The Earliest Sunset is Now…

We are so disconnected from the reality of the heavens that most people don’t know the moon’s phase (many don’t know you can see the moon in daylight). So the really subtle stuff gets overlooked.  But folks who rise with the sun probably notice that Northern Hemisphere (NH) sunrise keeps getting later after the solstice on the 21st of December. And, conversely, the earliest NH sunset is about now, in early December (a point in time GG really notices–it is nice to have it start to be light later as we approach New Years).

How can that be?

If you know a little astronomy, it is straightforward, and the cool thing is that it is a direct demonstration of the ellipticity of Earth’s orbit.  In northern hemisphere winter, we are near perihelion (closest point to the Sun). And so by Kepler’s Laws, it means we are traveling faster around the Sun than other times of the year. But Earth rotates on its axis at the same rate all year long, which is why latest sunrise and earliest sunset don’t occur on the solstice.

That might not be obvious, so let’s do a couple simple drawings and see how this goes:

Let’s start with an “average” day.  Earth spins around once every 23 hours and 56 minutes (more or less) relative to distant stars.  So every night the stars are in the same position 4 minutes earlier by our clocks than the night before. So why does our day last 24 hours and not 23 hour 56 minutes? Well, we’ve moved a little farther in our orbit around the Sun, so if we were looking down from far above the South Pole we would see something like this:

Earth viewed from above on two days with the Earth in the same position relative to the stars

Earth viewed from above on two days with the Earth in the same position relative to the stars

OK, on the left we have the Earth as viewed from above (below?), white were sunlit and gray where it is night, and the point in black is a point experiencing sunrise.  We go forward 23 hours and 56 minutes and the black point has rotated around once and is in the exact same spot relative to the stars–for our view, it has returned to the righthand side of the Earth.  But because Earth has moved in its orbit about the Sun just under 1 degree, that point is now in darkness.  It would take another 4 minutes of rotation to bring that point into sunrise.  23 h 56 m + 4 m = 24 hours.

OK, no biggy.  But now this time of year we are nearer the Sun and so moving in our orbit faster.  So now things look like this:

Earth viewed from above at two points 23h and 56m apart but when the Earth is nearer the Sun.

Earth viewed from above at two points 23h and 56m apart but when the Earth is nearer the Sun.

Because we are moving faster than before, Earth is now at that farther righthand point after 23h 56 m and the point is even farther from sunrise: we have to wait even more than our usual 4 minutes to get sunrise.  So on our 24 hour clock, sunrise comes a bit later–and so does sunset. So while the shortest day (in the northern hemisphere) comes on the day when the North Pole is pointing away from the Sun, the change in the length of day gets to be small enough this time of year that sunrise and sunset are more affected by this more rapid motion of Earth in its orbit.  So for about the next month, most of the change in sunrise and sunset has to do with that motion and so both become somewhat later everyday.  So the earliest sunset is about now, when half of the change in the length of the day about equals the extra time needed to rotate to compensate for that extra distance travelled in the orbit.  And the latest sunrise in early January similarly is about when about half the daily increase in the length of the day equals that extra distance.  Thus there is about a month between the earliest sunset and the latest sunrise.

As a last piece of trivia, if you have a globe with what looks like a figure 8 drawn in the eastern Pacific, you can now understand that thing–it is showing the position on the globe directly under the Sun at noon for a specific longitude.  It moves north and south because of the tilt of Earth’s axis, but it moves east and west because of the ellipticity of Earth’s orbit.

Nothing earth shattering, just one of those pieces of trivia that GG has always found amusing.  And, maybe, you can win a bar bet by betting on sunset being later on the shortest day of the year than it is right now. (And it kind of works the same in the southern hemisphere, though you have to word the bet differently: sunrise is later on longest day of the year than right now).

[Note: Relabeled figure as originally described, it had Earth rotating backwards.  Sorry]

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One response to “The Earliest Sunset is Now…”

  1. cjonescu says :

    Reblogged this on The Grumpy Geophysicist and commented:

    Once again that time of the year…for the earliest Northern Hemisphere sunset (and earliest Southern Hemisphere sunrise)–both of which occur not on the solstice but now, in early December. Here was a discussion from last year…

    Like

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