_The Phenomena of the Heavens as seen from different Parts of the Solar
System._
132. So vastly great is the distance of the starry Heavens, that if
viewed from any part of the Solar System, or even many millions of miles
beyond it, its appearance would be the very same to us. The Sun and
Stars would all seem to be fixed on one concave surface, of which the
Spectator’s eye would be the centre. But the Planets, being much nearer
than the Stars, their appearances will vary considerably with the place
from which they are viewed.
133. If the spectator is at rest without their Orbits, the Planets will
seem to be at the same distance as the Stars; but continually changing
their places with respect to the Stars, and to one another: assuming
various phases of increase and decrease like the Moon. And,
notwithstanding their regular motions about the Sun, will sometimes
appear to move quicker, sometimes slower, be as often to the west as to
the east of the Sun; and at their greatest distances seem quite
stationary. The duration, extent, and points in the Heavens where these
digressions begin and end, would be more or less according to the
respective distances of the several Planets from the Sun: but in the
same Planet they would continue invariably the same at all times; like
pendulums of unequal lengths oscillating together, the shorter move
quick and go over a small space, the longer move slow and go over a
large space. If the observer is at rest within the Orbits of the
Planets, but not near the common center, their apparent motions will be
irregular, but less so than in the former case. Each of the several
Planets will appear bigger and less by turns, as they approach nearer or
recede farther from the observer; the nearest varying most in their
size. They will also move quicker or slower with regard to the fixed
Stars, but will never be retrograde or stationary.
134. Next, let a spectator in motion view the Heavens: the same apparent
irregularities will be observed, but with some variation resulting from
his own motion. If he is on a Planet which has a rotation on it’s Axis,
not being sensible of his own motion he will imagine the whole Heavens,
Sun, Planets, and Stars to revolve about him in the same time that his
Planet turns round, but the contrary way; and will not be easily
convinced of the deception. If his Planet moves round the Sun, the same
irregularities and aspects as above will appear in the motions of the
Planets: only, the times of their being direct, stationary and
retrograde will be accelerated or retarded as they concur with, or are
contrary to his motion: and the Sun will seem to move among the fixed
Stars or Signs, directly opposite to those in which his Planet moves;
changing it’s place every day as he does. In a word, whether our
observer be in motion or at rest, whether within or without the Orbits
of the Planets, their motions will seem irregular, intricate and
perplexed, unless he is in the center of the System; and from thence,
the most beautiful order and harmony will be observed.
[Sidenote: The Sun’s center the only point from which the true motions
and places of the Planets could be seen.]
135. The Sun being the center of all the Planets motions, the only place
from which their motions could be truly seen, is the Sun’s center; where
the observer being supposed not to turn round with the Sun (which, in
this case, we must imagine to be a transparent body) would see all the
Stars at rest, and seemingly equidistant from him. To such an observer
the Planets would appear to move among the fixed Stars, in a simple,
regular, and uniform manner; only, that as in equal times they describe
equal Areas, they would describe spaces somewhat unequal, because they
move in elliptic Orbits § 155. Their motions would also appear to be
what they are in fact, the same way round the Heavens; in paths which
cross at small Angles in different parts of the Heavens, and then
separate a little from one another § 20. So that, if the solar
Astronomer should make the Path or Orbit of any one Planet a standard,
and consider it as having no obliquity § 201, he would judge the paths
of all the rest to be inclined to it; each Planet having one half of
it’s path on one side, and the other half on the opposite side of the
standard Path or Orbit. And if he should ever see all the Planets start
from a conjunction with each other[28]; Mercury would move so much
faster than Venus as to overtake her again (though not in the same point
of the Heavens) in a quantity of time almost equal to 145 of our days
and nights; or, as we commonly call them, _Natural Days_, which include
both the days and nights: Venus would move so much faster than the Earth
as to overtake it again in 585 natural days: the Earth so much faster
than Mars as to overtake him again in 778 such days: Mars is much faster
than Jupiter as to overtake him again in 817 such days: and Jupiter so
much faster than Saturn as to overtake him again in 7236 days, all of
our time.
[Sidenote: The judgment that a solar Astronomer would probably make
concerning the distances and bulks of the Planets.]
136. But as our solar Astronomer could have no idea of measuring the
courses of the Planets by our days, he would very probably take the
period of Mercury, which is the quickest moving Planet, for a measure to
compare the periods of the others by. As all the Stars would appear
quiescent to him, he would never think that they had any dependance upon
the Sun; but could naturally imagine that the Planets have, because they
move round the Sun. And it is by no means improbable, that he would
conclude those Planets whose periods are quickest to move in Orbits
proportionably less than those do which make slower circuits. But being
destitute of a method for finding their Parallaxes, or, more properly
speaking, as they could have no Parallax to him, he could never know any
thing of their real distances or magnitudes. Their relative distances he
might perhaps guess at by their periods, and from thence infer something
of truth concerning their relative bulks, by comparing their apparent
bulks with one another. For example, Jupiter appearing bigger to him
than Mars, he would conclude it to be much bigger in fact; because it
appears so, and must be farther from him, on account of it’s longer
period. Mercury would seem bigger than the Earth; but by comparing it’s
period with the Earth’s, he would conclude that the Earth is much
farther from him than Mercury, and consequently that it must be really
bigger though apparently less; and so of the rest. And, as each Planet
would appear somewhat bigger in one part of it’s Orbit than in the
opposite, and to move quickest when it seems biggest, the observer would
be at no loss to determine that all the Planets move in Orbits of which
the Sun is not precisely in the center.
[Sidenote: The Planetary motions very irregular as seen from the Earth.