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I viewed sunset times in some major Eastern U.S. cities including Boston, New York, Philadelphia, Boston / D.C. and Miami. It seems that the earliest sunset time starts around the first week of December, stays the same for the next week, and starts to get later again by December 18.

The winter solstice is around Dec. 22, which is considered the shortest day (i.e. length of sunlight) of the year. So, it seems that the earliest sunset time is a few weeks ahead of the shortest day. Why is that?

Chicago sunset/sunrise from http://www.sciquill.com/analemma/ http://www.sciquill.com/analemma/

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  • $\begingroup$ I really like the explanation from sciquill.com/analemma/page2.html. It is quite intuitive. Also, the page analemma.com provides a great explanation and a very cool set of animations of how this effect occurs in other planets of the Solar System. $\endgroup$ – arkaia Dec 15 '15 at 1:57
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If you follow the sunset timing for the equator you will notice that although the day remains pretty much the same all year round, the sunset moves up and down twice a year. The biggest difference is about a half hour.

This is due to the changing speed of the orbit of Earth. Earth's orbits is slightly oblong. Therefore on the longer parts the planet picks up speed. Which means that although Earth made a full rotation, since it is further in its orbit it is still seeing the sun.

This happens twice, on both longer stretches of the orbit. However the effect on one is more pronounced than the other. This is because in one leg Earth is speeding away from the sun, which slows it a bit, and in the other it is coming toward the sun.

The longest and shortest days are on both points of the oblong circle but the later sunset is before that point.

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So, it seems that the earliest sunset time is a few weeks ahead of the shortest day. Why is that?

It's because we measure time by a clock rather than by a sundial. That the latest sunrise and earliest sunset don't occur on the winter solstice pretty much (but not completely) vanishes when one looks at time as measured by a sundial.

From early December to mid January, a solar day (the time from one local noon to the next) is 24 or more seconds longer than is a day as measured by a clock, with the discrepancy peaking at about 30 seconds on December 25 or so. While the timing of sunrise and sunset is more or less symmetrical about local solar noon, this discrepancy between the length of a day as measured by a sundial versus the 24 hour day as measured a clock means that in the northern hemisphere, the earliest sunset (as measured by a clock) occurs sometime before the winter solstice and that the latest sunrise occurs sometime after the winter solstice. (How much depends on latitude.)

This discrepancy between solar time and clock time is a consequence of the Earth's axial tilt and the eccentricity of the Earth's orbit. How these two two effects combine is called the equation of time. The axial tilt results in a sine wave with a period of six months and zero crossings at the solstices and equinoxes. The eccentricity results in a sine wave with a period of a year (plus a tiny bit) with zero crossings at perihelion (when the Earth is closest to the Sun) and aphelion (furthest from the Sun).

Right now, perihelion occurs a couple of weeks after the northern hemisphere winter solstice. (This is just coincidental. In 5000 years or so, perihelion will occur around the time of the spring equinox, and in 10000 years, around the time of the summer solstice.) The difference between length of day as measured by a sundial vs a clock currently reaches its peak in late December because of that coincidence.

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The length of the day is the distance between sunrise and sunset. Sunrise and sunset around the longest and shortest day of the year do not go in the same "direction" as the earth makes the "turn" around the closest and farthest points of the oval orbit. That is, the sunrise and sunset times are a function of both the rotation of the earth as well as the movement of the Earth in its orbit around the sun (and the angle of the axis to the plane of the orbit).

Thus, there are times when sunset starts getting later (which would make the day longer) but sunrise is still getting later. For example, in Baltimore Maryland sunrise/sunset on December 5 was 7:11/4:43, December 12 was 7:17/4:44, December 19 is 7:22/4:46, and December 26 will be 7:25/4:49

The earliest sunset in Baltimore was at 4:43 from December 4 through December 12, while sunrise while sunrise continues getting later until January 10 (7:27 A.M.)

This means that the length of the day continues getting shorter until sunrise gets later less than sunset is getting later and then (sunrise) starts getting earlier.

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