Meteorologists in the Upper Midwest consider winter to be the months of December, January and February. Following this idea, winter, the season of frozen beauty and survival, gave way Wednesday to spring, the season of hope and renewal.

Now, on the third day of meteorological spring, we have many spring signs to prove it. Crocuses bloom on south sides of homes. Some eastern bluebirds already have been claiming nesting boxes. And more eastern chipmunks are out from their winter homes underground.

The record-breaking six-day warm spell we just experienced from Feb. 17-22 resulted in spring-in-winter in myriad ways: the first garden plants, the common snowdrops, were blooming. Honey bees visited those flowers and, too, those of the first blooming shrub, the witch hazel. In addition, the warmth triggered early waterfowl migration into Minnesota and the production of the first batches of maple syrup. So we might venture to say that biological spring began Feb. 17 this year.

Astronomers, on the other hand, would have us wait until March 20 at 5:29 a.m., the vernal equinox, for spring to begin. Latin words, vernal means “belonging to spring” and equinox means “equal night.” Our astronomical spring in the Northern Hemisphere begins on the vernal equinox. At that time the sun reaches the celestial equator, an imaginary line through the sky above the Earth’s equator. What happens? Every place on Earth is ready to receive 12 hours of sunshine and 12 hours of night. But because Earth’s axis is tilted 23 ½ degrees from an upright position with respect to the sun, this moment of equipoise is a fleeting one. From then until June 20, nights in the Northern Hemisphere continue to grow shorter while the days become longer.

If Earth were not tilted on its axis, climates would still vary from place to place, but there would be no spring, summer, fall or winter. Because Earth is tilted, at times the North Pole is leaning toward the sun. When the North Pole leans toward the sun, the rays of the sun strike the Northern Hemisphere in a more direct and concentrated manner per unit, bringing summer, the season of sunlight and possibilities, to the northern half of Earth.

Jim Gilbert’s Nature Notes are heard on WCCO Radio at 7:15 a.m. Sundays. His observations have been part of the Minnesota Weatherguide Environment Calendars since 1977, and he is the author of five books on nature in Minnesota. He taught and worked as a naturalist for 50 years.