bobjkelly t1_j7bkvwa wrote
The cycle is pretty consistent year to year but there are some long term cycles. These cycles take thousands of years, however, so probably not much difference from 1718 to here. One is that the tile of the earth relative to its orbit varies between 22.1% and 24.5% in a cycle of 41,000 years. It currently is about in the middle at 23.4% but is slowly decreasing. This would impact amount of daylight. Also, there is a cycle where the earths axis wobbles. And a cycle where the earth's orbit changes from almost round (where it is now) to slightly more elliptical. And a cycle where the closest approach to the sun (currently Jan 3) and farthest distance from the sun (about July 4) slowly change dates. All of these (and probably others) will impact daylight durations but probably not very much in a few hundred years.
gstormcrow80 t1_j7bp6fn wrote
The ‘wobble’ you mentioned is the answer I came here looking for.
The earth rotates like a spinning top, and the axial precession is on a 26,000 year cycle. The most noticeable effect is a change in the position of stars year to year, and it will eventually cause Polaris to be replaced as the ‘north star’ in another 3,200 years. Changes in day length are negligible, but present.
Humans have been aware of it and attempts to precisely measure and define it go back more than 2,000 years. The ancient Greek and Indian (ACTUAL Indian, not Native American) cultures both recorded attempts to define it.
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