Tilting toward spring: the Vernal Equinox arrives

We are in constant motion on this almost round, spinning planet and curiously, it is not the motion that we sense but the results of that motion.

The revolution of the Earth on its axis results in the daily phenomenon of night and day. The revolution of the earth around the sun produces the effect that we know as seasons. But there is another motion that happens over a much longer period and that is wobble or “precession”. The earth is currently tilted in its plane with the sun at 23.5 degrees, but that has varied over time by more than a degree in each direction. The result is that the heating and cooling of the hemispheres and the character and length of the seasons change. Ice age events may be linked to the changing tilt of the planet. But in the scale of a human lifetime, we can count on 23.5 degrees plus or minus a fifth of a degree.

You can celebrate the changes that result from the earth's motion on Saturday March 20th at 6:48 am when the earth will be oriented with its equatorial plane pointed directly at the sun. For that moment, both hemispheres will have nearly equal exposure to the sun. Day length will consequently be nearly equal north and south. We know this as the Vernal Equinox and in the northern hemisphere, the arrival of spring. In the next weeks and months, the northern hemisphere will “tip” increasingly toward the sun and reach its maximum on the summer solstice the third week in June.

The graph shows the variation in the tilt of the Earth's axis over the last 750,000 years. The blue line traces the tilt. The orange line shows today's value for comparison. The data are from Berger and Loutre (1991) as found on Illinois State Museum web site (www.museum.state.il.us).

Today's photo is of a wetland sunrise, and was taken during the Vernal Equinox of 2001, in Mercer County by WPC Volunteer Photographer John M. Karian. John is having an exhibition "Allegheny Waterway and Wetland Splendors" at the Silver Eye Center for Photography, 1015 E. Carson Street in Pittsburgh, through April 17th.

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