Q: Why doesn't spring start on the same date every year?

A: According to the astronomical definition, spring occurs when the sun's rays strike the equator at noon at an angle that is directly overhead. This time varies from year to year due to variations in the Earth's orbit around the Sun.

In the northern hemisphere, the vernal equinox occurs between March 19 and 23, but usually on March 21. This year, astronomical spring arrived in Madison on Friday at around 6:45 a.m. During the equinoxes, all locations on Earth experience 12 hours of daylight and 12 hours of darkness. The sun rises due east and sets due west.

But there are other definitions of spring. The seasons result from the tilt of the Earth and its yearly circling of the sun. Spring marks the transition from winter to summer. So meteorologists often define the three months of spring as March through May. By that definition, spring would begin on March 1. We might also define spring as the day on which, if there is precipitation, it is more likely to be in the form of rain than snow. For southern Wisconsin, that occurs later in March. We may also define spring based on the appearance of a particular flower, the blooming of certain trees, or the return of specific migrating birds.

Whatever the definition, during spring the length of daylight hours is increasing and the air is warming. That's welcome news for many people.

Steven A. Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA Radio (970 AM) the last Monday of each month at 11:45 a.m.