Andrew James wrote:
>According to Britannica's article on Hipparchus, " ... observed the
>positions of the stars and compared his results with those of Timocharis of
>Alexandria about 150 years earlier and with even earlier observations made
>in Babylonia.... He proposed precession to account for the size of the
>difference and he gave a value of 45 or 46 (seconds of arc) for the annual
>change, very close to the figure of 50.26 accepted today. ... Knowledge of
>precession enabled him to obtain a better value for the length of the year
>... his value for the tropical year was too great by only 6 1/2 [minutes?]".
>
>So, indeed, he had access to a long series of observations, but it doesn't
>answer the question of exactly how discovering precession led him to this
>value. Can anyone explain?
>
>To note just two of his other achievements, he produced the first star
>catalogue and was the first to specify position on the Earth by longitude
>and latitude; surely, he was a very smart fellow indeed.
>
The most accurate observations of the sun relative to the stars , which is
necessary for the determination of the year, is via the Heliacal rising of
a star. This could be pinned down to a single day. The anchients noticed
that this is changing relative to the Vernal equinox which is determined by
observations of the altitude of the sun and by the path a shadow takes. It
being a straight line on the equinoxes. This leads to two diffent "years".
The tropical year based on the seasons, vernal euqinox to vernal equinox
and the sidereal year which is based on revolution relative to the stars,
determined by heliacal rising. Due to precession, the tropical year is
20mins shorter than the sidereal year. 20minutes is a 26298th part of a
year. Thus the anchient Greeks, Indians and Chinese realized that there was
a cycle of about 26298 years. This is the time taken by the Earth's axis to
precess through 360 degrees.
This has to be sorted out first so you know what year you are measuring.
Following Julius Caesar we now use the tropical year. Prior to Caesar
people, such as the Egyptians, often used the sidereal year as they used
the Heliacal rising of a star, Sirius in the case of the Egyptians.
Cheers,
John
Professor John P.G.Shepherd
Physics Department
University of Wisconsin-River Falls
410 S. 3rd. St.
River Falls,WI 54022
Phone (715)-425-4689, eve. (715)-425-6203
Fax (715)-425-0652
44.88 degrees N, 92.71 degrees W.
