But this difficulty being surmounted, and the true figure of the earth discovered, a free space would now be granted for the sun, moon, and stars to perform their diurnal motions on all sides of it; unless perhaps at its extremities to the north and south; where something would be thought necessary to serve as an axis for the heavens to revolve on. This Mr. Crantz in his very entertaining history of Greenland informs us, is agreeable to the philosophy of that country, with this difference perhaps, that the high latitude of the Greenlander makes him conclude one pole only, necessary: He therefore supposes a vast mountain situate in the utmost extremity of Greenland, whose pointed apex supports the canopy of heaven, and whereon it revolves with but little friction.
A free space around the earth being granted, our infant astronomer would be at liberty to consider the diurnal motions of the stars as performed in intire circles, having one common axis of rotation. And by considering their daily anticipation in rising and setting, together with the sun’s annual rising and falling in its noon day height, swiftest about the middle space, and stationary for some time when highest and lowest, he would be led to explain the whole by attributing a slow motion to the sun, contrary to the diurnal motion, along a great circle dividing the heavens into two equal parts, but obliquely situated with respect to the diurnal motion. By a like attention to the moon’s progress the Zodiac would be formed, and divided into its several constellations or other convenient divisions.
The next step that astronomy advanced, I conceive, must have been in the discovery attributed to Pythagoras;[[A13]] who it is said first found out that Hesperus and Phosphorus, or the Evening and Morning Star, were the same. The superior brightness of this planet, and the swiftness of its motion, probably first attracted the notice of the inquisitive: and one wandering star being discovered, more would naturally be looked for. The splendor of Jupiter, the very changeable appearance of Mars, and the glittering of Mercury by day light, would distinguish them. And lastly, Saturn would be discovered by a close attention to the heavens. But how often would the curious eye be directed in vain, to the regions of the north and south, before there was reason to conclude that the orbits of all the planets lay nearly in the same plane; and that they had but narrow limits assigned them in the visible heavens.
From a careful attendance to those newly discovered celestial travellers, and their various motions, direct and retrograde, the great discovery arose, that the sun is the centre of their motions; and that by attributing a similar motion to the earth, and supposing the sun to be at rest, all the phænomena will be solved. Hence a hint was taken that opened a new and surprizing scene. The earth might be similar to them in other respects. The planets too might be habitable worlds. One cannot help greatly admiring the sagacity of minds, that first formed conclusions so very far from being obvious; as well as the indefatigable industry of astronomers, who originally framed rules for predicting eclipses of sun and moon, which is said to have been done as early as the time of Thales;[[A14]] and must have proved of singular service to emancipate mankind from a thousand superstitious fears and notions, which juggling[juggling] impostors (the growth of all ages and countries) would not fail to turn to their own advantage.
For two or three centuries before and after the beginning of the Christian era, astronomy appears to have been held in considerable repute; yet very few discoveries of any consequence were made, during that period and many ages following.
The ancients were not wanting in their endeavours to find out the true dimensions of the planetary system. They invented several very ingenious methods for the purpose; but none of them were at all equal, in point of accuracy, to the difficulty of the problem. They were therefore obliged to rest satisfied with supposing the heavenly bodies much nearer to the earth than in fact they are, and consequently much less in proportion to it. Add to this, that having found the earth honoured with an attendant, while they could discover none belonging to any of the other planets, they supposed it of far greater importance in the Solar System than it appears to us to be: And the more praise is due to those few, who nevertheless conceived rightly of its relation to the whole.
Tycho took incredible pains to discover the parallax of Mars in opposition; the very best thing he could have attempted in order to determine the distances and magnitudes of the sun and planets. But telescopes and micrometers were not yet invented! so that not being able to conclude any thing satisfactory from his own observations, he left the sun’s parallax as he found it settled by Ptolemy, about twenty times too great. And even after he had reduced to rule the refraction of the atmosphere, and applied it to astronomical observations, rather than shock his imagination by increasing the sun’s distance, already too great for his hypothesis, he chose to attribute a greater refraction to the sun’s light, than that of the stars, altogether contrary to reason; that so an excess of parallax might be balanced by an excess of refraction. Thus when we willingly give room to one error, we run the risk of having a whole troop of its relations quartered upon us. But Kepler afterwards, on looking over Tycho’s observations, found that he might safely reduce the sun’s parallax to one minute; which was no inconsiderable approach to the truth. Alhazen,[[A15]] an Arabian, had some time before, discovered the refraction of light in passing through air; of which the ancients seem to have been entirely ignorant. They were indeed very sensible of the errors it occasioned in their celestial measures; but they, with great modesty, attributed them to the imperfections of their instruments or observations.
I must not omit, in honour of Tycho, to observe that he first proved, by accurate observations, that the comets are not meteors floating in our atmosphere, as Aristotle,[[A16]] that tyrant in Philosophy, had determined them to be, but prodigious bodies at a vast distance from us in the planetary regions; a discovery the lateness of which we must regret, for if it had been made by the ancients, that part of Astronomy (and perhaps every other, in consequence of the superior attention paid to it), would have been in far greater perfection than it is at this day.
I had almost forgot to take notice of one important discovery made in the early times of Astronomy, the precession of the equinoxes. An ancient astronomer, called Timocharis, observed an appulse of the Moon to the Virgin’s Spike, about 280 years before the birth of Christ. He thence took occasion to determine the place of this star, as accurately as possible; probably with a view of perfecting the lunar theory. About four hundred years afterwards, Ptolemy, comparing the place of the same star, as he then found it, with its situation determined by Timocharis,[[A17]] concluded the precession to be at the rate of one degree in an hundred years; but later astronomers have found it swifter.
Whatever other purposes this great law may answer, it will produce an amazing change in the appearance of the heavens; and so contribute to that endless variety which obtains throughout the works of Nature. The seven stars that now adorn our winter skies, will take their turn to shine in summer. Sirius, that now shines with unrivalled lustre, amongst the gems of heaven, will sink below our horizon, and rise no more for very many ages! Orion too, will disappear, and no longer afford our posterity a glimpse of glories beyond the skies! glittering Capella, that now passes to the north of our zenith, will nearly describe the equator:[[A18]] And Lyra, one of the brightest in the heavens, will become our Polar Star: Whilst the present Pole Star, on account of its humble appearance, shall pass unheeded; and all its long continued faithful services shall be forgotten! All these changes, and many others, will certainly follow from the precession of the equinoxes; the cause of which motion was so happily discovered and demonstrated by the immortal Newton: A portion of whose honors was nevertheless intercepted by the prior sagacity of Kepler, to whom I return.