The celestial worlds discover'd

by Christiaan Huygens

Fig. 1 SYSTEMA COPERNICI.

p. 11.

Copernicus’s Syſtem explained. And now becauſe the chief Argument for the Proof of what we intend will be taken from the Diſpoſition of the Planets, among which without doubt, the Earth muſt be counted in the Copernican Syſtem, I ſhall here firſt of all draw two Figures. The firſt is a Deſcription of [[12]]the Orbs the Planets move in, in that order that they are placed round the Sun, drawn as near as can be in their true Proportions, like what you have ſeen in my Clock at home. The ſecond ſhows the Proportions of their Magnitudes in reſpect of one another and of the Sun, which you know is upon that ſame Clock of mine too. In the firſt the middle Point or Center is the Place of the Sun, round which, in an order that every one knows, are the Orbits of Mercury, Venus, the Earth with that of the Moon about it; then thoſe of Mars, Jupiter and Saturn: and about the two laſt the ſmall Circles that their Attendants move in: about Jupiter four, and about Saturn five. Which Circles as well as that of the Moon are drawn larger than their true Proportion would admit, otherwiſe they could not have been ſeen. You may eaſily apprehend the Vaſtneſs of theſe Orbits by this, that the diſtance of the Earth from the Sun is ten or twelve thouſand of the Earth’s Diameters. Almoſt all theſe Circles are in the ſame Plane, declining very [[13]]little from that in which the Earth moves, call’d The Plane of the Ecliptick. This Plane is cut obliquely by the Axis upon which the Earth turns it ſelf round with reſpect to the Sun in 24 Hours, whence ariſe the Succeſſions of Day and Night: The Axis of the Earth always keeping the ſame Inclination to the Ecliptick (except a ſmall Change beſt known to Aſtronomers) while the Earth itſelf is carried in its yearly Courſe round the Sun, cauſes the regular Order of the Seaſons of the Year: as you may ſee in all Aſtronomers Books. Out of which I ſhall tranſcribe hither the Periods of the Revolutions of the Planets, viz. Saturn moves round the Sun in 29 Years, 174 Days, and 5 Hours: Jupiter finiſhes his Courſe in 11 Years, 317 Days, and 15 Hours: Mars his in about 687 Days. Our Year is 365 Days 6 Hours: Venus’s 224 Days 18 Hours: and Mercury’s 88 Days. This is the now commonly received Syſtem, invented by Copernicus, and very agreeable to that frugal Simplicity Nature ſhows in all [[14]]Arguments for the Truth on’t. her Works. If any one is reſolved to find fault with it, let him firſt be ſure he underſtands it. Let him firſt ſee in the Books of Aſtronomers with how much greater Eaſe and Plainneſs all the Motions of the Stars, and Appearances in the Heavens are explained and demonſtrated in this than either in that of Ptolomy or Tycho. Let him conſider that Diſcovery of Kepler, that the Diſtances of the Planets from the Sun, as well of the Earth as the reſt, are in a fix’d certain proportion to the Times they ſpend in their Revolutions. Which Proportion it’s ſince obſerved that their Satellites keep round Jupiter and Saturn. Let him examine what a contradictory Motion they are fain to invent for the Solution of the Polar Star’s changing its Diſtance from the Pole. For that Star in the end of the little Bear’s Tail which now deſcribes ſo ſmall a Circle round the Pole, that it is not above two Degrees and twenty Minutes, was obſerved about 1820 Years ago, in the Time of Hipparchus, to be above 12: [[15]]and will within a few Ages more be 45 Degrees diſtant from it: and after 25000 Years more will return to the ſame Place it is now in. Now if with them we allow the Heavens to be turned upon their own Axis, at this rate they muſt have a new Axis every Day: a Thing moſt abſurd, and repugnant to the Nature of all Motion. Whereas nothing is eaſier with Copernicus than to give us Satisfaction in this Matter. Then he may impartially weigh thoſe Anſwers that Galilæus, Gaſſendus, Kepler, and others have given to all Objections propoſed, which have ſo ſatisfied all Scruples, that generally all Aſtronomers now-a-days are brought over to our Side, and allow the Earth its Motion and Place among the Planets. If he cannot be ſatisfied with all this, he is either one whoſe Dulneſs can’t comprehend it, or who has his Belief at another Man’s Diſpoſal.

Fig. 2.

p. 15.

In the other Figure you have the Globes of the Planets, and of the Sun, repreſented to your Eyes as placed near one another. Where [[16]]The Proportion of the Magnitude of the Planets, in reſpect of one another, and the Sun. I have obſerved the ſame Proportion, of their Diameters to that of the Sun, that I publiſhed to the World in my Book of The Appearances of Saturn: namely, the Diameter of the Ring round Saturn is to that of the Sun as 11 as to 37; that of Saturn himſelf about as 5 to 37; that of Jupiter as 2 to 11; that of Mars as 1 to 166; of the Earth as 1 to 111; and of Venus as 1 to 84: to which I ſhall now add that of Mercury obſerved by Hevelius in the Year 1661, but calculated by my ſelf, and found to be as 1 to 290.

If you would know the way that we came to this Knowledge of their Magnitudes, by knowing the Proportion of their Diſtances from the Sun, and the Meaſures of their Diameters, you may find it in the Book before-mentioned: And I cannot yet ſee any Reaſon to make an Alteration in thoſe I then ſettled, altho’ I will not ſay they are without their Faults. The Lamellæ more convenient than Micrometers. For I can’t yet be of their Mind, who think the Uſe of Micrometers, as they call them, is beyond that of [[17]]our Plates, but muſt ſtill think that thoſe thin Plates or Rods of which I there taught the Uſe, not to detract from the due Praiſes of ſo uſeful an Invention, are more convenient than the Micrometers.

In this proportion of the Planets it is worth while to take notice of the prodigious Magnitude of the Sun in compariſon with the four innermoſt, which are far leſs than Jupiter and Saturn. And ’tis remarkable, that the Bodies of the Planets do not increaſe together with their Diſtances from the Sun, but that Venus is much bigger than Mars.