Kepler has also supplied us with another mode of arriving at the distance and size of the sun and the planets: he has hazarded a conjecture, that the squares of the times of the revolution of the earth and the other planets are in proportion to the cubes of their distances from the sun, their common centre; and, as by observation we can arrive with tolerable certainty at a knowledge of the times of their revolutions, we may from hence proceed to the other matters we are desirous to ascertain. And that which Kepler seemed, as by a divine inspiration, to hazard in the way of conjecture, Newton professes to have demonstratively established. But the demonstration of Newton has not been considered as satisfactory by all men of science since his time.
Thus far however we proceed as we may, respecting our propositions on the subject of the solar system. But, beyond this, all science, real or pretended, deserts us. We have no method for measuring angles, which can be applied to the fixed stars; and we know nothing of any revolutions they perform. All here therefore seems gratuitous: we reason from certain alleged analogies; and we can do no more.
Huygens endeavoured to ascertain something on the subject, by making the aperture of a telescope so small, that the sun should appear through it no larger than Sirius, which he found to be only in the proportion of 1 to 27,664 times his diameter, as seen by the naked eye. Hence, supposing Sirius to be a globe of the same magnitude as the sun, it must be 27,664 times as distant from us as the sun, in other words, at a distance so considerable as to equal 345 million diameters of the earth(60). Every one must feel on how slender a thread this conclusion is suspended.
(60) Encyclopaedia Londinensis, Vol. 11, p. 407.
And yet, from this small postulate, the astronomers proceed to deduce the most astounding conclusions. They tell us, that the distance of the nearest fixed star from the earth is at least 7,600,000,000,000 miles, and of another they name, not less than 38 millions of millions of miles. A cannon-ball therefore, proceeding at the rate of about twenty miles in a minute would be 760,000 years in passing from us to the nearest fixed star, and 3,800,000 in passing to the second star of which we speak. Huygens accordingly concluded, that it was not impossible, that there might be stars at such inconceivable distances from us, that their light has not yet reached the earth since its creation(61).
(61) Ibid, p. 408.
The received system of the universe, founded upon these so called discoveries, is that each of the stars is a sun, having planets and comets revolving round it, as our sun has the earth and other planets revolving round him. It has been found also by the successive observations of astronomers, that a star now and then is totally lost, and that a new star makes its appearance which had never been remarked before: and this they explain into the creation of a new system from time to time by the Almighty author of the universe, and the destruction of an old system worn out with age(62). We must also remember the power of attraction every where diffused through infinite space, by means of which, as Herschel assures us, in great length of time a nebula, or cluster of stars, may be formed, while the projectile force they received in the beginning may prevent them from all coming together, at least for millions of ages. Some of these nebulae, he adds, cannot well be supposed to be at a less distance from us than six or eight thousand times the distance of Sirius(63). Kepler however denies that each star, of those which distinctly present themselves to our sight, can have its system of planets as our sun has, and considers them as all fixed in the same surface or sphere; since, if one of them were twice or thrice as remote as another, it would, supposing their real magnitudes to be equal, appear to be twice or thrice as small, whereas there is not in their apparent magnitudes the slightest difference(64).
(62) Encycl. Lond. Vol. II, p. 411.
(63) Ibid, p. 348.
(64) Ibid, p. 411.