212. The _English_ Astronomers have made Tables for shewing the times of
the Eclipses of Jupiter’s Satellites to great precision, for the
Meridian of _Greenwich_. Now, let an observer, who has these Tables with
a good Telescope and a well-regulated Clock at any other place of the
Earth, observe the beginning or ending of an Eclipse of one of Jupiter’s
Satellites, and note the precise moment of time that he saw the
Satellite either immerge into, or emerge out of the shadow, and compare
that time with the time shewn by the Tables for _Greenwich_; then, 15
degrees difference of Longitude being allowed for every hour’s
difference of time, will give the Longitude of that place from
_Greenwich_, as above § 210; and if there be any odd minutes of time,
for every minute a quarter of a degree, east or west must be allowed, as
the time of observation is before or after the time shewn by the Tables.
Such Eclipses are very convenient for this purpose at land, because they
happen almost every day; but are of no use at sea, because the rolling
of the ship hinders all nice telescopical observations.
[Sidenote: Fig. II.