The Autumnal Equinox

The Autumnal Equinox is one of the two points where the Ecliptic crosses the Celestial Equator. At the Autumnal Equinox the sun appears to be moving across the equator from the Northern Celestial Hemisphere to the Southern Celestial Hemisphere. The other point where the sun crosses the Celestial Equator is the Vernal Equinox.

Recall that it is the apparent motion of the sun eastwards around the Ecliptic that creates the seasons that we experience at different times of the year. The arrival of the sun at the Autumnal Equinox on or about September 23 marks the beginning of Autumn. Because the number of days in the year is not a whole number, the sun does not arrive at the Autumnal Equinox at the same date every year. The date of the Autumn Equinox may vary by more than two days. See the discussion of the Vernal Equinox. For the purposes of ASTR106, we will take the date of the Autumnal Equinox to be Sept. 23. Since the Autumnal Equinox represents the beginning of the fall season, we can also refer to it as the Fall Equinox.

Since the sun is on the Celestial Equator at the Autumnal Equinox, it has a declination there of 0°. Recall that the right ascension at the Vernal Equinox is 0 h. Since the Autumnal Equinox is diametrically opposite to the Vernal Equinox on the Celestial Sphere, the Autumnal Equinox must lie at 12 h of right ascension. You should then remember the equatorial coordinates of the Autumnal Equinox as 0°, 12 h.

On the Equatorial Sky Chart the Autumnal Equinox lies on the right edge and left edge of the chart. You must imagine that the chart closes around the sky, so that the line represented by the left edge of the map coincides with the line represented by the right edge. Each edge of the map represents the 12 h hour circle.