Astronomy Essentials

Year’s fastest sunsets happen around equinoxes

Fastest sunsets: White sun in orange-golden sky over glimmering orange sea.
Sunrise over the Red Sea, via Graham Telford.

Year’s fastest sunsets and sunrises

The September equinox will arrive tomorrow (September 22, 2021) at 19:21 UTC. And here’s a little-known equinox phenomenon: the sun 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. It’s 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.

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Why does the sun set so quickly around the equinoxes? It’s because, 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.

Year’s slowest sunsets and sunrises

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. Farther north or south on the Earth’s globe, the duration of sunset lasts longer. Closer to the equator, the duration is shorter. But let’s just consider one latitude, 40 degrees north, the latitude of Denver or Philadelphia in the United States; parts of Spain; and Beijing, China.

At that latitude, on the day of equinox, the sun sets in about 2 3/4 minutes.

On the other hand, the solstice sun sets in roughly 3 1/4 minutes at 40 degrees latitude.

Diagram of four Earths and Earth's orbit with sun in center and equinoxes and solstices labeled.
The equinox is an event that takes place in Earth’s orbit around the sun.

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

All you need to know about the September equinox

Are day and night equal on the equinox?

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Posted 
September 21, 2021
 in 
Astronomy Essentials

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Bruce McClure

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