From: joe@montebello.NoSubdomain.NoDomain (Joe Dellinger)
Subject: Re: lunar eclipse frequency?
Reply-To: joe@montebello.soest.hawaii.edu
Organization: School of Ocean and Earth Science Technology
Date: Sat, 12 Dec 1992 11:22:55 GMT

In article <ByyLvq.286@mtholyoke.edu>, kbourque@MtHolyoke.edu (Karen M. Bourque) writes:
|> How many total lunar eclipses are there per year?

Well, in 1982 there were three! But that was *very* unusual. (Won't happen
again for centuries.) In many years there are none at all.

During the time span 1980-1999 there are 16 total lunar eclipses.

|> Does it matter where you live, or does *everyone* see the eclipse?

Each total eclipse is visible from a little more than half the Earth.
(The half that experiences night sometime while the eclipse is happening:
since the moon and sun are directly opposite each other in the sky, else
the eclipse wouldn't be happening, if the sun is down the moon must be up.)

So from a given location you can expect to have a chance to see a total
lunar eclipse about once every two and a half years. (If you are lucky
it won't be cloudy that night.) Of course what really happens is you
may have 2 or 3 over a span of a year and a half or so, and then you
may go several years without having a chance to see even one. (As I recall
this total eclipse was the first visible for Western Europe in something
like 6 or 7 years, hence the anguish at clouds.)

[For purists only: The complete answer to "how often do eclipses happen"
depends on what time period you're talking about. There are several cycles
of different length that come into play, and they fortuitously make a
near-repeat after 18 years, 15 1/3 days. This is called the "saros interval".

So EXACTLY how many eclipses a year you are getting (averaged over decades)
depends on how those cycles happen to be falling relative to each other.
In particular, how many of the concurrently-running "saros cycles" are
currently producing total lunar eclipses.]


Total lunar eclipses vary quite a lot: the moon may penetrate deeply
into the Earth's shadow, or may just skim along the edge. The light
in Earth's shadow varies in brightness and color, depending on the
amount of dust and junk in the parts of the Earth's atmosphere where
the sun is near the horizon.

For most lunar eclipses the moon turns a bright red. It never gets
so dark that it is at all hard to find in the sky. However, THIS eclipse
happened with the Earth's atmosphere full of volcanic junk from mount
Pinatubo, so it was a dark one. (The previous really dark eclipse was in
December, 1982, when the Earth's atmosphere was full of junk from an
eruption of El Chichon in Mexico. You can only expect a few really dark
eclipses per century, so count yourself lucky if you saw this one.)

PS:
Intense Saros cycle trivia: after one Saros interval you get a near-repeat
of an eclipse, shifted 1/3 of the way West around the Earth. But there
is also a HALF Saros cycle that turns lunar eclipses into solar eclipses,
and vice versa, and only shifts 1/6 of the way around the Earth (on average).
So lunar eclipses are a great way to predict a solar eclipse 9 years and
7 days later (or earlier) visible from somewhere near your longitude.
In fact you can do better then that: you can predict the _latitude_ of
centrality of the solar eclipse by noting the track of the Earth's
shadow across the moon. If the moon goes through the Northern part of
Earth's shadow, the associated solar eclipse will be in the Northern
part of the Earth (as seen from the sun at that season), and vice versa.
(The deeper the lunar eclipse, the more central the solar eclipse.)

Let's try it out:

	I remember seeing a slight partial lunar eclipse in December, 1991.
	The moon was due overhead for Hawaii, with just a nick out of the
	Southern edge. Thus I can "predict" that in December 1982 there must
	have been a solar eclipse, probably partial (since the lunar eclipse
	was so slight), that must have been best visible in the far North, at
	a latitude about 60 degrees East of Hawaii's... North Central Canada?
	(Can anyone look this up to verify? I don't know for sure, I'm
	just extrapolating backwards from my observation using what I know
	of eclipse cycles!)

	I remember seeing a deep partial lunar eclipse in June, 1992, already
	past the midpoint at moonrise for Hawaii. This time the Northern part
	of the moon was darkened. Since it was already mostly over at moonrise
	for Hawaii, it must have been centered at a longitude around, say, 40
	degrees West or so? (???)
	So... from that one observation we can "predict" an eclipse in June
	1983, probably central (total or annular), occuring at noon for a
	longitude about around 20 degrees East. (???) It would have been
	somewhat South of the Earth's center as seen from the Sun, but since
	the Earth at that month has the North pole tilted 23 degrees towards
	the sun it was probably in the equatorial regions.

	More importantly from my one observation I can also predict a central
	solar eclipse occuring at noon in the equatorial regions at around
	longitude 100 degrees West in June 2001...

	Well, how did I do? I don't have the "answers" here to check myself
	with. I expect a fair amount of slop in longitude, of course, since
	the half Saros isn't an integral number of lunar anomalistic months
	(perigee to perigee), so the eclipse may happen a little early or
	late of the simple-minded half-saros prediction as a result, causing
	my longitude predictions to be in error.

	But it's sort of interesting, isn't it? Could the ancients of
	predicted eclipses this way?

     /\    /\    /\/\/\/\/\/\/\.-.-.-.-.......___________
    /  \  /  \  /Hawaii Institute of Geophysics, Honolulu\/\/\.-.-....__
___/    \/    \/Joe Dellinger, Internet: joe@montebello.soest.hawaii.edu\/\.-.__
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