And I trust there will not be wanting men of genius, to arise in this new world, whose talents may be particularly adapted to astronomical enquiries. Indeed I am persuaded that nature is by no means so nigardly in producing them, as we are apt to imagine. Some are never tempted forth from obscurity, some are untimely snatched away by death, a striking instance whereof we have in Horrox; and many are accidentally led to other pursuits.
The Astronomy of comets is still in its infancy; not that the attention of the learned and ingenious has at all been wanting for more than a century past; but because it will necessarily require many ages to bring it to perfection. I wish we were in a condition to promote it in some degree, by carefully observing such comets as may appear. As yet we scarce dare affirm that any one has or will return a second time. It has never, that I know of, been certainly proved by observation, that a comet has descended within a parabolic orbit, and until that is done we have only a coincidence of periods and orbits (none of which have been very precise) to depend on for their return. Far less are astronomers able to determine the changes that may, and probably do, happen in their orbits[[A36]] and velocities in every period, so as to predict their nearer or more remote approach to the earth or any planet. Whether their business be to repair or destroy, whether they are worlds yet in formation or once habitable worlds in ruins; whether they are at present habitable and regular attendants of our Sun only, or whether they are the vast links that connect the distant parts of creation by surrounding more suns than one, we know not.
If we descend to the Planetary System, there are still many things wanting to compleat Astronomy.
The orbits of the primary planets have at one time been supposed moveable with various irregularities, at other times fixed and permanent. It seems now generally granted, that according to the theory of gravity they must change their situations; yet not long since, some great astronomers warmly contended that this change was altogether insensible.
According to the best tables we now have, the planes of the orbits of Jupiter, the Earth and Mercury are immoveable, though the orbits themselves have a progressive motion in their planes. On the contrary, the poles of the orbits of Saturn, Mars and Venus are supposed to revolve about the poles of the earth’s orbit, with such velocities as at present nearly reconcile calculation to appearances. But there is good reason to apprehend that such a supposition is not true in fact, and a mistake in this matter will have some important consequences. More probable is it, that the poles of the orbits of all the planets, the earth not excepted, revolve about some common centre. The several quantities of these motions, I am confident, are to be had from observation, and not from theory alone. If such a motion of the earth’s orbit be admitted, it will account for the diminution[[A37]] of the obliquity of the ecliptic; which seems now incontestible; and that in whatever manner we divide the forces producing such motion, amongst the two superior planets and Venus, or even amongst all of them. And I should suspect the further diminution of obliquity, from this cause, will amount to about one degree and an half.
But as Astronomy now stands, it seems doubtful whether this change is owing to a deviation in the diurnal or annual motion of the earth; which introduces a very disagreeable uncertainty in conclusions drawn from some nice and useful observations.
The Lunar Astronomy has been brought so much nearer to perfection, by the celebrated[celebrated] Mayer,[[A38]] than could have been expected, that I shall mention no deficiency in it, but this. We do not certainly know whether that apparent acceleration of the moon’s motion, which Mayer with other great astronomers has admitted, ought to be attributed to a real increase of velocity in the moon, or to a diminution of the earth’s diurnal motion. If to the former, the destruction of this beautiful and stupendous fabric, may from thence be predicted with more certainty than from any other appearance in Nature: But if to the latter, it may be prettily accounted for, by Dr. Halley’s ingenious hypotheses concerning the change of variation in the magnetical needle. The Doctor supposes the external crust or shell of the earth to contain a nucleus detatched from it, and that the impulse which first caused the diurnal motion, was given to the external parts, and from thence in time communicated to the internal nucleus, by means of an intervening fluid; but not so as perfectly to equal the velocity of the superficial parts of the globe. Whence it will follow, that the external shell of the earth is still communicating motion to the internal parts, and losing motion itself proportionably. The diurnal motion must therefore become slower and slower, yet can never be retarded, by this cause, beyond certain limits; nor can we conceive that any inconvenience will follow.
There is another physical question relating to the moon, which to me appears extremely curious; it is this—Whence is it that the moon always turns the same side to us? or, which is the same thing, How comes the moon’s rotation on her axis, and her monthly revolution about the earth, to be performed in the same time? None I believe will suppose it to be accidental, nor will the astronomer be easily satisfied with a final cause. Was it not originally brought about by a natural cause which still subsists? Can the attraction of any foreign body change a rotatory motion into a libratory one, and a libratory motion into rest, in spaces so very free from all resistance as those wherein the planets move? There are other defects in Astronomy that are purely optical. Removing of those, depends on the further improvement of telescopes, or rather on the more judicious use of them, at times and places the most favourable.
In speaking of telescopic discoveries I purposely reserved those made on Venus for this place, because they are still uncertain. Burratini in Poland first discovered spots in Venus, then Cassini in Italy; and afterwards Bianchini got a sight of them. But from all their observations it is uncertain, whether Venus revolves on its axis once in 23 hours, or once in 24 days. Perhaps it does neither. Nor is their determination of the axis’ situation much more satisfactory. These spots on Venus are not to be seen but through an excellent telescope and a pure atmosphere.
In the year 1672 and 1676 Cassini saw a small star near Venus, which he thought might be a satellite attending on her. It appeared to have the same phase with Venus. In 1740 Mr. Short with a telescope of 16 inches saw a small star at the distance of ten minutes from Venus, which from its apparent shape he likewise thought might be a satellite. And in 1761 Mr. Montaigne, in France, saw what he took to be the satellite of Venus, on the 3d, 4th, 7th and 11th of May.[[A39]] But whether Venus has a satellite or not, must still be left amongst the doubtful things of Astronomy.