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From: karn@allegra.UUCP (Phil Karn)
Newsgroups: net.astro
Subject: Re: Solstice =/= Earliest Sunset?
Message-ID: <2139@allegra.UUCP>
Date: Sat, 24-Dec-83 03:54:30 EST
Article-I.D.: allegra.2139
Posted: Sat Dec 24 03:54:30 1983
Date-Received: Mon, 26-Dec-83 01:13:28 EST
References: <4696@decwrl.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 24

The reason is due to the earth's non-circular orbit. Perihelion (closest
approach to the sun) occurs during early January, and aphelion (furthest
distance from the sun) occurs during early July. Because of this, the
earth does not move at a constant rate around the sun (Kepler's laws and
all that.)  The "mean" sun corresponds to the position the sun would
occupy at a given time of year if the earth's orbit were perfectly
circular; the "real" sun will be either ahead or behind, depending on
the time of year.

Between July and January, when it is falling toward perihelion, the earth
is behind and "catching up" with it's mean position (which it will overtake
in January); during this time, the sun appears to rise and set earlier
than it would if the earth's orbit were circular.

Conversely, between January and July, when the earth is climbing toward
aphelion, the earth is "falling back" towards its mean position (which
will overtake it in July) and the sun appears to rise and set later than
it would if the earth's orbit were circular.

Combine this effect with the symmetrical shortening of the day
(later sunrise, earlier sunset) you get at the winter solstice, and you
can see why the earliest sunset of the year occurs before the solstice.

Phil Karn