the-astrology-book

Fixed Stars
equinoxes or the solstices. Ptolemy was primarily an astronomer, and therefore this
problem was one of his central preoccupations. The problem of the day was that two
sets of data were required to answer the question on precession. The first was the accurate
position of stars for one period in time, and the second was the same list of stars
but measured for a later period. By comparing the two lists, and knowing the time
period between the two, the rate of precession could be found.
The most common method of locating a star in the sky, before Ptolemy, was to
use regularly repeating lunar cycles: noting the date and time, and the Moon ’s degree
of longitude and latitude, then marking its orientation to a star. This was a cumbersome
method, as the following excerpt from Ptolemy’s Almagest shows:
Again, Timnicharis says he observed in Alexandria that in the year 36
of the First Callippic Period exactly at the beginning of the tenth hour,
the moon appeared to overtake with its northern arc the northern star
of those in the Scorpion’s forehead. And this date is the year 454 of
Navonassar, Egyptian wise Phaophi 16–17, 3 seasonal hours after midnight
and 3 2 ⁄ 5 equatorian hours, because the sun was 26° within the
Archer, but 3 1 ⁄ 6 hours with respect to regular solar days. At that hour
the true position of the moon’s centre was 31 1 ⁄ 4 ° from the autumn
equinox and 1 1 ⁄ 3 ° north of the ecliptic.
Ptolemy initially proceeded according to this method, finding the position of a
particular lunation for his current date, and then calculating the star’s movement. But
it was tedious and not particularly accurate, so Ptolemy decided to develop a better
system for recording the position of stars. His logic was that if he could clearly lay
down a technique for measuring stars, and use that technique for measuring “as many
stars as we could up to those of the sixth magnitude” (Almagest), then he would be
able to produce a list of stars that could be used by future generations of astronomers
to check his estimates of the rate of precession .
His method was simple. He first developed an instrument that would enable
him to make the needed measurements. He then found the poles of the ecliptic, and
projected every star onto the ecliptic via the lines of longitude from these poles. The
point where the projected star cut the ecliptic he carefully measured, and noted the
star’s latitude north or south of the ecliptic. By this method, he measured 1,022 stars,
and published this list in his Almagest.
It was an ingenious system. It meant that the position of a star could be accurately
and simply recorded. It could be reproduced in years to come so that any change
in the ecliptical position of the star could be easily noted. It was a huge advance for
astronomers and placed Ptolemy among the giants of astronomy. However, it also
altered the way that astrologers worked with fixed stars. Until that time, evidence suggests
that the predominant method for working with stars in astrology was via their
risings, culminations, and settings. But within several hundred years after Ptolemy,
astrologers had taken his convenient list of stars with their ecliptical degrees, and were
routinely using it in their horoscopes, forsaking the older, more tedious, observational
methodologies. Ptolemy had developed the list as an astronomer, for astronomical
needs. He was, afterall, an astronomer and the book in which he published this listing
THE ASTROLOGY BOOK
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