daemon@decwrl.UUCP (12/22/83)
From: In.from.the.ENET, sent by Ed Featherston <roll::featherston>
Begin Forwarded Message:
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Newsgroup : net.astro
>From : DVINCI::FISHER
Organization : Digital Equipment Corp.
Subject: Solstice =/= Earliest Sunset?
One of our local (Boston) boob tube weather "personalities" made the claim that
although the winter solstice, and thus the shortest day of the year, occurs
on Dec. 21st, the date of the earliest sunset was actually last week. Several
of us have been racking our brains trying to figure out a plausible explanation
for this alleged phenomenon, but have so far been unsuccessful. Can anyone
out there in net.astronomy-land deny or confirm and explain this claim?
Thanks,
Burns Fisher
UUCP: ... decvax!decwrl!rhea!dvinci!fisher
or ...allegra!decwrl!rhea!dvinci!fisher
or ... ucbvax!decwrl!rhea!dvinci!fisher
ARPA: decwrl!rhea!dvinci!fisher@Berkeley
or decwrl!rhea!dvinci!fisher@SU-Shasta
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End Forwarded Messagekarn@allegra.UUCP (Phil Karn) (12/24/83)
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
ajs@hpfcla.UUCP (12/30/83)
#R:decwrl:-469600:hpfcla:37600001:000:830 hpfcla!ajs Dec 25 17:10:00 1983 Length of day is dependent on the angle of the Earth's axis away from the Sun. The minimum occurs when the Earth passes through the point in its orbit where the North Pole is furthest "out". However, the small ellipticity of the Earth's orbit causes it to be a little "ahead" or "behind" of where a circular orbit would put it at certain times. This is expressed in the "equation of time", which gives the deviation (up to 14 minutes, if I recall correctly). So, it's possible that the sunrise time could get a little earlier for a time near the Solstice, even as the days got shorter. So much for my layman's explanation; let's see how the experts correct me. Alan Silverstein, Hewlett-Packard Fort Collins Systems Division, Colorado ucbvax!hplabs!hpfcla!ajs, 303-226-3800 x3053, N 40 31'31" W 105 00'43"
ntt@dciem.UUCP (Mark Brader) (01/04/84)
It appears that the explanations given by hpfcla!ajs (Alan Silverstein) and allegra!karn (Phil Karn) are both correct; I checked a couple of books and saw explanations like this: ...the days are not of equal length. The earth's orbit is elliptical, not circular, and the earth moves more rapidly when near the sun and more slowly when farther away. Also, the earth's axis of rotation is tipped relative to the orbit. These two phenomena cooperate to make the `apparent' sun first run ahead of and then lag behind its average position. To avoid the inconvenience of changing the rate of our clocks from day to day, we employ `mean' instead of `apparent solar time' for practical use. --Donald H. Menzel in `A Field Guide to the Stars & Planets' Incidentally, the fact that the orbit is elliptical has another effect besides the variation of orbital speed; it means that the sun is not at the center of the orbit, but is about 1.5 million miles from it, i.e., at one focus. I don't really understand how these effects combine to give the actual result, though, which is more complicated than Phil Karn said. In fact, when I think about it, I don't understand why the axial tilt matters at all, except perhaps to modify the *amount* of the effects of the elliptical orbit. Those effects alone, I should think, will give longest and shortest apparent days (i.e. noon to noon, nothing to do with longest and shortest periods of daylight, which depend only on the axial tilt) at the aphelion and perihelion, which is to say, the equation of time should be changing most rapidly then. The actual equation of time, from Menzel's book, however, exhibits FOUR, not two, lobes in the year, and all of them are unequal. A table is given, which I will summarize now. Again, `equation of time' means apparent time minus mean time, and is the correction to *subtract* from the time on a sundial to give the actual time (after allowing for longitude and daylight time!). I start at Feb. 14 when the ET is at its minimum. All the dates are estimated because the table is every 5 days. However, it is clear that neither the solstice/equinox dates of about the 21st of Mar./June/Sep./Dec. (based on axial tilt) nor the aphelion/perihelion dates of about the 3rd of Jan./July (based on elliptical orbit) appear directly in the table. In the first column I give days from Feb. 14. 0(365) Feb 14 ET = -14.3 min. major minimum 61 Apr 16 ET = 0 (and increasing 14 seconds/day) 90 May 15 ET = +3.7 min. lesser maximum 121 Jun 15 ET = 0 (and decreasing 13 seconds/day) 165 Jul 29 ET = -6.4 min. lesser minimum 200 Sep 2 ET = 0 (and increasing 19 seconds/day) 265 Nov 6 ET = +16.3 min. major maximum 315 Dec 26 ET = 0 (and decreasing 30 seconds/day) Mark Brader