An original theory or new hypothesis of the universe /

Concerning the Nature, Magnitude, and Motion of the Planetary Bodies round the Sun, &c.

SIR,

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.

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.

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.

PLATE IV.

Is a true Delineation of the solar System, with the Trajectories of three of the principal Comets, whose Periods and Orbits have been accurately determined, and are represented in their true Proportion and Position to one another, and the Order of the Planets round the Sun, marked with their respective Characters, viz. ♄, for Saturn, ♃, Jupiter, ♂, Mars, ♁, the Earth, ♀, Venus, and ☿, Mercury. The Scale being nearly five hundred and eighteen Millions of Miles to an Inch.

PLATE V.

Is a true Projection of the System of the known Comets; in which are represented nine of the chief Trajectories, from their Aphelii to their Perihelii, all in just Proportion and Position to the Orbits of Saturn and Jupiter, which are also represented by the two concentric Circles, supposed to be drawn round the Sun as their Center.

The Ellipsis, or Trajectory, marked A, shews the Position and Path of the Comet which appeared in the Year 1684, whose Period is supposed to be about fifty Years, and has been observed within the Region of the Planets once.

That mark'd B, is the Way of the Comet of 1682;
The Period conjectured to be about seventy-five
Years and a half, and has been observed thrice.
  C, Way of the Comet of 1337;
The Period about 100 Years, observed once.
  D, That of the Comet of 1661;
The Period about 129 Years, observed twice.
  E, Tract of the Comet of 1618;
The Period about 160 Years, observed once.
  F, Way of the Comet of 1677;
The Period about 200 Years, observed once.
  G, Way of the Comet of 1744;
The Period about 300 Years, observed once.
  H, Way of the Comet of 1665;
The Period about 400 Years, observed once.
  I, Way of the Comet of 1680;
The Period about 575 Years, observed thrice.

The Scale of this System is equal to one Third of the former.

Here I must observe to you, as a Thing I judge may prove of great Consequence with regard to the System of Comets, which is as yet very imperfect: That I am strongly of Opinion, that the Comets in general, through all their respective Orbits, describe one common Area, that is to say, all their Orbits with regard to the Magnitude of their proper Planes, are mathematically equal to one another; which, if it once could be proved, and confirmed by Observation, the Theories of all the Comets that have been justly observed, might easily be perfected, and their Periods at once determined, which now we can only guess at, or may wait whole Ages for more Certainty of. What leads me to believe, that this may prove to be really the Case is this.

I find by Calculation, that the Orbits of the two last Comets, whose Elements have been most corrected by Sir Isaac Newton and Dr. Hally, are to one another, according to their Numbers, nearly as [J]13 to [K]17, notwithstanding one of them is one of the most erratick that ever came under our Observation; and the other one of the most neighbouring to the Sun.

But it is well known to all Mathematicians, that the first of these Comets moved in so eccentric a Trajectory, that the least Error in its almost incredible Proximity to the Sun will produce a very sensible Difference in the Area of the Orbit: And accordingly, if we moderate the Perihelion Distance of this Comet, by making it but 1000 instead of [L]612, which is but increasing it a 1/35000th Part of the great Radius of the Orbit, (which is an Error every Astronomer will readily grant is very easily made) and we shall find the Orbits of the said two Comets to be exactly equal.

Further, I must inform you, that the Comet of 1682, which the above is compared with, seems to have been so accurately observed, that it does not appear to have altered its Perihelion Distance half a 68th Part in one intire Revolution. Now, if we can with any Show of Reason, and a Probability on our Side, bring the Areas of these two extream Comets, as I may call them, to an Equality, sure we may conclude, it is a Subject highly worthy to be more considered and enquired into.

PLATE VI.

Is a true Representation of the satellite Systems, proportionable to one another, and to the Orb of the Sun's Body, that a just Idea of the Distances of those secondary Planets, may be easier had from their respective primary ones.

S represents the solar Body with its Atmosphere. Fig. 1. is the System of Saturn from the same Scale. Fig. 2. that of Jupiter from ditto. And Fig. 3. the Orbit of the Moon round the Earth, in the same Proportion.

But as you can have but a very imperfect Idea of the Magnitude of these last Circles, with regard to the Body of the Earth or Moon,

PLATE VII.

Is a true Projection of their real Globes, at their proper Distance from each other, with their common Center of Gravity, and the Point and Line of equal Suspension betwixt them, viz.

A, represents the Globe of the Earth.
B, that of the Moon.
C, Point, and C D, Line of equal Suspension betwixt them.
E, Common Center of Gravity, which describes the Orbis Magnus.
E, F, and B, G, is the Orbit of the Moon.

Farther, that nothing may be wanting to give a true Notion of the whole together,

PLATE VIII.

Is a proportional Drawing of all the primary and secondary Planets together, distinguished by their Characters, proper to attend a Globe of twelve Inches Diameter, such a one being supposed to represent the Sun.

PLATE IX.

Is an exact Scheme of the principal known Comets, in just Proportion, to the Globe of the Earth represented at A, with the Nucleus, and Part of the Tail of the Comet of 1680, B, as it was observed in its Assent from the Sun, viz. a a the Comet's natural Atmosphere, z z z, the Denser Matter winding itself into the Axis of the Train x x, the inflam'd Atmosphere and Tail dilated near the Sun. C, represents the Ball of the Comet of 1682, D, that of 1665, E, that of 1742, and F, the Head of the Comet of 1744.

And again, that you may have some Notion of the apparent Magnitudes of all these Planets and Comets, &c. as they appear at the Earth,

PLATE X.

Represents the Sun and Moon in the just Proportion of their mean Diameters, with two of the Comets A and B, and the five erratick Planets, as they are observed at the Earth, in a middle State of their Distances from it.

For a more full and particular Description of all the Parts of the solar System, and of the home Elements of Astronomy in general, I refer you to my Clavis Cœlestis, &c. where every thing concerning the Planets, Comets, and Stars; and their real and apparent Motions, are at large represented, explained, and accounted for, for the Benefit of such as have not made the Mathematicks their regular Study.

Now, to convince you that the Planets are all in their own Nature no other than dark opaque Bodies, reflecting only the borrowed Light of the Sun, I must recommend to your Observation, this natural and simple Experiment, which almost any Opportunity of seeing the Moon a little before the Full, will put into your Power to make; but best and easiest when the Sun is in any of the North Signs, i. e. in Summer.

At such a time, the Sun being near setting, the Moon will appear in the eastern Hemisphere; and if there be any bright Clouds northward, or southward near her, you will plainly perceive, that the Light of the one is of the same Nature with that of the other; I mean the Light of the Moon, and that of the Cloud. To me there never appeared any Difference at all; and I am perswaded, were you to make but two or three Observations of this kind, which is from Nature itself, a sort of ocular Demonstration, you cannot fail of being convinced, that the Moon's Light, such as it is, without Heat, can possibly proceed from no other Cause than that which illumines the Cloud: For if the Clouds, whose Composition we know to be but a thin light Fluid, formed of condensed Vapours only, is capable of remitting so great a Lustre, how much more may we not allow the Moon, which, Length of Time, and many other Circumstances, have long confirmed to be a durable and solid Body.

The Increase of her Lustre, indeed, during the Absence of the Sun from us, to a less penetrating Genius than your's, may possibly afford some trifling Ground of Objection to the above Conclusions, as being drawn from the Phænomena of Day-light only; by reason in the Night, we have no Clouds in equal Circumstances to compare with her.

But this I need not tell you, is all owing to her being seen through a darker Medium, and not to any real Increase of natural Light emitted from the Sun. As a Proof of which, were it necessary, you need only, shut out the Rays of the Atmosphere, by the Help of a sufficiently long Tube; and the Moon, or any other celestial Body, will appear through it, as bright in the Day-time as in the Night.

Thus all light Bodies of inferior Lustre, whether shining by their own natural Radiences, or by a borrowed Reflection, partake of the same Advantage, when removed from the more potent Influence of a superior one; and hence it is, that the [M]Aura Ætherea shines out most manifest, when the Body of the Sun himself is hid, the Stars, and the Via Lactea most lively and numerous in the Absence of the Moon, and those Exhalations, or Meteors, vulgarly called Falling-stars, become only visible (like Glow-worms) in the Night.

Here it may not be improper to tell you, that the Clouds are to us in effect no other than as so many Moons, whereby we have our artificial Day prolonged to us several Hours after the Sun is set, and likewise produced as much sooner before he rises; and were they to ascend by still stronger Power of Exhalation to an Elevation, all round the Atmosphere, so as to form a Sphere equal to four Times the Globe of the Earth, there would then be no such Thing as real nocturnal Darkness to any Part of the World.

The lunar Light then we may very justly conclude, proceeds originally from the Sun: And notwithstanding many more Arguments might be drawn from the Demonstration of her Phases, Eclipses, &c. to prove it, yet none of them need here be added, to what has been already said, to convince you of the Truth of it. This being granted, let us now consider what Effect this, or a like Quantity of borrowed Light, would have, when removed to a much greater Distance.

I may, I think, suppose, that you know so much of Opticks as to understand, that all visible Objects apparently decrease in Magnitude, as their Distance from the Eye increases. Consequently, that, if the Moon's Orbit was placed as far again from the Earth as it really is, her Globe, or rather Disk, would then seem to be but half as big as to us she now appears to be, and of course still farther, were she placed at ten times the Distance she is known to revolve at, her apparent Diameter would be reduced to a tenth Part only of what it now appears to be in her present Orbit, that is, one hundred Times less in visible Magnitude than her neighbouring Disk is found to be where it now is seen. And such, but something less, the two Planets Venus and Jupiter, which are frequently, in their Turns, our Morning and Evening Stars, appear to be through a common Telescope.

Now these two Planets, together with the other three, which we find moving in regular Orbits round the Sun, are all found subject to the same [N]Changes of Phænomena, in their various Aspects with the Sun; and who can doubt but that they are all of the same or like Nature? But you'll say, perhaps, how are we sure that Venus and Jupiter have no native Light of their own, since many of the ancient Philosophers, and in particular Anaximander, allowed even the Moon to have some; and besides, in Philosophy, as well as in Logick, I think you hold there is no proving a Negative, at least at such a Distance.

To make you conceive the Impossibility of such a Light, and next to a Demonstration, convince you of the Unnaturalness of such a Supposition, I must put you in mind, that some time ago, when I was last in the Country with you, I think it was about the latter End of Autumn, near the Winter Solstice, as we were walking one Evening, I bid you take notice of the Moon, which was then near setting, and about two Days old. You may remember, her whole Globe appeared to us very conspicuously within a manifest Circle. You immediately told me, that that kind of Phænomenon the Country People called a Stork, or the old Moon in the new one's Arms. This I then endeavoured to explain to you, and I think made you sensible it was intirely an Effect of the Earth's, and an Appearance always to be expected at that Time of the Year. The Earth being then in the State of a Full-Moon to that Part of the lunar Orbit, and near her Perihelion, at which time, the Earth sends back a Reflection to the [O]Moon twenty-five times more potent than that of the Moon to us.

Now the Planet Venus, from undeniable Principles of Geometry, is allowed to be nearly such another Globe as the Earth is; and since the Earth, as I have just now related, is found to reflect much more Light to the Moon, by reason of her superior Magnitude, than the Moon can possibly reverberate to Earth again; and since also 'tis plain, the Earth has no Light of its own, why then should we imagine Venus to be endowed with a Lustre, which we can prove to be no more than a similar Body, and governed by the same Laws as the Earth is?

Anaximander's Mistake, in supposing the Moon in some small Degree a radiant Body of itself, lay, in not considering, that the faint Illumination here described, and visible all over her Globe, soon after almost every Conjunction with the Sun; and probably in Eclipses, also proceeded from the Earth; but the thing I think is too evident to expect any sort of Contradiction, therefore I hope you will admit it as a Truth, and consequently take it for granted, that the planetary Bodies in general, are meer terrestrial, if not terraqueous Bodies, such as this we live upon; which is the Thing I have chiefly in this Letter attempted to demonstrate, or have rather explained; and now I hope, for the future, you will receive the Idea of a Plurality of Worlds more favourably, and look upon astronomical Conjectures in a less ridiculous Light than you used to do, especially since you must allow, they give our unlimited Imaginations a like all endless Field of Contemplation, not only full of the wonderful Works of Nature, but also of a visible Providence.

I think I cannot conclude this Letter to you more properly, than with the following fine Lines of Mr. Addison's from the Spectator, Vol. VI. No. 465, which I hope you are not so polite as to look upon as an unfashionable Quotation.

And am, &c.