Latin lesson here: Our astronomical spring in the Northern Hemisphere begins on the vernal ("belonging to spring") equinox ("equal night").
The Earth has reached the point in its annual path around the sun when each place receives 12 hours of sunshine and 12 hours of darkness. Until June 21, however, nights will continue to shorten and days will get longer.
The vernal equinox is the time when the sun reaches the celestial equator, an imaginary line through the sky above the Earth's equator. As the sun's center crosses this line, the season officially changes. With that, astronomers say that spring begins Friday at 5:45 p.m.
With respect to the sun, the Earth's axis is tilted 23½ degrees from an upright position. This tilt remains throughout the Earth's daily rotation and its annual revolution around the sun. Because of the Earth's tilt, the North Pole at times leans toward the sun and conversely away from it, too. When the North Pole leans toward the sun, rays strike the Northern Hemisphere in a more direct manner, bringing summer to the northern half of the Earth.
Meteorologically speaking, spring started March 1 and runs until May 31. As a naturalist who has tracked spring, I agree that the first astronomical day of spring isn't necessarily the first spring day. By March 20, we each year have experienced dozens of spring signs.
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.