The September equinox and the first quarter moon both fall on September 22 this year. In 2012, the moon reaches its half-lit first quarter phase only about five hours after this year’s September equinox.
The equinox happens today at 14:49 Universal Time, which is 9:49 a.m. Central Daylight Time for us in the central U.S. The September equinox takes place whenever the sun is at zenith – straight overhead – at the Earth’s equator. This magical moment signals the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere. On the day of an equinox, the sun rises due east and sets due west all over the world.
The last quarter Earth, as seen from the first quarter moon (2012 September 22 at 19:41 Universal Time). Image credit: Earth and Moon Viewer
To celebrate the change of seasons, enjoy the moon on this, the first evening of a Northern Hemisphere autumn – or Southern Hemisphere spring. Everyone around the world will see the moon at or in close vicinity of first quarter phase as the sun sets tonight. Any first quarter moon that falls on the day of the September equinox shows you the sun’s place in front of the stars of the Zodiac for three months hence – or on the December solstice. That’s because the first quarter moon resides 90o (one-quarter circle) east of the sun as measured on the great circle of the ecliptic.
Because the September equinox is the Northern Hemisphere’s autumn equinox, tonight’s moon approximates the position of the sun on the winter solstice. At sunset today in the Northern Hemisphere, look for tonight’s moon to mimic the noonday winter solstice sun.
In the Southern Hemisphere, on the other hand, the September equinox is their spring equinox. At sunset in the Southern Hemisphere, today’s first quarter moon assumes the lofty position of the noonday summer solstice sun.
The September equinox – the Northern Hemisphere’s autumn equinox and the Southern Hemisphere’s spring equinox – occurs on or near September 22 every year. Yet, the close alignment of the first quarter moon with the September equinox only happens in cycles of nineteen years.
Take advantage of today’s rather rare alignment of the September equinox and the first quarter moon. At sunset today, the first quarter moon enables you to envision the position of the noonday December solstice sun!