Another had been observed
seventy-six years earlier than 1456, viz., in 1380, and another
seventy-five years before that, in 1305.
As Halley thus found that a comet had been recorded on several
occasions at intervals of seventy-five or seventy-six years, he was
led to the conclusion that these several apparitions related to one
and the same object, which was an obedient vassal of the sun,
performing an eccentric journey round that luminary in a period of
seventy-five or seventy-six years. To realise the importance of this
discovery, it should be remembered that before Halley's time a comet,
if not regarded merely as a sign of divine displeasure, or as an omen
of intending disaster, had at least been regarded as a chance visitor
to the solar system, arriving no one knew whence, and going no one
knew whither.
A supreme test remained to be applied to Halley's theory. The
question arose as to the date at which this comet would be seen
again. We must observe that the question was complicated by the fact
that the body, in the course of its voyage around the sun, was
exposed to the incessant disturbing action produced by the attraction
of the several planets. The comet therefore, does not describe a
simple ellipse as it would do if the attraction of the sun were the
only force by which its movement were controlled.
Pages:
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203