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. To learn more about the fast sunsets around now, and what to look for in the sky after sunset, follow the links below.
Why does the sun set so quickly around the equinoxes? At every equinox, the sun rises and sets due west. That means – on the day of an equinox – the 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.
Although the sunset duration varies by latitude, the equinox sun sets in about 2 and 3/4 minutes at 40o latitude (Denver, Philadelphia).
Meanwhile, at 40o latitude, the solstice sun sets in roughly 3 and 1/3 minutes.
What can I see in the sky after sunset? After the quick sunset on Friday, September 20 – or any night around now – be sure to look westward for the close pairing of the planets Venus and Saturn. These two are in the same field of view of your binoculars from September 15 to September 21.
Venus is by far the brighter of these two solar system worlds, though both Venus and Saturn will be easily visible to the eye alone around 45 to 60 minutes after sunset.
If for any reason (poor sky conditions?) 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.
When is the next equinox? 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 in September 2013, the planets Venus and Saturn appear close in the western sky.