Answer to the Question 05/98

UNEQUAL HALVES

The question was:

Why is the time from autumn to spring equinox in the Northern hemisphere three days shorter than the time from the spring to autumn equinox?


The problem has been solved qualitatively by K. MacArthur (e-mail macarthu@firsttrust.com) and by O. Melinger. A slightly more quantitative solution is presented below.


Solution:

The orbit of the Earth around the Sun is almost, but not quite, circular. In January the distance to the Sun is 147 million km, while in July it is 152 million km. According to Keplers laws, this causes more than 3 percent difference in maximal and minimal speeds of the Earth, and more than one percent difference in the mean speeds in the two half-year periods. At the same time the distance traveled by the earth in the "winter half-year" is slightly shorter. Those two effects together cause the "winter half-year" to be about two percent shorter than the "summer half-year".

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