01 March 2006

Vernal Equinox

Spring is in the air. Well, you might not realize that from the cold and the rain. But in fact you can tell by the changes in the sky. In particular, the days are getting longer and if you hadn't yet noticed, the pace of change in the amount of daylight is rapid.

The Vernal Equinox (the first day of Spring) takes place this year on March 20th. This is the day when the Sun stands directly over the Earth's equator at noon and begins its six month journey in the northern skies. From our perspective the Sun will be 52 degrees above the horizon at noon (which by the way is determined by subtracting our latitude in San Francisco from 90 degrees - a bit tricky to explain in writing so if you want to know more about this come to my lecture on March 21st!).

What does this mean for us right now? Each day the time of sunrise is arriving about 1 1/2 minutes earlier and the time of sunset is occurring about 1 minute later than the day before. So in a single week in March we experience more than 15 minutes of additional daylight compared to the week before. The Vernal Equinox is the day in which this daily change is fastest -- that is, the days are getting longer at the most rapid rate. After the Vernal Equinox the days will continue to get longer but the daily change will be slower.

All of this happens because of the tilt of the Earth on its axis. The tilt of the Earth gives us longer and shorter days and hence warmer and colder seasons. And it means that the location of the Sun, Moon and planets is not a simple, predictable line across the sky. We will look at some of the special motions of our Solar System neighbors in a coming issue about eclipses.

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