Part 101

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Footnote A36:

This I know has been pretended to. But it is easy to make geometrical conclusions come out as we would have them, when the data they are founded on, are so uncertain that we may chuse them as suits our purpose.

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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 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.

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Footnote A37:

This circumstance tends gradually to lessen the variety of the seasons.

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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 MAYER, 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.

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Footnote A38:

This was Tobias Mayer, who was born at Marbach in the principality of Wurtemberg, in the year 1723: he rendered himself celebrated in astronomy, by having calculated the best tables of the moon, and by an excellent catalogue of stars. He died at Gottingen in 1762, at the age of thirty-nine years. W. B.

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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. But whether Venus has a satellite or not, must still be left amongst the doubtful things of Astronomy.

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Footnote A39:

It may happen that any of the planets, about the time they become stationary, shall describe a loop about some small fixed star, in such manner as might be easily mistaken for the star making part of a revolution about the planet. This I suspected to have been the case with the above observation of Montaigne. But the times set down do not confirm the suspicion.

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The spots on the sun, and those on the surfaces of several planets, have been many years observed without our approaching any nearer towards discovering their nature and cause. Dr. Wilson of Glasgow, has lately succeeded in advancing one step at least, with respect to those of the sun. He has proved from observation that those spots are vast cavities, whose bottoms lie far below the general surface of the sun, and whose sloping sides form the border which we generally see surrounding them. If I should venture to add one conjecture of my own, to those of this ingenious gentleman, I would suppose that those prodigious cavities in the surface of the sun, some of them capable of containing half our earth, are not repeatedly formed by unaccountable explosions of a semifluid substance, but permanent and solid, like the cavities within the moon. And that it is the dark matter sometimes lodging in them, that distinguishes them, and is only accidental.

The diurnal rotations of Saturn and Mercury are yet unknown; but when further improvements shall be made in the art of using telescopes, this circumstance will hardly escape the vigilance of astronomers.

These are a few of the many things that are still left to the industry of the ingenious in this science.

But if all higher and more sublime discoveries are not reserved for us in a future and more perfect state; if Astronomy shall again break those limits that now seem to confine it, and expatiate freely in the superior celestial fields; what amazing discoveries may yet be made amongst the fixed stars! That grand phænomenon the Milky-Way seems to be the clue that will one day guide us. Millions of small stars compose it, and many more bright ones lie in and near it, than in other parts of heaven. Is not this a strong indication that this astonishing system of worlds beyond worlds innumerable, is not alike extended every way, but confined between two parallel planes, of immeasureable, though not infinite extent? Or rather, is not the Milky-Way a vein of a closer texture, running through this part of the material creation? Great things are sometimes best explained by small and small by great. Material substances, such as we daily handle, have been thought composed of impenetrable particles in actual contact: then again it has seemed necessary to suppose them at a distance from each other, and kept in their relative situations by attraction and repulsion. Many appearances require that those distances should be very great in proportion to the size of the particles. Hence some, with no small reason, have concluded that matter consists of indivisible points endued with certain powers. Let us compare these smaller portions of it with that great aggregate of matter which is the object of Astronomy; Light will then appear to have as free passage through a piece of glass, as the comets have in the planetary regions; and several other new considerations will arise.