Why is sunset in New York earliest on Dec 12 and not on Dec 21?

Question

Look at http://www.timeanddate.com/worldclock/astronomy.html?n=179&month=12&year=2007&obj=sun&afl=-1&day=1

It shows that sunset in New York is at 16:28 on Dec 12 but at 16:31 on Dec 21. I thought sunset is earliest on the winter solstice. The site does show that it is the shortest day, however.

Answer 1

Hi Mr. President!

Lots of people think that the earliest sunset, the latest sunrise and the shortest day are all on the same day, the winter solstice, which this year falls just after 1 a.m. on December 22 here in New York. In reality, though, each falls on a different day.

For most of the year, the apparent northward and southward motion of the sun governs the changing times of sunrise and sunset. When the sun is moving southward, as it is in November, days in the Northern Hemisphere grow shorter.

The path of the sun across the dome of the sky is known to geometry buffs as a "sine curve." In September, when the sun is moving southward at maximum speed, the days grow shorter by over 3 minutes per day. In December, however, the sun "levels off". The southward motion slows and comes to a stop. For about a week, the sun appears to stand still above the Tropic of Capricorn. The shortening of the day's length slows and stops on December 22.

The north-south motion is not the only one, however. Two others are at work.

One is based on a simple principle of geometry.

Look at the “sine curve” again. The northward/southward component is only a small part of the sun’s daily motion. Mostly, the sun is moving eastward day by day. In December, the sun is moving almost purely eastward. In September, however, a helping of southward is mixed in with the eastward motion.

Think about that. If the sun covers about the same distance in the sky each day, it’s going to make better eastward time in December, when it’s moving purely eastward, than in September when some of its motion is devoted to moving south too. But to our clocks, each day is exactly the same length, 24 hours, the sun’s average time. This means that, since the sun is moving eastward more in December than September, the earth must spin a little longer to catch up with the sun’s farther-than-average eastward motion. In December, it takes about 20 extra seconds of rotation for the earth to catch up, compared to the time it would take in September.

As the shortening of the days finally damps out in early December, the sunset is no longer getting much earlier each afternoon. But the sun is still both rising and setting about 20 seconds later each day than the day before because of the sun's greater-than-average eastward motion. The point is reached, in about the second week of December in north temperate latitudes, where that 20 seconds a day starts to exceed the fading shortening of the day. When that happens, the sunset slowly reverses and begins to come later, even though the solstice is still almost two weeks away.

In the morning, the opposite is happening. The sunrise is still moving later and later each morning, both because of the shortening day and also the extra 20 seconds needed to catch up with the sun’s maximum eastward motion. The shortening stops in time for the solstice, December 22nd, but because of the extra 20 seconds a day, the sunrise continues to move later, even after the solstice. In fact, it skids all the way into the first week of January until the lengthening of days after the solstice finally begins to drown out the extra 20 seconds a day. That’s why the latest sunrise is in the first week of January in north temperate latitudes.

A similar thing happens in June, because in June the sun is also covering more eastward territory each day than at other times. The effect in December is more pronounced than the one in June, however, due to the earth’s elliptical orbit.

In December and January, the earth is moving faster in its orbit than at any other time of the year. This makes the sun appear to move farther each day than at any other season, about 10 seconds further than average. Much the same as with the 20 seconds-per-day, this delays both the sunrise and sunset in December and January, making the earliest sunset here in New York December 8, and the latest sunrise January 4th, about 27 days apart.

In June, however, the earliest sunrise comes on June 15th, while latest sunset is June 27th, only 12 days apart. This is because in June and July, the earth is moving the slowest in its orbit. The difference between the 27 days in December-January and and the 12 days in June is caused by the contrast between the earth’s speed in December and June.

Answer 2

Whether it heralds a brand new establishing or an abrupt finish is open to hypothesis. Some do say that the following Pope is prophesied to be the final earlier than the Judgment. The Vatican holds very intently to the Prophecy of St. Malachy. This Medieval Monk had special visions of 112 Popes till Doomsday, sometimes called The Biblical Apocalypse. The present Pope, Benedict XVI is the 111th named Pope, just one Pope stays so the date 12-21-12 would be an overly big date to the Judgement day. I do not feel that Global warming will height by means of the yr 2012 so no, we can no longer fry in 2012.

Answer 3

It's due to the Earth's orbit being elliptical, and Earth's axial tilt. Another way of looking at it is that at some times of year a sundial runs fast, and at other times of year a sundial runs slow. In the tropics the effect is even more pronounced, with earliest sunset being as early as November.

At my latitude the earliest sunset is on the 15th, and the latest sunrise is on the 1st.

The article on AOL is a good summary. And now you know what that figure 8 thing on globes is for!

Answer 4

It is because Earth's orbit is elliptical, not a perfect circle. Earth is near perihelion in December and so is moving faster than average in its orbit, which makes sunset (and sunrise) occur later than average. Dec 12 has sunrise at 7:10am and sunset at 16:28pm for a 9 hour 18 minute long day. Dec 21 has sunrise at 7:16am and sunset at 16:31pm for a 9 hour 15 minute long day, 3 minutes shorter than Dec 12.

Answer 5

don't confuse "earliest sunset" with "shortest day." The length of the day depends on the times of both sunrise and sunset.

Answer 6

New York has a curfew.