LETTER the THIRD.
Concerning the Nature, Magnitude, and Motion of the Planetary Bodies round the Sun, &c.
SIR,
T
The younger Pliny, if I remember right, somewhere says, that there is, or ought to be, a wide Difference betwixt writing to a Friend, and writing to the Publick: I have indeed pleased myself with the one, but am far from thinking myself qualified for the other; I must therefore rather intreat you, though perhaps you cannot possibly overlook all my Faults as an Author, to excuse them at least in the Friend, and by such kind of unlimited Indulgence, you will give me a much greater Chance to do the Subject some Justice, though I own I despair in this first Attempt, to reconcile every thing I advance to your more cool and impartial Reasoning. But to the Business:
As I have no Ambition to have the Substance of my Theory more admired by you than understood, which is too often the Case in Works of this Nature, I must beg leave to repeat to you Part of a former Discourse, which will refresh in your Ideas the principal Laws of the System of our Sun, and make you properly acquainted with such Things as are necessary to be known in the now-established Astronomy of [I]Copernicus, &c. before I proceed to any new Matter.
[I] Nicolaus Copernicus, stiled by Bulialdus, Vir absolutæ subtilitatis, was a Native of Thorn in Polish Prussia, and Canon of the Church of Frawenburgh; he was Scholar to Dominicus Maria of Ferrara, to whom he was Assistant in his astronomical Observations at Bologne, and Professor of the Mathematicks at Rome, in his noble Work, De Revolutionibus Orbium Cælestium; he fortunately revived, happily united, and formed into an Hypothesis of his own, the several Opinions of Philolaus, Heraclides Ponticus, and Ecphantus Pythagoreus, viz. after the Opinion of Philolaus he made the Earth to move about the Sun, as the Center of its annual Motion; and according to Heraclides and Ecphantus, he likewise gave it a diurnal Rotation round its own Axis: Which System has withstood all Opposition; and as Ricciolus, (though a Dissenter from it) observes, Per damna, per cædes, ab ipso sumit opes, animumque ferro.
The Sun, you are not to learn, is the reputed Center of our Planetary System, and may remember, that the Earth on which we live, and these five following Erratic Stars, viz. Saturn, Jupiter, Mars, Venus and Mercury, have been demonstrated to move round him in the Order and Manner following.
Saturn is found to complete one Revolution round the Sun in twenty-nine Years, one hundred and seventy-four Days, six Hours, and thirty-six Minutes; at the Distance of about seven hundred and seventy-seven Millions of Miles. Jupiter performs a like Revolution in about eleven Years, three hundred and seventeen Days, twelve Hours, and twenty Minutes; distant from the Sun about four hundred and twenty-four Millions of Miles. Mars compleats his Circuit in one Year, three hundred and twenty-one Days, twenty-three Hours, and twenty-seven Minutes; and his mean Distance is about one hundred and twenty-three Millions of Miles.
These three are called superior Planets, as being farther from the Sun than the Earth, and circumscribing its Orbit.
The Earth circumambulates her Orbit in one solar Year, viz. in three hundred and sixty-five Days, five Hours, forty-eight Minutes, and fifty-seven Seconds; at the mean Distance of eighty-one Millions of Miles.
The Radius of Venus's Orbit is about fifty-nine Millions of Miles; and that of Mercury nearly thirty-two Millions, ditto.
The Heliocentric Revolution of Venus, is made in two hundred and twenty-four Days, sixteen Hours, forty-nine Minutes, and twenty-seven Seconds; and that of Mercury, in eighty-seven Days, twenty-three Hours, fifteen Minutes, and fifty-four Seconds. These two last Planets are called inferior Ones, as being circumscribed by the Earth.
The Diameter of the Sun being demonstrated to be nearly seven hundred and sixty-three thousand Miles:
The proportional Magnitudes of all the above Planets will be found nearly as follows, viz.
| The Diameter of the Globe, | |||
| Of Mercury | 4,240 |
| Miles |
| Venus | 7,900 | ||
| the Earth | 7,970 | ||
| Mars | 4,440 | ||
| Jupiter | 81,000 | ||
| and Saturn | 61,000 | ||
Thus much I have thought proper to premise, and for your immediate Inspection, have added the following Schemes, that nothing may be wanting to give a general Idea of the Order of the celestial Bodies in our own System, before I attempt to lead you through the neighbouring Regions of the Stars to the more remote Tracts of Infinity.