March 19, 1999

     Vernal Equinox

Our astronomical spring in the Northern Hemisphere begins on the vernal equinox, which occurs at 7:46 p.m. Saturday this year and on March 20 or 21 each year.  It's 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.

It's also the time when the earth has reached the point in its annual path around the sun when each place on our planet receives 12 hours of sunshine and 12 hours of night.  From now until June 21, nights in the Northern Hemisphere continue to grow shorter than 12 hours, and days become longer.

That's the genesis of the name vernal equinox.  Vernal is from Latin meaning "belonging to spring." Equinox is from Latin meaning "equal night."

The tilt of the earth's axis is the reason for the seasons.  The earth's axis, in relation to the sun, is tilted 23.5 degrees from an upright posistion.  This angle of tilt remains the same throughout the daily rotation on the axis and the annual revolution around the sun.  If the earth were not tilted on its axis there would be no spring, summer, fall or winter, but there would be varied climates.  Because the earth is tilted, at times the North Pole is leaning towards the sun and at other times it leans away from 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, bringing summer to the northern half of the earth.

The vernal equinox marks the beginning of spring astronomically but not meteorologically.  For meterologists, spring runs from March 1 to May 31.