138. Dr. LONG, in his first volume of _Astronomy_, has given us figures
of the apparent paths of all the Planets separately from CASSINI; and on
seeing them I first thought of attempting to trace some of them by a
machine[29] that shews the motions of the Sun, Mercury, Venus, the Earth
and Moon, according to the _Copernican System_. Having taken off the
Sun, Mercury, and Venus, I put black-lead pencils in their places, with
the points turned upward; and fixed a circular sheet of paste-board so,
that the Earth kept constantly under it’s center in going round the Sun;
and the paste-board kept its parallelism. Then, pressing gently with one
hand upon the paste-board to make it touch the three pencils, with the
other hand I turned the winch which moves the whole machinery: and as
the Earth, together with the pencils in the places of Mercury and Venus,
had their proper motions round the Sun’s pencil, which kept at rest in
the center of the machine, all the three pencils described a diagram
from which the first Figure of the third Plate is truly copied in a
smaller size. As the Earth moved round the Sun, the Sun’s pencil
described the dotted Circle of Months, whilst Mercury’s pencil drew the
curve with the greatest number of loops, and Venus’s that with the
fewest. In their inferiour conjunctions they come as much nearer the
Earth, or within the Circle of the Sun’s apparent motion round the
Heavens, as they go beyond it in their superiour conjunctions. On each
side of the loops they appear Stationary; in that part of each loop next
the Earth retrograde; and in all rest of their paths direct.
[Illustration: Plate III. _J. Ferguson delin._ _J. Mynde Sc._]
[Sidenote: PLATE III.]
If _Cassini_’s Figures of the paths of the Sun, Mercury and Venus were
put together, the Figure as above traced out, would be exactly like
them. It represents the Sun’s apparent motion round the Ecliptic, which
is the same every year; Mercury’s motion for seven years; and Venus’s
for eight; in which time Mercury’s path makes 23 loops, crossing itself
so many times, and Venus’s only five. In eight years Venus falls so
nearly into the same apparent path again, as to deviate very little from
it in some ages; but in what number of years Mercury and the rest of the
Planets would describe the same visible paths over again, I cannot at
present determine. Having finished the above Figure of the paths of
Mercury and Venus, I put the Ecliptic round them as in the Doctor’s
Book; and added the dotted lines from the Earth to the Ecliptic for
shewing Mercury’s apparent or geocentric motion therein for one year; in
which time his path makes three loops, and goes on a little farther;
which shews that he has three inferiour, and as many superiour
conjunctions with the Sun in that time, and also that he is six times
Stationary, and thrice Retrograde. Let us now trace out his motion for
one year in the Figure.
[Sidenote: Fig. I.]
Suppose Mercury to be setting out from _A_ towards _B_ (between the
Earth and left-hand corner of the Plate) and as seen from the Earth his
motion will then be direct, or according to the order of the Signs. But
when he comes to _B_, he appears to stand still in the 23d degree of ♏
at _F_, as shewn by the line _BF_. Whilst he goes from _B_ to _C_, the
line _BF_ goes backward from _F_ to _E_, or contrary to the order of
Signs; and when he is at _C_ he appears Stationary at _E_; having gone
back 11-1/2 degrees. Now, suppose him Stationary on the first of
_January_ at _C_, on the tenth thereof he will appear in the Heavens as
at 20, near _F_; on the 20th he will be seen as at _G_; on the 31st at
_H_; on the 10th of _February_ at _I_; on the 20th at _K_; and on the
28th at _L_; as the dotted lines shew, which are drawn through every
tenth day’s motion in his looped path, and continued to the Ecliptic. On
the 10th of _March_ he appears at _M_; on the 20th at _N_; and on the
31st at _O_. On the 10th of _April_ he appears Stationary at _P_; on the
20th he seems to have gone back again to _O_; and on the 30th he appears
Stationary at _Q_ having gone back 11-1/2 degrees. Thus Mercury seems to
go forward 4 Signs 11 Degrees, or 131 Degrees; and to go back only 11 or
12 Degrees, at a mean rate. From the 30th of _April_ to the 10th of
_May_, he seems to move from _Q_ to _R_; and on the 20th he is seen at
_S_, going forward in the same manner again, according to the order of
letters; and backward when they go back; which, ’tis needless to explain
any farther, as the reader can trace him out so easily through the rest
of the year. The same appearances happen in Venus’s motion; but as she
moves slower than Mercury, there are longer intervals of time between
them.
Having already § 120. given some account of the apparent diurnal motions
of the Heavens as seen from the different Planets, we shall not trouble
the reader any more with that subject.