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Year’s fastest sunsets at equinoxes

Sunrise over the Red Sea by EarthSky Facebook friend Graham Telford

Tonight for September 21, 2015

Photo top of post by EarthSky Facebook friend Graham Telford.

Here’s a natural phenomenon you might never have imagined. That is, the sun actually sets faster around the time of an equinox. The fastest sunsets (and sunrises) occur at or near the equinoxes. And the slowest sunsets (and sunrises) occur at or near the solstices. This is true whether you live in the Northern or Southern Hemisphere.

And, by the way, when we say sunset here, we’re talking about the actual number of minutes it takes for the body of the sun to sink below the western horizon. Follow the links below to learn more:

Why does the sun set so quickly around the equinoxes?

When is the next equinox?

Equinoxes and solstices, via Geosync

Equinoxes and solstices, via Geosync

Why does the sun set so quickly around the equinoxes? At every equinox, the sun rises due east and sets due west. That means – on the day of an equinox – the setting sun hits the horizon at its steepest possible angle. In other words, the sun is dropping almost straight down from above.

Meanwhile, at a solstice, the sun is setting farthest north or farthest south of due west. The farther the sun sets from due west along the horizon, the shallower the angle of the setting sun. That means a longer duration for sunset at the solstices.

The sunset duration varies by latitude, but let’s just consider one latitude, 40o North, the latitude Denver or Philadelphia in the United States, or Beijing in China. At that latitude, on the day of equinox, the sun sets in about 2 and 3/4 minutes.

Meanwhile, at 40o latitude, the solstice sun sets in roughly 3 and 1/4 minutes.

The lunar calendars are almost here! They’ll help you know the moon phases throughout the year.


When is the next equinox? In 2015, the Northern Hemisphere’s autumn equinox (Southern Hemisphere’s spring equinox) falls on September 23 at 8:21 Universal Time. In North America, that translates to September 23, at 4:21 a.m. Eastern Time, 3:21 a.m. Central Time, 2:21 a.m. Mountain Time and 1:21 a.m. Pacific Time.

Everything you need to know about the autumnal equinox of 2015

Bottom line: The fastest sunsets of the year are happening now, around the time of the September equinox.

Are day and night equal on the equinox?

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