Autumnal Equinox – 23rd September 2014

Earth lighting equinox

Illumination of the Earth during an equinox. Image Credit: http://en.wikipedia.org/wiki/File:Earth-lighting-equinox_EN.png

02:29 UT (03:29 BST) sees the Southward equinox.  For ourselves in the Northern Hemisphere, it is known as the Autumnal equinox and the Vernal or Spring equinox for residents in the Southern Hemisphere. The Sun falls on the Celestial Equator during the Autumnal equinox resulting in equal lengths of day and night across the planet.  From today on-wards  as the name suggests, the Sun is seen to be moving southwards, resulting in shorter days and longer nights for us.

celestial sphere

The Celestial Sphere. Image credit: http://www.kirchdorferweb.com

The point on the Celestial Equator where the Sun lies during the Vernal equinox is known as the First Point of Aries.  However, due to precession (the continued change in the rotation of the planet on its axis), the Sun is no longer in the constellation of Aries and has not been since around 100 BC.  At the moment, the Sun is in the constellation of Pisces and by the year 2600 it will be in the neighbouring constellation of Aquarius.

As the Sun rises at an equinox, the Sun will appear to cross the horizon due east and recrosses the horizon at Sunset due west.  Over the next 3 months leading up to the Winter or December Solstice, the Sun will appear to rise and set further to the south of these points as it appears to follow the path of the ecliptic.  It will then move back north approaching the northward equinox in March when the Sun will again rise and set due east and west respectively.

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