The spring equinox — also called the vernal equinox — marks the beginning of the spring season in the Northern Hemisphere and the autumn season in the Southern Hemisphere.
This year the equinox arrived at 4:37 a.m. Saturday.
The equinoxes (equi for “equal,” and nox “night”) occur when the sun’s rays strike the equator at noon at an angle of 90 degrees. During the spring and fall equinoxes, the sun is above the horizon for all locations on Earth for 12 hours.
The fastest sunsets and sunrises of the year happen at the equinoxes. By fastest we mean the length of time it takes for the sun to sink below the horizon. At the equinoxes, the sun rises due east and sets due west, no matter where you live. If you live at the equator, the sun appears overhead at noon.
The tilt of the Earth’s axis is responsible for the seasonal variation in the amount of solar energy distributed at the top of the atmosphere. The Earth’s axis is tilted at an angle of 23.5 degrees from its orbital plane.
Because the Earth’s axis always points in the same direction — toward the North Star — the orientation of the Earth’s axis to the sun is always changing as the Earth orbits around the sun. As this orientation changes throughout the year, so does the distribution of sunlight on the Earth’s surface at any given latitude, and this is the cause of the seasons.
On the equinoxes the axis is not pointed at or away from the sun. This results in all areas experiencing a little more than 12 hours of daylight.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at firstname.lastname@example.org or email@example.com.