Wednesday, September 23, 2015

Fall Equinox – First day of Autumn 2015

Copyright © Françoise Herrmann

Today’s Google Doodle (in the US) reminds everyone that Sept. 23, 2015 is the first day of Autumn. Indeed the Fall semester at NYU is starting soon at the School of Professional Studies.

Here are a few key concepts of Astronomy… just in case you find patents in the sky…(and you will if you are covering climate change..)!

The first day of Autumn corresponds to the Fall Equinox, which is the day and time of the year when the sun is directly positioned on the Equator.  This precise position and time of the sun over the Equator (for example, in California, at 1:22 am on Sept. 23, 2015) divides the duration of day and night into equal length, which explains the term “equinox” derived from the Latin aequinoctium combining aequus meaning “equal”, and nox meaning “night”. This special position of the sun happens just two times each year, once in the Fall, called the Fall Equinox, and once in the Spring, called the Spring Equinox.

The Summer and Winter Solstices, in contrast, mark the shortest and longest days of the year, depending on the hemisphere of the earth. The Solstice is recorded on the day the sun’s position is the farthest or at it’s zenith relative to the elliptical orbit of the Earth. In the Northern hemisphere this position occurs in June, marking the Summer Solstice relative to the Northern hemisphere and the Winter solstice relative to the Southern hemisphere. In the Southern hemisphere, the sun is at it’s zenith or the farthest from the Earth’s equator, in December, marking the Summer solstice relative to the Southern hemisphere and the Winter solstice relative to the Northern Hemisphere.  

 Because of the Earth’s tilt relative to the sun, when the sun is at it’s zenith in the Northern hemisphere, it is positioned directly over the Tropic of Cancer. In the Southern hemisphere, when the sun is at it’s zenith, it is positioned directly over the Tropic of Capricorn.

Below, a figure drawing of the solstices and equinoxes, extracted from the online version of the Encyclopedia Britannica.

Google Doodles
Encyclopedia Britanica (Article on Solstice and Zenith) - What is the September Equinox?

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