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I just read an article in the 'Science' journal online about Monarch butterfly migration. (Mysterious monarch migrations may be triggered by the angle of the Sun , by Elizabeth Pennisi , Dec. 18, 2019) It says that the butterflies closely follow the Sun as it moves southward during the late summer and early fall, trying to keep the Sun's height above the horizon to between 47 and 58 degrees (Sun angle at solar noon, or SASN).

Then comes a shocker (to me, at least...) It says that the butterflies '...speed up from about 17 kilometers per day to about 47 immigration. Then, farther south, they slowed back down to 17 kms/day.'

Huh? They have to more than double their speed to keep pace with the sun?

I know that some days during the year are slightly longer or shorter than others, and the Earth moves slightly faster around the Sun during (N. Hemisphere) winter, but..... Huh?

It links to another article (the original paper) that says, essentially, the same thing. (frontiers in.org/articles/10.3389/fevo.2019.00442)

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Due to Earth's axial tilt, the height of the Sun in the sky changes with the seasons, and thus the length of daylight to night also changes. But the 24 hour day does not significantly change. The 24 hour day is based on the spin of the Earth, which, while very minutely slowing due to tidal drag, does not change seasonally.

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  • $\begingroup$ However, due to the differen speed with which the Earth orbits the Sun, the solar day (time it takes the sun to complete one cycle relative to an observer on Earth) varies ever so slightly throughout the year. In Dec./Jan., when we're closest to the Sun, the solar day is about 1--2 minutes longer. Although the seasonal variations are by far the dominant factor in determining the amount of daylight. $\endgroup$ Dec 23 '19 at 15:38

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