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Slowest sunsets around solstices

Here’s a natural phenomenon you might never have imagined. That is, the sun actually sets more slowly around the time of a solstice.

Peter Lowenstein in Mutare, Zimbabwe captured this beautiful sunset on December 16, 2017, a few days before the December solstice – his summer solstice.

Here’s a natural phenomenon you might never have imagined. That is, the sun actually sets more slowly around the time of a solstice. 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. Follow the links below to learn more:

When is the solstice?

Why does the sun set so slowly around the solstices?

Adrian Strand captured this photo on a beach in northwest England.

When is the solstice? In 2018, the Northern Hemisphere’s summer solstice (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 Eastern Time, 5:07 a.m. Central Time, 4:07 a.m. Mountain Time, 3:07 a.m. Pacific Time, 2:07 a.m. Alaskan Time and 12:07 a.m. Hawaiian Time.

Equinoxes and solstices, via Geosync

Equinoxes and solstices, via Geosync

Why does the sun set so slowly around the solstice? At the December (or June) solstice, the sun rises and sets farthest south (or north) of due east and 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.

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, 40o north, the latitude Denver or Philadelphia in the United States, or Beijing in China. At that latitude, on the day of solstice, the sun sets in about 3 and 1/4 minutes.

On the other hand, at 40o north latitude, the equinox sun sets in roughly 2 and 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 (65o north latitude), the duration of a solstice sunset lasts about 15 minutes; at the equator (0o latitude), the solstice sun takes a little over 2 and 1/4 minutes to set. Regardless of latitude, however, the duration of sunset is always longest at or near the solstices.

Actually, the sunset and sunrise are a tad bit longer on the December solstice than they are on the 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. Additionally, the closer December sun moves eastward upon the ecliptic at a faster clip, helping to retard the December solstice sunset (and sunrise) even more. For instance, at 50o 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.

equinox_solstice_610

Bottom line: The slowest sunsets of the year are happening now, around the time of the June solstice.

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Bruce McClure

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