row2k Features
Sunrise, Sunset - Why the Latest Sunrise and Earliest Sunset Are Not on The Solstice
December 3, 2018
Ed Hewitt,

Boston locals may notice that this photo was not taken on the solstice, but it seemed like good photo to go with the article

With the December solstice just under three weeks away, rowers are enjoying or enduring the longest and shortest days of the year, depending on which hemisphere they live in. Rowers are certainly extremely tuned in to sunrise and sunset times, but many still assume that the earliest sunset and latest sunrise occur on the solstice, which will occur on December 21 at 5:23pm Eastern time. But the earliest sunset and latest sunrise actually occur several days on either side of the solstice, a phenomenon caused by differences in the length and timing of the solar day (which shifts around quite a bit) versus the "clock day" (which does not).

The date of the latest sunrise and earliest sunset also vary considerably by latitude; in the United States alone, the earliest sunset varies by almost two weeks, occurring on November 29 in Key West FL (with the latest sunrise on Jan 13), and December 11 in Bellingham WA (with the latest sunrise on Dec 31). EarthSky gives a nice lay explanation here.

Physicist Steve Rabinowitz, a former Columbia coxswain who gave up crew for physics but still gets to apply his experience to analyzing naval systems (including one opportunity to steer a nuclear submarine), explains further:

"At this time of year many people whose activities depend on sunrise and sunset times take note of the shortening days culminating in the Winter Solstice, with its shortest period of daylight, which occurs on December 21. It is only natural to assume that this day would feature the latest sunrise and earliest sunset, but actually in New York the earliest sunset is on December 8, while the latest sunrise is January 4. The explanation requires understanding a bit about the Earth's motion and the way we tell time.

"Our annual trip through the solar system can be quite dizzying, rather like sitting on a swivel chair on a moving carousel (oh, and the chair is tilted just to make things interesting). If you imagine looking down on the solar system from above, the Earth rotates counterclockwise on its axis and also revolves in a counterclockwise orbit around the Sun. It's pretty easy to see that the rotation itself makes the Sun appear to move from East to West every day, but the revolution part is a bit trickier.

"Think of the Earth as not rotating but still orbiting around the Sun. We'll start with a specific location, say New York City, facing towards the Sun and put the Earth at 12 o'clock on our imaginary clock face. Clearly when the Earth is at the bottom, at 6 o'clock, NYC is now facing directly away from the Sun. In an intermediate position, when the Earth is at 9 o'clock, NYC is at the bottom of the globe. If you think for a moment about the relative position of the two bodies you can see that the Sun seems to be moving from West to East. Since the Earth's yearly revolution is a slower effect than its daily rotation, this eastward contribution is small but has an important effect, causing the Sun to appear at its highest point in the sky a few minutes later each day.

"But wait, there's more! The Earth's axis of rotation is also tilted 23.5 degrees. The Sun appears to trace a path across the sky from East to West, but that path shifts throughout the year. On the dates of the spring and fall equinox, the Sun rises due East and sets due West, but on the upcoming winter solstice it will be following its southernmost arc (just as at the summer solstice it will be at its northernmost point).

"These two motions conspire to produce the different sunrise and sunset extremes. Prior to the winter solstice, the Sun's eastward motion makes sunrise later but the southward motion makes sunset earlier. Around December 8th we reach the earliest sunset but sunrise continues to get later, aided by the southward motion. At the solstice the Sun begins to reverse its North-South motion and start heading northward again. After that the sunsets start getting later but sunrises also continue to get later. Around January 4th, the eastward motion finally causes the latest sunrise, after which the northward component balances it out and drives sunrises earlier again."

So for the afternoon rowers among us, our darkest day is just a few days away – for the morning rowers, it will be another month before the sun starts to swing in your favor.


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