Tuesday, March 16, 2010

The Days Are Getting Longer, Fast!

Even the most casual observers have noticed that the days are getting longer fast at this time of year. The table to the right is derived from sun rise and set data provided by the US Naval Observatory for White Plains, NY, and a quick study of the data reveals why.

On 3/25, the Sun will rise 17 minutes earlier and set 10 minutes later than it did on 3/15 - a gain of 27 minutes of daylight in just 10 days!

While the daylight period has been getting longer since the winter solstice on December 21, a look at the Sun rise and set times for late December reveal why those long winter nights seem to drag on for so long...10 days after the solstice, the sun was rising 3 minutes later and setting 6 minutes later, for a hardly noticeable gain of only 3 minutes of daylight!

The geometry of the Sun's path among the stars is responsible for the variability of the change of the length of daylight.  The length of daylight changes quickly around the equinoxes in March and September, and very little around the solstices in June and December.

Notice too that the gains are not symmetrical around noon -  in March the morning gains are almost twice as great as the evening gains, and in December the sun rises later each day even as the days get longer - the lengthening is all in the evening!

This is because the Sun speeds up and slows down relative to the clock (which is in sync with the average time the sun takes to cross the sky). In the spring the Sun is "speeding up" relative to the clock while in December the Sun slows so dramatically that sunrise lags behind the clock even as the days get longer!  This is all a result of both the changing velocity and axial tilt of the Earth in its orbit around the Sun (see this analemma discussion for more information on the relationship between solar and clock time).

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