Sunday, September 21, 2014

Tonight's Sky for September 22: Autumnal Equinox


For anyone not keeping track of the calendar, fall 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 Autumnal 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 Autumnal Equinox, the shortening of the days will continue (for us in the Northern Hemisphere) until the Sun finally reaches its most Southerly rise/set on the Winter Solstice, the shortest day of the year., which is around December 20. From that point on, the Sun will only get stronger, once again having an Equinox, the Vernal, around the 20th of March before culminating in its most Northerly rise of the year, the Summer Solstice, around June 20.

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