The Sun sets near the domes of McDonald Observatory, creating a brilliant sundog above the telescopes. Even though the winter solstice is still a few weeks away, the Sun will soon begin to set a bit later each evening from the United States, working from south to north.
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The days are continuing to get shorter here in the northern hemisphere. That provides a lot of time to get out and enjoy the night skies. But over the next few weeks, one part of that trend will reverse. The Sun will begin to set a little later each night — even though the shortest day of the year is still weeks away.
The shortest day is the winter solstice, on December 21st. So between now and then, the total amount of time between sunrise and sunset will shrink. Yet the date of earliest sunset comes before the solstice.
There are a couple of reasons for that. One is Earth’s tilt on its axis, which causes the angle and location of sunrise along the horizon to change from day to day. The other is Earth’s lopsided orbit, which causes the Sun to return to the same point in the sky a little earlier or later in the day depending on whether we’re close to the Sun or far away.
When you add these factors together, you find that the date of earliest sunset works its way from south to north. For the latitudes of Honolulu and Miami, the earliest sunsets happen about now. For higher latitudes, such as Denver, they’ll take place in another week or so. And for cities that are even farther north, like Anchorage, earliest sunset happens just a few days before the solstice.
To balance things out, the date of the latest sunrise comes after the solstice, working from north to south — a few more minutes of darkness before the dawn of another day.
Script by Damond Benningfield