The days are getting noticeably shorter with the sun setting earlier and earlier. You may have noticed that but have you noticed the sun setting faster as well?

Tonight – at sunset – 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? 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, 40^o 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 40^o latitude, the solstice sun sets in roughly 3 and 1/4 minutes.

When is the next equinox? The September equinox will arrive on September 22, 2016 at 1421 UTC. Although the equinox happens at the same moment worldwide, your clock times will depend on your time zone. For time zones in the continental U.S., this equinox comes at 10:21 a.m. EDT, 9:21 a.m. CDT, 8:21 a.m. MDT or 7:21 a.m. PDT. Translate to your time zone.

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