Autumnal Equinox, 2017

At 20:02 UTC (4:02 PM, EDT) today, the Autumnal Equinox occurs, 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.

Today the sun transits the celestial equator (Declination: 0º, Right Ascension: 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.

Since today, 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.

The sun as it crosses the meridian at this year’s Autumnal Equinox. The ecliptic is red and the Celestial Equator is blue.

Above, the Autumnal Equinox, 2017 as illustrated by Stellarium. At every equinox the sun, as it appears to move eastward along the ecliptic, crosses the Celestial Equator, the analog of the Earth’s equator projected onto the sky. In three short months, the sun will reach its lowest point on the sky in the Northern Hemisphere during the Winter Solstice, occurring at 11:28 AM, EST, Thursday, 21 December.

Take note of six solar system objects including the sun, all visible on this date, as objects east of the sun, following it as it sets: the Moon and Jupiter and as objects west of the sun, rising before it at sunrise: Mercury, Mars and Venus. Note beautiful, red shimmering Arcturus, low in the northwest after sunset.


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