88. In Fig. 5th, we have a view of the bulks of the Planets in
proportion to each other, and to a supposed globe of two foot diameter
for the Sun. The Earth is 27 times as big as Mercury, very little bigger
than Venus, 5 times as big as Mars; but Jupiter is 1049 times as big as
the Earth, Saturn 586 times as big, exclusive of his Ring; and the Sun
is 877 thousand 650 times as big as the Earth. If the Planets in this
Figure were set at their due distances from a Sun of two feet diameter,
according to their proportional bulks, as in our System, Mercury would
be 28 yards from the Sun’s center; Venus 51 yards 1 foot; the Earth 70
yards 2 feet; Mars 107 yards 2 feet; Jupiter 370 yards 2 feet; and
Saturn 760 yards two feet. The Comet of the year 1680, at it’s greatest
distance, 10 thousand 760 yards. In this proportion, the Moon’s distance
from the center of the Earth would be only 7-1/2 inches.
[Sidenote: An idea of their distances.]
89. To assist the imagination in conceiving an idea of the vast
distances of the Sun, Planets, and Stars, let us suppose, that a body
projected from the Sun should continue to fly with the swiftness of a
cannon ball; _i. e._ 480 miles every hour; this body would reach the
Orbit of Mercury, in 7 years 221 days; of Venus, in 14 years 8 days; of
the Earth, in 19 years 91 days; of Mars, in 29 years 85 days; of
Jupiter, in 100 years 280 days; of Saturn, in 184 years 240 days; to the
Comet of 1680, at it’s greatest distance from the Sun, in 2660 years;
and to the nearest fixed Stars in about 7 million 600 thousand years.
[Sidenote: Why the Planets appear bigger and less at different times.]
90. As the Earth is not the center of the Orbits in which the Planets
move, they come nearer to it and go farther from it and at different
times; on which account they appear bigger and less by turns. Hence, the
apparent magnitudes of the Planets are not always a certain rule to know
them by.
[Sidenote: Fig. I.]
91. Under Fig. 3, are the names and characters of the twelve Signs of
the Zodiac, which the Reader should be perfectly well acquainted with;
so as to know the characters without seeing the names. Every Sign
contains 30 degrees, as in the Circle bounding the Solar System; to
which the characters of the Signs are set in their proper places.
[Sidenote: The Comets.]
92. The COMETS are solid opaque bodies, with long transparent trains or
tails, issuing from that side which is turned away from the Sun. They
move about the Sun, in very excentric ellipses; and are of a much
greater density than the Earth; for some of them are heated in every
Period to such a degree, as would vitrify or dissipate any substance
known to us. Sir ISAAC NEWTON computed the heat of the Comet which
appeared in the year 1680, when nearest the Sun, to be 2000 times hotter
than red-hot iron, and that being thus heated, it must retain it’s heat
until it comes round again, although it’s Period should be more than
twenty thousand years; and it is computed to be only 575. The method of
computing the heat of bodies, keeping at any known distance from the
Sun, so far as their heat depends on the force of the Sun’s rays, is
very easy; and shall be explained in the eighth Chapter.
[Sidenote: PLATE I.
Fig. I.
They prove that the Orbits of the Planets are not solid.
The Periods only of three are known.