2016 Autumnal Equinox

At 14:21 UTC (10:21 EDT) yesterday, the Autumnal Equinox occured, the Astronomical beginning of Autumn. This is the second time this year, and every year, that marks the date and time when there are equal hours of daylight and darkness, the first being the Vernal Equinox, the Astronomical beginning of Spring.

Yesterday the sun transited the celestial equator (Declination: 0º, Right Ascention: 12h:00m), midway between the solstices (Summer, in June and Winter, in December). Note in the image below, the location of the sun and the meridian and that the Equinox occurs east (2h, 24m) of local noon for locations along the North American Eastern seaboard. Local noon is defined as the transit of the sun across the meridian.

On this day, and likewise on the Vernal Equinox, the sun crosses the celestial equator with equal hours of daylight and darkness, 12 hours each, it will rise due east and set due west. The Celestial Equator is the celestial analog of the Earth’s equator projected onto the sky and thus, we define and derive the Cardinal Directions (N, E, S, W) from the equator. With each passing day, the sun will rise slightly more south until the Winter Solstice occurs on 21 December this year.

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This year’s Autumnal Equinox occurs at 14:21 UTC (10:21 EDT). Note the coordinates of the sun and the locations of Venus, Jupiter and Mercury. Image via Stellarium.

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