March 20th is the March equinox, one of four important days in the year that define the relation between the sun and the Earth.
Most of the year, the Earth’s axis is tilted relative to the Sun and the Sun strikes the Earth at varying angles, depending upon the latitude. When the northern hemisphere is tilted AWAY from the sun, we get less direct sunlight, shorter days, and, therefore, winter. The southern hemisphere is just opposite.
However, on the two equinoxes, one in March and one in September, the Earth is not tilted relative to the Sun.
In the northern hemisphere, we are entering spring; however, the southern hemisphere is entering autumn. For this reason, it is becoming more common to call this the March equinox, rather than the spring equinox.
Equinox comes from the Latin meaning “equal night”. We might think that this means that the day and night are each 12 hours long. While, for practical purposes, this is suitable, it’s not technically accurate.
"Sunrise" is defined as when the upper edge of the sun's disk becomes first visible above the true horizon, or the horizon you see if you were on a large body of water. In the same sense, "sunset" refers to when the upper edge disappears below the true horizon. In both instances, the center of the sun is below the horizon, and therefore the equinox day lasts a little longer than 12 hours and the night is slightly less.
Note that an equinox can occur on any planet that has a tilt in its axis relative to its orbit around the Sun. The most extreme example of this is Uranus. While the Earth’s axis is tilted about 23 degrees, Uranus is tilted 98 degrees, meaning it’s almost lying on its side. The seasons there are quite variable and long.
Western Slope Skies is produced by members of the Black Canyon Astronomical Society.