It's due to 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. In its course around the sun, however, the earth moves further than average in December and January, making the earth seem to get "ahead" of the sun a little bit each day, in 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 distance we've traveled over December and January days.
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 (in the Southern Hemisphere) this motion is still making the sunrise 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 making everything happen later and later each day. In the first week of December (the 6th of December at Adelaide), the point is reached where that 1/3 of a minute per day starts to exceed the fading seasonal change and the sunrise seems to start happening later each morning.
The reverse happens in the evening (in the Southern Hemisphere). In the first half of December, both the seasonal change and the Equation of time are pulling in the same direction, making the sun set later each evening. 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 7th in Adelaide, when the northward movement of the sun finally wins out and the sunset starts getting earlier.
The reason the sunrise has gotton so much later by early March, while the sunset is not all that much earlier, is because the sunrise had a month's head start. But watch each evening to see by how much the sunset is now hurtling backward. It's by nearly two minutes a day now, while the sun rises later by just over one minute. By early June, the sunset will have made up for lost time, such that the earliest sunset in June is over a week before the latest sunrise!
By the way, the same thing happens in the Northern Hemisphere too, and for the same reason, but in reverse. At their summer solstice (June in the Southern) the earliest sunrise happens several days before the longest day of the year and the latest sunset several days later. The Northern winter solstice (December) saw the earliest sunset in early December and the latest sunrise not until the first week of January.
2007-03-03 02:08:08
·
answer #1
·
answered by Anne Marie 6
·
1⤊
0⤋
specific, it particularly is a properly straightforward visual attraction. The length between break of day and sundown particularly is shortest on Dec 21 and longest on June 21. however the cases of sunsets and sunrises themselves do not fall on presently! the reason of it particularly is rather straightforward because of the fact the "Equation of Time". There are 2 clarification why our sunlight isn't optimum interior the sky at precisely close by midday: - The elliptic flow of Earth around sunlight. - The flow of sunlight alongside the Ecliptic on the sky, yet making an attitude with the Celestial Equator. this implies the sunlight on an afternoon is minutes at the back of, on yet another day minutes forward. So this additionally potential short variations interior the moments of sunrises and sunsets.
2016-12-18 04:45:30
·
answer #2
·
answered by ? 4
·
0⤊
0⤋
Because the revolution track is an ellipse, has near solar the placealso to have far solar the place. The Earth's axis not vertical isslanting, has the included angle with the sunlight.
2007-03-03 02:08:57
·
answer #3
·
answered by daweiwang1001 1
·
0⤊
0⤋