Part 96

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, 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.

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Footnote A15:

Alhazen was one of the greatest of the Arabian astronomers. He went, about the year 1100, to Spain, where many of his nation had established themselves in the eighth century, and carried thither their knowledge of astronomy; yet, from the year 800 down to about 1300, science remained shrowded with the darkest ignorance, throughout Europe.

Mr. Lalande observes, that the theory of Refractions is an important one, in astronomy; although it was considered of little consequence until the time of Alhazen. W. B.

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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, 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.

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Footnote A16:

Aristotle, as though he had been of the race of the Ottomans, thought he could not reign except he first killed all his brethren. Insomuch as he never nameth or mentioneth an ancient author or opinion, but to confute or reprove. Bacon. Advancement.

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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, concluded the precession to be at the rate of one degree in an hundred years; but later astronomers have found it swifter.

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Footnote A17:

Timocharis of Alexandria endeavoured, with Aristillus, a philosopher of the same school, to determine the places of the different stars in the heavens, and to trace the course of the planets. Dr. Lempriere places him 294 years before Christ; and the Abbé Barthelemy has inserted his name in the list of illustrious men, who flourished in the fourth century before the Christian era: he probably lived some time after the commencement of that century. W. B.

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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: 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.

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Footnote A18:

By its peculiar situation it will continue to do so for a long time.

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Kepler’s love of harmony encouraged him to continue his pursuits, in spite of the most mortifying disappointments, until he discovered that admirable relation which subsists between the periodic times of the primary planets, and their distances from the sun; the squares of the former being as the cubes of the latter. This discovery was of great importance to the perfection of Astronomy; because the periods of the planets are more easily found by observation, and from them their several relative distances may be determined with great accuracy by this rule. He likewise found from observation, that the planets do not move in circles; but in elipses, having the sun in one focus. But the causes lay hid from him, and it was left as the glory of Sir Isaac, to demonstrate that both these things must necessarily follow from one simple principle, which almost every thing in this science tends to prove does really obtain in Nature: I mean, that the planets are retained in their orbits by forces directed to the sun; which forces decrease as the squares of their distances encrease.

Kepler also discovered that the planets do not move equally in their orbits, but sometimes swifter, sometimes slower; and that not irregularly, but according to this certain rule; That in equal times, the areas described by lines drawn from the planet to the sun’s centre, are equal. This, Sir Isaac likewise demonstrated must follow, if the planet be retained in its orbit by forces directed to the sun, and varying with the distance in any manner whatsoever. These three discoveries of Kepler, afterwards demonstrated by Newton, are the foundation of all accuracy in astronomical calculations.

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Footnote A19:

According to Lalande, Kepler was as celebrated in astronomy by the consequences he drew from the observations of Tycho Brahé, as the latter was for the immense mass of materials which he had prepared for him: and the Abbé Delaporte (in his Voyageur François) represents him as precursor of Descartes in opticks, of Newton in physicks, and as a law-giver (“legislateur”) in astronomy.

John Kepler, for this was the name of that famous mathematician, was born at Wiel, in the duchy of Wirtemberg, in the year 1571; and the Abbé Delaporte says, his family was illustrious. He died at Ratisbon, in 1630. W. B.

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