Ptolemy's ingenuity, however, pointed out a thoroughly
satisfactory method by which the times of sunset at two places could
be compared. He was acquainted with the fact, which must indeed have
been known from the very earliest times, that the illumination of the
moon is derived entirely from the sun. He knew that an eclipse of
the moon was due to the interposition of the earth which cuts off the
light of the sun. It was, therefore, plain that an eclipse of the
moon must be a phenomenon which would begin at the same instant from
whatever part of the earth the moon could be seen at the time.
Ptolemy, therefore, brought together from various quarters the local
times at which different observers had recorded the beginning of a
lunar eclipse. He found that the observers to the west made the time
earlier and earlier the further away their stations were from
Alexandria. On the other hand, the eastern observers set down the
hour as later than that at which the phenomenon appeared at
Alexandria. As these observers all recorded something which indeed
appeared to them simultaneously, the only interpretation was, that
the more easterly a place the later its time. Suppose there were a
number of observers along a parallel of latitude, and each noted the
hour of sunset to be six o'clock, then, since the eastern times are
earlier than western times, 6 p.
Pages:
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28