Spring marks the transition from winter to summer. In astronomical terms, the date that marks the spring in the Northern Hemisphere is the vernal equinox.
This year the spring equinox occurred on at 10:33 a.m. Sunday. This is the first day in 2022 that the sun is above the horizon for all locations on Earth for 12 hours. The second time will be at the fall equinox.
The sun rises due east and sets due west at the equinox, no matter where you live. The length of time it takes for the sun to sink below the horizon is shortest during the equinoxes. Looking at Earth from space at an equinox, you’ll see the Earth’s terminator – the dividing line between day and night – is vertical and connects the north and south poles.
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 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, which results in all areas experiencing a little more than 12 hours of daylight.
Besides astronomy, there are other ways to mark spring. Meteorologists refer to March, April and May as the three months of spring. Phenological signs of spring can include the blooming of cherry trees or the return of specific migrating birds. Some commuters mark spring by the increase in the number of potholes.
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 stevea@ssec.wisc.edu or jemarti1@wisc.edu.
