why is dawn still so late?

Question

Why is it that three weeks after the Solstice dawn is still later than it was then? Sure, the daylight is longer because sunset is later in the afternoon, but the latest dawn doesn't arrive until about New Year's Day and today's is only 3 minutes earlier than it was on Jan 1. Why don't dawn and sunset widen after the shortest day at an equal pace?

Answer 1

The cause of these is a phenomenon called the Equation of time.

The earth revolves around the sun at changing speeds, caused by the fact that it has an elliptical orbit. Kepler described exactly how in his Second Law of planetary motion. This motion is fastest in December and January, when the earth is closest to the sun.

The speed of rotation (the 24-hour day) cannot change but since, in its course around the sun, the earth moves further than average in December and January, this has the effect of making sunrise and sunset both happen about 1/3 of a minute later each day, cumulatively. Since the earth has moved farther than usual, it takes the sunrise and sunset about 1/3 of a minute more than normal to "catch up" with the extra amount we've traveled.

By early December, the north-south movement of the sun, caused by the seasons, is gradually starting to bottom out in a pattern known in trigonometry as a "sine curve." In November, this motion is still making the sunset happen earlier and earlier each day. By early December, however, it flattens out to the point where the seasonal change in the day's length becomes less than one-third of a minute per day.

Meanwhile, the Equation of time is still making everything happen later and later each day. In early December, the point is reached where that 1/3 of a minute per day starts to exceed the fading seasonal change and the sunset seems to start happening later each day.

The reverse happens in the morning. In the first half of December, both the seasonal change and the Equation of time are pulling in the same direction, making the sun rise later each morning. The seasonal movement fades completely by the solstice and even starts to reverse, but the 1/3 of a minute later each day continues to make its presence felt until about January 5th here in New York, when the northward movement of the sun finally wins out and the sunrise starts getting earlier.



P.S. It seems true what they say, that imitation is the sincerest form of flattery!

Answer 2

The cause of these is a phenomenon called the Equation of time.

The earth revolves around the sun at changing speeds, caused by the fact that it has an elliptical orbit. Kepler described exactly how in his Second Law of planetary motion. This motion is fastest in December and January, when the earth is closest to the sun.

The speed of rotation (the 24-hour day) cannot change but since, in its course around the sun, the earth moves further than average in December and January, this has the effect of making sunrise and sunset both happen about 1/3 of a minute later each day, cumulatively. Since the earth has moved farther than usual, it takes the sunrise and sunset about 1/3 of a minute more than normal to "catch up" with the extra amount we've traveled.

By early December, the north-south movement of the sun, caused by the seasons, is gradually starting to bottom out in a pattern known in trigonometry as a "sine curve." In November, this motion is still making the sunset happen earlier and earlier each day. By early December, however, it flattens out to the point where the seasonal change in the day's length becomes less than one-third of a minute per day.

Meanwhile, the Equation of time is still making everything happen later and later each day. In early December, the point is reached where that 1/3 of a minute per day starts to exceed the fading seasonal change and the sunset seems to start happening later each day.

The reverse happens in the morning. In the first half of December, both the seasonal change and the Equation of time are pulling in the same direction, making the sun rise later each morning. The seasonal movement fades completely by the solstice and even starts to reverse, but the 1/3 of a minute later each day continues to make its presence felt until about January 5th here in New York, when the northward movement of the sun finally wins out and the sunrise starts getting earlier

Answer 3

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