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From: jay@npois.UUCP (Anton Winteroak)
Newsgroups: net.astro
Subject: precession of perihelion
Message-ID: <183@npois.UUCP>
Date: Mon, 11-Feb-85 15:44:08 EST
Article-I.D.: npois.183
Posted: Mon Feb 11 15:44:08 1985
Date-Received: Tue, 12-Feb-85 06:39:04 EST
Organization: ATTIS, Neptune, NJ
Lines: 29


In the early twelfth century, al-Zarqali (the blue eyed one) measured
the rate of precession of "solar perigee" as 1 degree of arc per 299
years.
He had some tools to make astronomical measurements, including quadrant,
astrolabe, water clocks of his own devise, recent translations of the
works of the greeks, and large empty churches in Spain, where pinhole
images might be measured.
One of his documents from the greeks, gave a position in the sky for
the solar perigee (in those days), and an approxomate rate of change.

Since the suns apparent size only changes 2 percent in its annual cycle,
and since perihelion is not a time when it changes quickly, I am finding
it hard to think of ways that someone could have made this measurement.

Possibilites include studiously watching the sun go behind a tower of
such a size that on only one day of the year could you see the edge of
the sun on both sides of the tower at the same time, and figuring out
the suns position on this basis. Another possibility is measuring
the time between mean time and solar time for noon, and figuring that
when difference is greatest, that is perihelion. I don't know if his
clocks were able to measure small increments of time.

If anyone actually knows how he did it, or knows of a clever way to determine
the position of solar perigee that is accurate, using medieval tools,
please post it on the net


			Thanks in advance, for any help