal) equinox in 124 years!
Equinox means "equal night" in Latin because it seems like we get an even 12 hours of sunlight and darkness on the equinox, but that's not true everywhere on the globe. In Louisville our day of roughly 12 hours of sunlight and 12 hours of dark actually happened March 17th, so now we're adding daylight with every day that passes. What is true at every location on the equinox is that the sun rises at due east and sets at due west. The equinox is caused by tilt of earth's axis (roughly 23.5º). On the day of the equinox the tilt of the earth combined with it's orbit around the sun lines the position of the Earth up in such a way that the axis is lined up vertically, neither toward nor away from the sun.Â
If you want to get a little more technical...
On the day of the equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth. However, the Sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. At these times, the center of the disk is already below the horizon. Another reason why the day is longer than 12 hours today is because Earth's atmosphere refracts, or bends, light. This refraction causes the sun's upper edge to be visible several minutes before sunrise. This also happens at sunset, you can see the sun for a few minutes after it has dipped below the horizon. This means that everyday on Earth is longer because of this refraction, not just on the equinox.Â
