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N. Hemisphere? Watch for earliest sunsets

The earliest sunset for 40 degrees N. latitude is on December 7, 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.

Sunset in Pike County, Illinois on December 5, 2016. Photo via Russ Adams. Thanks Russ!

Sunset in Pike County, Illinois on December 5, 2016. Photo via Russ Adams. Thanks Russ!

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 2016 comes on December 21 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 7.

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.

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EarthSky Facebook friend Dutch McClintock in Livingston, Montana took this photo. Livingston’s latitude is about 45 degrees N., so – for Dutch and all those living at that latitude – the earliest sunset will happen closer to the December solstice.

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 7, 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 7.

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 7 7:09 a.m. 11:52 a.m. 4:35 p.m. 9 hours 26 minutes
December 21 7:19 a.m. 11:59 a.m. 4:39 p.m. 9 hours 20 minutes

Solstice tale of two cities: New York, NY, and St. Augustine, FL

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.

Click here for more details on why the earliest sunset doesn’t fall on the shortest day.

Hong Kong sunset , at the Hong Kong Science Park, from EarthSky Facebook friend Kins Cheung. Hong Kong is at 22 degrees N. latitude, so the earliest sunset there has already happened.

Sunset in Manila by EarthSky Facebook frieind Jv Noriega. Manila is at 14 degrees N. latitude, so the earliest sunset there happens even earlier than in Hong Kong

Bottom line: The 2016 solstice comes on December 21, but the earliest sunsets at mid-northern latitudes – say, 40 degrees N. happen on or near December 7. 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.

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Bruce McClure