Sunday, March 20, 2016

Tonight's Sky for March 20: Vernal Equinox (First Day of Spring)

For anyone not keeping track of the calendar, spring arrives today, which begs a question: why do we have seasons at all? Answer: it all has to do with the Earth’s 23 degree tilt.

If the Earth were spinning on its axis with no tilt at all, everyone would be treated to days of identical length every day of the year, with latitudes nearer the equator having longer days than those nearer the poles. However, with the tilt, the angle of the Earth relative to the Sun changes as or planet moves about its orbit. On the Vernal Equinox, the Sun will rise/set exactly due East/West and the day and the night will be exactly 12 hours long (Equinox means 'equal night').

After the Vernal Equinox, the lengthening of the days will continue (for us in the Northern Hemisphere) until the Sun finally reaches its most Northerly rise/set on the Summer Solstice, the longest day of the year, which is around June 20. From that point on, the Sun will only get weaker, once again having an Equinox, the Autumnal, around the 20th of September before culminating in its most Southerly rise of the year, the Winter Solstice, around December 20.

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