Here’s a natural phenomenon you might never have imagined. That is, the sun actually sets more slowly around the time of a solstice.
It’s true. The slowest sunsets (and sunrises) occur at or near the solstices. The fastest sunsets (and sunrises) occur at or near the equinoxes. 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.
When is the solstice? In 2018, the Northern Hemisphere’s summer solstice – and Southern Hemisphere’s winter solstice – will fall on June 21 at 10:07 UTC.
In the United States, that translates to June 21 at 6:07 a.m EDT, 5:07 a.m. CDT, 4:07 a.m. MDT, 3:07 a.m. PDT, 2:07 a.m. Alaska Daylight Time and 12:07 a.m. Hawaii-Aleutian Daylight Time. Translate to your time zone.
Why does the sun set so slowly around the solstice? As viewed from the entire Earth, the sun rises and sets farthest north at the June solstice and farthest south at the December solstice.
Now consider that 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.
Meanwhile, at an 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.
The sunset duration varies by latitude, but let’s just consider one latitude, 40 degrees north, the latitude of Denver or Philadelphia in the United States, or Beijing in China. At that latitude, on the day of a solstice, the sun sets in about 3 1/4 minutes.
On the other hand, at 40 degrees north latitude, the equinox sun sets in roughly 2 3/4 minutes.
At more northerly temperate latitudes, the sunset duration is greater; and at latitudes closer to the equator, the sunset duration is less. Near the Arctic Circle (65 degrees north latitude), the duration of a solstice sunset lasts about 15 minutes. At the equator (0 degrees latitude), the solstice sun takes a little over 2 1/4 minutes to set.
Regardless of latitude, however, the duration of sunset is always longest at or near the solstices.
As it turns out, the sunset and sunrise are a tad longer on a December solstice than they are on a June solstice. That’s because the sun is closer to Earth in December than it is in June. Therefore, the sun’s disk looms a bit larger in our sky in December, and so it takes slightly longer to set.
Additionally, the closer December sun moves eastward upon the ecliptic at a faster clip, helping to slow down the December solstice sunset (and sunrise) even more. For instance, at 50 degrees north latitude, the winter solstice sunset (sunrise) lasts about 4 minutes and 18 seconds, or about 8 seconds longer than the sunset (sunrise) on the summer solstice.
Bottom line: Here’s a natural phenomenon you might never have imagined. That is, the sun actually sets more slowly around the time of a solstice.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.