Enjoying EarthSky? Subscribe.

164,312 subscribers and counting ...

Sun at zenith over Earth’s equator on the September equinox

Simulated image of Earth (without clouds) at the instant of the September 23 equinox.

Tonight for September 22, 2015

The 2015 northern autumnal equinox (southern spring equinox) takes place on Wednesday, September 23, at 8:21 UTC. If you could view earth from the sun at that moment, at the instant of the 2015 autumnal equinox, you’d see the hemisphere of Earth shown on the image at the top of this post facing in the sun’s direction.

Although the equinox happens at the same instant for everyone worldwide, the clock time for the equinox varies by time zone. In the U.S., the local clock time for the September 23 equinox will be 4:21 a.m. EDT, 3:21 a.m. CDT, 2:21 a.m. MDT or 1:21 a.m. PDT.

At this special moment – the instant of the September equinox – the midday sun will be at zenith, or straight overhead, at the Earth’s equator. That’s the meaning of equinox. The September equinox sun crosses the sky’s equator, going from north to south. Because the path of the sun is heading southward, this equinox signals the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere.

On the day of the equinox, the sun rises due east and sets due west all over the world, with everyone worldwide receiving approximately equal portions of day and night.

When October comes rolling around that’ll change dramatically. By then, the sun will rise noticeably south of due east and will set noticeably south of due west. That’ll mean shorter days and longer nights for the Northern Hemisphere, yet longer days and shorter nights in the Southern Hemisphere.

After the equinox, the sun and the migrating birds will continue to travel southward to the southern climes. Arctic sea ice will begin to freeze; Antarctic ice will start melting. The great wheel of the seasons will continue to turn.

How to celebrate? Try to watch as the sun rises due east and sets due west on the great day of the equinox.

Read more: Everything you need to know about the September equinox 2014

Day and night sides of Earth at instant of September 2015 equinox

Day and night sides of earth at the instant of the September equinox (2015 September 23 at 8:21 Universal Time). Image credit: Earth and Moon Viewer

Day and night sides of earth at the instant of the September equinox (2015 September 23 at 8:21 Universal Time). Image credit: Earth and Moon Viewer

Fastest sunsets of the year around equinox time