Of the Nature of the heavenly Bodies continued, with the Opinions of the Antients concerning the Sun and Stars.
SIR,
Y
You tell me you begin to be a tolerable good Copernican, and would now be glad to have my Opinion further upon the Nature of the Sun and Stars, with regard to the Suggestion of their being like Bodies of Fire. This you say will go a great Way towards confirming you in the Notion you have begun to embrace of a Plurality of Systems, and a much greater Multiplicity of Worlds than our little solar System can admit of. Besides, shewing in a very evident Light, that the Authorities cited in my first Letter are founded upon the clearest Reason.
Anaxagoras, you say, believed the Sun to be a Lump of red-hot Iron; Euripides thought it a Clod of Gold; and others still more ridiculously have imagined it to be a dark Body, void of all Heat. That the Sun is a vast Body of blazing Matter, notwithstanding the various Opinions of those primitive Sages, will, I think, hardly admit of a Question: Since the known Warmth of his prolifick Beams, and the visible Effect of the Burning-glass, puts it quite out of the Power of our present Set of Senses, at least to argue against it; and how reasonably we may imagine the Stars to be all of the same or like Nature, will sufficiently appear from these following Considerations: First, it is well known to all Mathematicians, that any visible Object of any determined Magnitude may be reduced to the Appearance of [P]a physical Point, by removing the Eye of the Observer to a proper or proportionable Distance from it, within the finite View: And that the apparent Diameter of every luminous celestial Body, will always be diminished reciprocally, in Proportion to the Distance from the Eye, till they become altogether imperceptible.
[P] What is here meant by a physical Point, is a Point visible to the naked Eye, which human Art cannot divide; and so far it partakes of the Property of a mathematical one, which is only to be conceived, and not seen.
Thus the Disk of the Sun, which appears to us at Earth under an Angle of about half a Degree, if seen from the Planet Saturn, would appear not much bigger than the Planet Venus or Jupiter, in their most neighbouring Vicinity does to us; and consequently to an Eye placed in the Aphelion Point of the Orbit of the great Comet of 1680, his apparent Diameter would be so reduced as to seem but little bigger than the largest of the Stars; and by the same Analogy, or Way of Reasoning, admitting Space and Distance infinite, which I humbly apprehend is not to be disputed, were all the Matter in the Universe united, and conglobed in one Mass, with respect to ocular Sensation, it might be diminished so near to a mathematical Punctum, as to be almost adequate to our Ideas of Nothing.
This to any tolerable Optician, must be an evident Conviction of the Truth of the modern Astronomy, which now universally allow all those radiant Bodies the Stars to be of the same Nature with the Sun; and that as certainly they are no other than vast Globes of blazing Matter, all undoubtedly shining by their own native Light.
But as you have often objected to what has been said of the Distance of the Stars in general, and may possibly from a Supposition, that they are, or may be, much nearer to us, infer, that their Light, like that of the Planets, may be also borrowed from the Sun, or from some other radiant Body, which, from the Nature of the Supposition, must of Consequence be invisible to us, I judge it will not be amiss to throw a few demonstrative Arguments in your Way, in order to lead you a little out of the Path of an early Prejudice, and draw you as it were by Degrees through the Dawn of astronomical Reasoning, out of your original Error, and rescue your Imagination from the false Notions imbibed from Phænomena only in your younger Years. This I guess cannot fail of reconciling you to this more rational Way of Thinking, and make you acquainted with Truths of much Consequence, which perhaps you have yet been an intire Stranger to. The grand Deceptio Visus, which I must first endeavour to remove, and which as a sort of Paradox in Nature, has, as I may say, imprisoned the Understanding of many superficial Reasoners, and in general all incurious Men, is this.
Most People are too apt to think originally, that as the Heavens appear to be a vast concave Hemisphere, that the Stars must of course, as of Consequence, be fixed there, like so many radiant Studs of Fire, of various Magnitudes; and take it for granted, chiefly designed for no other Purpose than to deck and adorn the Canopy of our Night. This was long ago the Opinion of Thales the Milesian, and wants not the Authority of many of the Antients to back it. Others, in particular [Q]Ptolomy of Pelusium in Africa, who from his Experience in this Science, is called by some the Prince of Astronomers, believed them to be Loop-holes in the vast solid celestial Firmament, emitting the Light of the Crystalline Heaven through it to all within it. The famous Diogenes, Cotemporary with Plato, conceived them to be of the Nature of Pumice-stones, and inclined to an Opinion, that they were the Spiracula, or Breathing-holes of Heaven. Anaxagoras thought them Stones snatched up from the Earth by the Rapidity of its Motion, and set on Fire in the upper Regions above the Moon.