David Madore's WebLog: Welcome, Winter

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(Monday) · Winter Solstice (2003-12-22T07:03:48.55Z)

Welcome, Winter

For those of us who live in the northern hemisphere, now is the darkest moment of the year: today is the shortest day, with only 29495 seconds (8h11m35s) from (astronomical) sunrise to sunset at the Paris observatory. Now we are in winter.

Southern hemisphere dwellers would do ill to laugh now: for the northern hemisphere has a definite advantage for those who like summer better, namely that it is longer. Specifically, in the northen hemisphere, the mean duration of seasons for the J2000 epoch is:

92.758 days
93.649 days
89.842 days
88.993 days

(And, of course, summer in the southern hemisphere lasts as long as winter in the northern, and so on.)

This inequality is due to the ellipticity of the Earth's orbit: perihelion (the point where the Earth is closest to the Sun) is reached in the early days of january (next passage of the Earth at perihelion is around 2004-01-04T01:05Z) and this is the point where (by Kepler's law of areas) its angular velocity around the Sun is greatest. Since it falls near the winter solstice, this makes autumn and winter substantially shorter than spring and summer in the northern hemisphere (and the other way around in the southern).

This is changing, of course: the Earth's perihelion advances on the Earth's orbit, with respect to distant fixed stars, at a rate of one rotation every 1116 (Julian) centuries, whereas the seasons regress on the orbit (the so-called precession of the equinoxes) over a period of 258 centuries. So, with a period of 209 centuries, the length of the seasons oscillates: some 10000 years in the past or in the future, winter was or will be the longest season in the northern hemisphere.

Merry winter everybody!

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