The earliest sunset for 40 degrees N. latitude is on December 8, and the year’s earliest sunset comes around then for everyone near this latitude. The exact date of the Northern Hemisphere’s earliest sunset or the Southern Hemisphere’s earliest sunrise varies by latitude. But, at temperate latitudes, both of these annual hallmarks in our sky come some weeks before the December solstice, not on the solstice as you might expect.
Every year, toward the end of the first week of December – that is, around now – mid-temperate latitudes in the Northern Hemisphere have their earliest sunsets. That would be the latitude of New York City (shown in Jerry Ferguson‘s photo, top of post); Philadelphia, Pennsylvania; Kansas City, Missouri; Denver, Colorado; Reno, Nevada; Beijing, China; Madrid, Spain; Naples, Italy.
Suppose you live at 40o South latitude? Then, you’d be waking up to the earliest sunrises of the year.
What if you’re not at 40o North or 40o South latitude? At latitudes closer to the equator, the earliest sunset or earliest sunrise comes at an earlier date.
Closer to the Arctic and Antarctic Circles, the earliest sunset and earliest sunrise happen nearer the December solstice.
The next solstice in 2015 comes on December 21 or 22 (depending on your time zone) and marks an unofficial beginning for winter in the Northern Hemisphere. For this hemisphere, this upcoming solstice brings the shortest day and longest night of the year. And yet the earliest sunsets for middle latitudes in the Northern Hemisphere happen around December 8.
It seems paradoxical. At middle latitudes in the U.S. – and throughout the Northern Hemisphere – the earliest sunsets of the year come about two weeks before the solstice and the shortest day of the year.
Why isn’t the earliest sunset on the year’s shortest day? It’s because of the discrepancy between the clock and the sun. A clock ticks off exactly 24 hours from one noon to the next. But an actual day – as measured by the spin of the Earth, from what is called one “solar noon” to the next – rarely equals 24 hours exactly.
Solar noon is also called simply midday. It refers to that instant when the sun reaches its highest point for the day. In the month of December, the time period from one solar noon to the next is actually half a minute longer than 24 hours. On December 8, the sun reaches its noontime position at 11:52 a.m. local standard time. Two weeks later – on the winter solstice – the sun will reach its noontime position around 11:59 a.m. That’s 7 minutes later than on December 8.
The later clock time for solar noon also means a later clock time for sunrise and sunset. The table below helps to explain.
For Philadelphia, Pennsylvania
|Date||Sunrise||Solar Noon (Midday)||Sunset||Daylight Hours|
|December 8||7:09 a.m.||11:52 a.m.||4:35 p.m.||9 hours 26 minutes|
|December 22||7:19 a.m.||11:59 a.m.||4:39 p.m.||9 hours 20 minutes|
As you might have guessed, the latest sunrises and sunsets aren’t on the day of the solstice either. For middle latitudes in the Northern Hemisphere, the latest sunrises come in early January.
So there’s variation in the exact dates, but the sequence is always the same for both hemispheres. First: earliest sunset before the winter solstice, the winter solstice itself, latest sunrise after the winter solstice. Half a year later: earliest sunrise before the summer solstice, the summer solstice itself, latest sunset.
The earliest and latest sunsets and sunrises are lovely phenomena that happen around every solstice. People around the world notice them and often ask about them.
Bottom line: The 2015 solstice comes on December 22, but the earliest sunsets at mid-northern latitudes – say, 40 degrees N. happen on or near December 8. Latitudes closer to the equator had their earliest sunset in late November, or earlier in December. Latitudes closer to the Arctic Circle have their earliest sunset closer to the December solstice.