Wednesday, March 19, 2008



At 5:48 UT tomorrow, March 20, the Vernal Equinox will occur. This is actually 0:48 CDT here in Winnipeg (don't forget we are now on Daylight Savings Time, so the usual 6 hour difference is now only 5 hours). If you want to find the time in your own time zone consult the Time and Date website. The Equinox is supposed to be a day when day and night are equal. this is untrue because the Sun is not a point source of light. In actual fact the day is longer than the night at this time of year because the upper half of the Sun's disc is already above the horizon at sunrise before the centre comes to the horizon point. Similarly the upper half of the disc is still above the horizon during sunset. This results in a time difference of about 14 minutes at the equator. As you go towards the poles the difference becomes greater and greater. The term "equinox" refers to an exact time not a day. The rate of change between day and night is greatest at the times of the equinoxes, and the difference is greater the further you get away from the equator.The exact time of the equinoxes fall about 6 hours later each year but are "reset" by the leap years.

An interesting fact is that our "seasons" are not of equal length. It is 94 days from the June solstice to the September equinox, but only 89 days from the December solstice to the March equinox. That makes summer 5 days longer than winter. Now aren't you grateful ? The difference is because of the variable speed of the Earth during its revolution about the Sun. During the winter months it is closer to the Sun, and therefore moves faster. There are also small perturbations in the times due to the influence of the Moon and the other planets.

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