210. To every place 15 degrees eastward from any given Meridian, it is
noon an hour sooner than on that Meridian; because their Meridian comes
to the Sun an hour sooner: and to all places 15 degrees westward it is
noon an hour later § 208, because their Meridian comes an hour later to
the Sun; and so on: every 15 degrees of motion causing an hour’s
difference in time. Therefore they who have noon an hour later than we,
have their Meridian, that is, their Longitude 15 degrees westward from
us; and they who have noon an hour sooner than we, have their Meridian
15 degrees eastward from ours: and so for every hour’s difference of
time 15 degrees difference of Longitude. Consequently, if the beginning
or ending of a Lunar Eclipse be observed, suppose at _London_, to be
exactly at mid-night, and in some other place at 11 at night, that place
is 15 degrees westward from the Meridian of _London_: if the same
Eclipse be observed at one in the morning at another place, that place
is 15 degrees eastward from the said Meridian.
[Sidenote: Eclipses of Jupiter’s Satellites much better for that
purpose.]
211. But as it is not easy to determine the exact moment either of the
beginning or ending of a Lunar Eclipse, because the Earth’s shadow
through which the Moon passes is faint and ill defined about the edges;
we have recourse to the Eclipses of Jupiter’s Satellites, which
disappear so instantaneously as they enter into Jupiter’s shadow, and
emerge so suddenly out of it, that we may fix the phenomenon to half a
second of time. The first or nearest Satellite to Jupiter is the most
advantageous for this purpose, because it’s motion is quicker than the
motion of any of the rest, and therefore it’s immersions and emersions
are more frequent.
[Sidenote: How to solve this important problem.