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Re: Equinoxes



At 20:31 10/18/97 -0700, David Abineri wrote:
When was precession of the equinoxes first observed and how was the
observation made? When was it verified?

to which I replied:

The discovery is attributed by Ptolemy to Hipparchus, ca. 135 BC.
Hipparchus must have had access to the measurements of an earlier
observer.

At 7:45 AM -0700 10/19/97, brian whatcott wrote:

I have no copy of Hipparchus, but can finally provide this direct
quotation from "On The Precession Of The Tropic And Equinoctal Points"
by that author:
"If then, for example, Spica was formerly 8 degrees in longitude
west of the autumnal equinox and is now 6 degrees west..."

Spica is now 24 degrees east of the autumnal equinox. (This coordinate
is most likely the celestial longitude rather than the right ascension
by the way. The former is measured along the ecliptic and the latter
along the equator.)

This would correspond to Hipparchus writing ca. 150 BC and referring
to an observation recorded ca. 295 BC, 145 years earlier! (Please
forgive the excess precision implied here. It's like writing out all
the digits from one's calculator display, I know, but the excess
precision comes free, and may well be worth exactly what it costs.)

One of the things which has attracted me to astronomy is its enduring
value. Imagine, an observation made by an anonymous astronomer 2300
years ago was used by another astronomer 150 years later and reported
to us by yet a third astronomer, Claudius Ptolemaeus of Alexandria
(Ptolemy), yet another 300 years after that! The work of that
anonymous early astronomer has scientific value still because there
is no way in which his observation can be improved upon by modern
techniques.

Leigh