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. What’s more, 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.
After the quick sunset on Friday, September 20, be sure to look in the southwestern sky for the close pairing of the planets Venus and Saturn. Venus is by far the brighter of these two solar system worlds, though both should be visible to the naked eye around 45 to 60 minutes after sunset. If you can see dazzling Venus but not Saturn, use binoculars to observe Saturn and Venus in the same binocular field together. But don’t wait around. At mid-northern latitudes, Venus and Saturn follow the sun beneath the horizon about one and one-half hours after sundown.
People often ask us why the sun sets so quickly around the equinoxes. When the sun sets due west – as it does on the day of an equinox – the sun hits the horizon at the steepest possible angle, which hastens the duration of sunset. Although the sunset duration varies by latitude, the equinox sun sets in about 2 and 3/4 minutes at 40o latitude (Denver, Philadelphia).
On the solstices, the sun sets farthest north or farthest south of due west. The farther the sun sets from due west, the shallower the angle of the setting sun. That means a longer duration for sunset. At 40o latitude, the solstice sun sets in roughly 3 and 1/3 minutes.
In 2013, the Northern Hemisphere’s autumn equinox (Southern Hemisphere’s spring equinox) falls on September 22 at 20:44 (8:44 p.m.) Universal Time. In North America, that translates to 4:44 p.m. Eastern Time, 3:44 p.m. Central Time, 2:44 p.m. Mountain Time and 1:44 p.m. Pacific Time.
Bottom line: The fastest sunsets of the year are happening now, around the time of the September equinox. At evening dusk on September 20, 2013, the planets Venus and Saturn have a close conjunction in the western sky.