ever notions of the inhabitants of earth are not contradictory to nature and to fact. It is ufual to denominate those planets nearer to the fun than ourfelves, inferior, and those further from the fun than ourselves, fuperior. The inferior planets confequently are Mercury, and Venus. These have fo many appearances in common, that when we have attained a distinct idea of either, we have little to add concerning the other.

I shall now request your attention to what is common to both, (1.) The courfe of their motions being within the courfe of our motion with regard to the fun, it will naturally happen that they fometimes are exacly between the sun and the earth, fo that looking to the fun we should (fuppofing their light ftrong enough) fee the planets alfo; and this must needs be their nearest approach to the earth, or Peri-Geo (Ge being the Greek name for the Earth-Vide a fimilar explanation, page 65); and not only may the planets be thus in conjunction with the fun, but if the conjunction be abfolutely perfect, a line drawn from the fun to the earth, will pafs through the body of the planet; hereby the planet intercepts fo many of the folar rays, as are proportionate to its magnitude, and eclipfes to a certain degree the fun from us. This paffage of a planet over the face of the fun, is called a Tranfit, and can only happen to inferior planets. Thus, Venus may fee a tranfit of Mercury; the earth of Venus and Mercury; Mars of the earth alfo, and fo on. But Mercury never fees a transit of Venus, because the path of that planet never lies within (but always without) his orbit. Tranfits are very rare. (2.) We have fuppofed the planet between the earth and the

fun

When a

rays pre

fun (this is called an inferior conjunction); but it is evident, that the same planet, in purfuing its course, muft fometimes be beyond the fun-from us: fo that the fun fhould be between us and the planet (this is a fuperior conjunction), infomuch that the fun may pafs over the face of the planet, as before the planet paffed over the face of the fun. planet is thus fituated, the cffulgence of the fun's vents its being visible in any degree whatever. There are therefore two periods at which the inferior planets are to us invifible, i. e. the conjunctions, when the fun's rays are coincident with the planet, and too powerful for it. Add to this, that in the inferior conjunction, the illuminated face of the planet being turned toward the fun, the dark part is confequently turned toward us. So that these planets pretty nearly resemble the courfe of our moon, being new (or dark), horned, halflight, full, and so on, in the progress of each revolution.

If the earth was not herself a planet, but fixed, these conjunctions would happen in every revolution of these planets round the fun; but as the earth moves as well as the planets, conjunctions can only happen when two planets are in the fame part of the heavens, feen from the sun.

But though the earth is a planet, we will for a while fuppofe her at reft: thus fituated, we attend to one revolution of the planet Venus; and we note, that she first paffes pretty brifkly in her courfe, according to the order of the figns; but in a little time her pace flackens, fhe ftands quite ftill, then the returns back in her course, till again the feems motionless, from which recovering the refumes her original progress, and

again advances in the order of the figns. This is the neceffary effect of her circular course.

To render these appearances ftill clearer, I have here a fmall wheel, which I lay flat on the table; then erecting this wire upon any part of the circle (or periphery) of the wheel, I put on the top of the wire a fmall object for convenience of feeing it; now I place a candle on one fide of the wheel:

1

trace, if you please, the shadow of the ball on the wainscot, as I turn the wheel round-the shadow moves first (you see) to the left, then it returns to the right, at each revolution of the wheel; and at each extreme it feems to move with diminifhed velocity.

The inferior planets can never be very much elongated from the fun, but their greatest distance from him will always be proportionate to the confined dimenfions of their orbits: thus Mercury, whofe orbit is finall, is fo conftantly a neighbour to the fun, that he was long unnoticed by the early astronomers. He is only to be feen for a little while after fun-fet, and a little while before fun-rife, and always in that quarter of the heavens where the fun is, infomuch, that during a great part of his course, he is obfcured by the folar beams. · On the fame principle Venus, thoug's feen later in the evening than Mercury (her orbit being larger), yet always fets before midnight.

If fuch peculiarities in the courfes of thefe planets are not fufficient to prove that they revolve round the fun, yet I think we may defy the acutest maintainer of that system which supposes them to revolve round the earth, to account for them

fairly

!

fairly; nor is it poffible on that fcheme to account for the different apparent magnitudes of thefe planets, in perigeo and apogeo. Venus, when neareft to the earth, is as feventeen in diameter, but when furtheft from the earth as only three. Mercury is in perigee to his diameter in apogee, as fourteen to

fix.

We will now, if you please, turn our attention to those planets, whofe orbits being without ours, are termed Superior Planets. Thefe have, in common with the foregoing, the appearances of being progreffive, then ftationary, then retrograde, and ftationary again. They are alfo in fome parts of their orbits nearer to the earth, in others more remote. They differ, in that they can never tranfit the fun's face, or be between us and him. A conjunction of a planet is, when the fun, that planet and the earth are in one exactly ftraight line : but, as in this cafe, to trace the progrefs of fuch a line, we muft, to view the planet, turn our backs on the fun, this fituation of these bodies is termed Oppofition; the proper conjunction of a fuperior planet, being only when it is behind the fun.

We fuppofed, that while an inferior planet went round the fun the earth continued ftill; let us now, if you pleafe, change the fuppofition, and imagine that while the earth revolves round the fun, a fuperior planet (Mars, as being nearest) remains permanent. The effect would be, that when the earth is fartheft diftant from the planet, the planet would seem direct in motion (i. e. according to the order of the figns); as the earth advanced in the turn of her courfe, as we may be allowed to call it, the planet would feem ftationary, and No. IV.

U

when

when the earth was in her nearest semi-circle, the planet would fecm retrograde. You eafily conceive these variations from what I have already faid. And you will please to remember, that as neither of thefe bodies do in fact ftand ftill, the appearances will be proportionately varied.

We have heretofore obferved, that the smaller the orbit of a planet, the fooner it accomplishes a revolution: Venus, for inftance, who finishes her courfe in feven months (while the earth takes twelve), will be advanced five months in another course, by the time the earth has concluded her old one; as the earth will complete a whole revolution, and almost a second, while Mars is fulfilling his orbit. The difference of the apparent velocities of two planets, is ufually calculated, by fuppofing a spectator in the fun to obferve their revolutions, and to estimate the daily difference of their motions. The daily mean motion of the earth, would appear to fuch a fpectator to be fifty-nine minutes, eight feconds; the daily mean motion of Venus, one degree, thirty-fix minutes, eight feconds; the difference thirty-feven minutes: now, therefore, as thirty-feven minutes is to the whole circle of three hundred and fixty degrees, fo is one day to the whole course of Venus, till fhe returns into the fame relative fituation to the earth (conjunction if you will), which requires about five hundred eighty-three days. The daily mean motion of Mars is thirty-one minutes, twenty-feven feconds; and the earth takes to recover any given fituation with respect to Mars, two years and fifty days. The mean motion of Jupiter is four minutes,