213. To explain this by a Figure, let _J_ be Jupiter, _K_, _L_, _M_, _N_
his four Satellites in their respective Orbits 1, 2, 3, 4; and let the
Earth be at _f_ (suppose in _November_, although that month is no
otherways material than to find the Earth readily in this scheme, where
it is shewn in eight different parts of it’s Orbit.) Let _Q_ be a place
on the Meridian of _Greenwich_, and _R_ a place on some other Meridian.
Let a person at _R_ observe the instantaneous vanishing of the first
Satellite _K_ into Jupiter’s shadow, suppose at three o’clock in the
morning; but by the Tables he finds the immersion of that Satellite to
be at midnight at _Greenwich_: he can then immediately determine, that
as there are three hours difference of time between _Q_ and _R_, and
that _R_ is three hours forwarder in reckoning than _Q_, it must be 45
degrees of east Longitude from the Meridian of _Q_. Were this method as
practicable at sea as at land, any sailor might almost as easily, and
with equal certainty, find the Longitude as the Latitude.
[Sidenote: Fig. II.
We seldom see the beginning and end of the same Eclipse of
any of Jupiter’s Moons.]
214. Whilst the Earth is going from _C_ to _F_ in it’s Orbit, only the
immersions of Jupiter’s Satellites into his shadow are generally seen;
and their emersions out of it while the Earth goes from _G_ to _B_.
Indeed, both these appearances may be seen of the second, third, and
fourth Satellite when eclipsed, whilst the Earth is between _D_ and _E_,
or between _G_ and _A_; but never of the first Satellite, on account of
the smallness of it’s Orbit and the bulk of Jupiter; except only when
Jupiter is directly opposite to the Sun; that is, when the Earth is at
_g_: and even then, strictly speaking, we cannot see either the
immersions or emersions of any of his Satellites, because his body being
directly between us and his conical shadow, his Satellites are hid by
his body a few moments before they touch his shadow; and are quite
emerged from thence before we can see them, as it were, just dropping
from him. And when the Earth is at _c_, the Sun being between it and
Jupiter hides both him and his Moons from us.
In this Diagram, the Orbits of Jupiter’s Moons are drawn in true
proportion to his diameter; but, in proportion to the Earth’s Orbit they
are drawn 81 times too large.
[Sidenote: PLATE VI.
Jupiter’s conjunctions with the Sun, or oppositions to him,
are every year in different parts of the Heavens.]
215. In whatever month of the year Jupiter is in conjunction with the
Sun, or in opposition to him, in the next year it will be a month later
at least. For whilst the Earth goes once round the Sun, Jupiter
describes a twelfth part of his Orbit. And therefore, when the Earth has
finished it’s annual period from being in a line with the Sun and
Jupiter, it must go as much forwarder as Jupiter has moved in that time,
to overtake him again: just like the minute hand of a watch, which must,
from any conjunction with the hour hand, go once round the dial-plate
and somewhat above a twelfth part more, to overtake the hour hand again.
[Sidenote: The surprising velocity of light.]
216. It is found by observation, that when the Earth is between the Sun
and Jupiter, as at _g_, his Satellites are eclipsed about 8 minutes
sooner than they should be according to the Tables: and when the Earth
is at _B_ or _C_, these Eclipses happen about 8 minutes later than the
Tables predict them. Hence it is undeniably certain, that the motion of
light is not instantaneous, since it takes about 16-1/2 minutes of time
to go through a space equal to the diameter of the Earth’s Orbit, which
is 162 millions of miles in length: and consequently the particles of
light fly about 164 thousand 494 miles every second of time, which is
above a million of times swifter than the motion of a cannon bullet. And
as light is 16-1/2 minutes in travelling across the Earth’s Orbit, it
must be 8-1/4 minutes in coming from the Sun to us: therefore, if the
Sun were annihilated we should see him for 8-1/4 minutes after; and if
he were again created he would be 8-1/4 minutes old before we could see
him.
[Sidenote: Fig. V.