Of the inferior Planets and the Sun

The Sun’s apparent Diameter has been observed of late Years, in the Absides of the Earth’s Orbit, to be, when greatest, 32^min. 48^2ds. and when least 31^min. 42^2ds. the Anomalia being in the first Place 0, and in the latter 6 Signs. And, at a mean Distance from the Earth, Sir Isaac Newton makes it 32^min. 15^2ds. But if Mouton’s Observations of the Sun’s Diameter throughout the Year 1660, be true, which seem to have been made with great Accuracy, the Sun appears to be least, when, by his Theory, he should not be so, namely in March and September.

For by a Pendulum’s vibrating 9550 times in an Hour.

Now as 955, is to 360;
So is each respective Number here found,
To the Seconds the Sun was in passing the Meridian at that Time.

And,
As 240 Seconds, is to 60 Minutes, or one Degree;
So is the Seconds thus found, to the Sun’s apparent Diameter.

Hence substracting 5^2ds. for the Sun’s momentary Advance, arising from the Earth’s annual Motion in her Orbit,
The Sun’s Diameter in March, will be found 31^min. 40^2ds.
In December, 35^min. 22^2ds. and in June 34^min. 14^2ds.

But in these Quantities, especially the former, the Dilation fo Light must be considered, in Proportion to the Elevation and Degrees of Refraction, which will make it something less, the Sun being seen thro’ a much denser Medium near the Horizon than towards the Zenith.

If these Observations may be depended upon, I think this manifestly proves the Sun’s visible Form to be elliptical, and his Body an oblate Spheroid, which Sir Isaac Newton, in some measure, concludes it must be from its Rotation round its Axis. Else how these apparent Contradictions will be accounted for, I have at present no Idea.

The Sun’s horizontal Parallax, or the Semi-angle under which the Earth would appear to an Eye at the Sun, tho’ much depends upon a true Knowledge of it, we have never yet been able to determine; the Minuteness of its Quantity, arising from the vast Distance of the two Bodies rendering it an almost insurmountable Difficulty to obtain.

Mr Cassini pretends to have made it 9^2ds.

Dr Gregory makes it a Mean betwixt that of Mercury and Venus, viz. 12^2ds. and Sir Isaac Newton in his Principia, has it a Mean betwixt this and the former, viz. 10^2ds. 30^3ds.

Was its true Quantity known, the exact Distance of the Sun from the Earth might, by the simple Solution of a right-angled Triangle, be easily found, which hitherto hath only been attempted by the Parallax of Mars in Perigeon.

The Sun is frequently observed by the Help of Telescopes, (and sometimes with the naked Eye in this Weather), to have many dark Spots upon his Disk, which are supposed to be thrown out from his Body into his Atmosphere, and are carried round him in a Vortex. Some of these Spots continue for a long Space of Time, others are broke into Pieces or separated, while the Eye is viewing them, tho’ the least of them are Masses and Matter, very near as big as our Earth. By the Revolution of these Spots along with the Sun, apparently from East to West, the Sun is found to have a rotation round his Axis in 25¼ Days, from West to East; the North Pole directing to 10 Degrees of Pisces, and the South to 10 Degrees of Virgo, with each 83 Degrees Latitude from the Ecliptic.

This Motion being oblique to the Plain of the Ecliptic, sometimes the South Pole of the Sun, and sometimes the North is shewn to the Earth, and those Spots apparently describe Elliptical Curves, all bending from the visible Pole.

But when the Earth is in 10 Degrees of Gemini, or as many Degrees advanced in Sagittarius, which are the two solar equinoctial Points, both Poles will be so near the Sun’s Verge, as to be neither visible, at which time the Spots will pass over the Disk, in the Direction of a right Line.

The Sun’s circumambient Atmosphere, on all Sides together, form a vast inflamed Region, called, or rather form that Medium from whence proceeds, the Aura Ætherea, and this luminous Sphere, chiefly produces our Morning Aurora, which generally shews itself in a semi-circular Figure, soon after the Dawn or Day-Break, and shines out most manifest in the Summer Mornings and Evenings, a little before Sun-rise, and sometime after he is set.

Besides this agreeable Phænomenon of the Aura Ætherea, there is a third kind of pellucid Matter always attending the Sun, and proceeding from him, called the Zodiac Light. This was first taken Notice of by Mr J. Childrey, about the year 1650; and was afterwards observed by Mr Cassini, in the Year 1683, a little before the vernal Equinox.

This extends from the Sun, both ways along the Ecliptic, to sometimes 60 and sometimes 90 Degrees; it’s greatest visible Breadth is about 30 Deg. and its two Extreams, where both Sides or Surfaces of it meet, form an Angle of 21 Deg. nearly. The Light of it is much like that of the Milky Way, and is pellucid, like the Tail of a Comet, and most conspicuous about the end of February and the Beginning of October.

Mr Cassini imagined this Brightness in the Heavens, to proceed from an infinite Number of little Planets near the Sun, strongly reflecting his Light; Mr Facio supposed the a vast Collection of Corpuscles, surrounding the Sun in the form of a Lens; the greatest Plain passing nearly parallel to the Ecliptic.

But, in all probability, the Particles that seem to fill up and form this luminous Space, may prove to be no other than the broken Pieces, or lesser solar Spots dispers’d all round the Sun, in the Direction of his Equator, and most fully reflecting his Light, as the inferior Planets do at their greatest Elongation.

Hence according to the Obliquity of the Sun’s Axis, it appears to deflect a little towards the North from the Ecliptic, and to be convex that way, the South Part not being so easily seen.

And from this also a Reason may be given, why both Sides of the Sun are not equally luminous.

Its greatest Breadth can never be seen, unless in a total Eclipse of the Sun of the greatest Duration, and could its outward Elongation be nicely observed, where it is most visible, its extremest Distance from the Sun may be found by the Solution of a right-angled Triangle, in the same Manner as that of Venus and Mercury.

The apparent Diameters of the Planets, were they to be seen from the Sun, would be nearly as follows.

The solar Zodiac, in which the Planets would appear to move, is about 15^deg. 40^min. broad.

The Velocity of the Sun’s inflam’d Surface round his Axis, is nearly equal to 4000 Miles per Hours.

For the Planets mean horizontal Parallaxes, as seen from the Sun. (See, to each his apparent Diameter).

Mercury is the first Planet we know of in the System, and, by reason of his great Vicinity to the Sun, is seldom within the Sphere of our Observation, he was seen in the great Eclipse fo 1715; and May the 17th, 1736, at 9 o’clock in the Evening, I saw him in Conjunction with Venus, distant about five Minutes of a Degree.

To Mercury, the Sun and Venus are the only two great Bodies of the Universe. He views Venus and all the rest of the Planets, as we do Saturn, Jupiter and Mars; but Venus shines upon him with great Lustre, and ’tis probable, her great Light in opposition to the Sun, serves him instead of a Moon.

To him the Sun appears above six times greater than to us at the Earth, but his Nychthemeron or natural Day is not yet known.

His geocentric prime Revolution, or Perigration, is perform’d in 46 Years, in which Period of Time he returns to the same Place in the Heavens, and may be found in his former Position in respect of the Sun.

Every 58 Days, he is nearly conjoin’d with the Sun, and his mean heliocentric Motion is 4^deg. 5^min. 32^2ds. per day.

Anno 1700, December the 1st.

There are Astronomers who give the Nodes of this Planet, a yearly Motion of 1^min. 36^2ds. but upon no real Foundation.

His Horizontal Parallax from the Earth is found,

Mercury appears thro’ the Telescope, to be sometimes round, and at aother times gibbous, and in one Revolution round the Sun, has all the Phases of our Moon.

Venus is the second Planet known in the System, and is the most splendid of all the Planetary Planets, being often seen in the Day time, several Hours before Sun-set. At her greates Elongation from the Sun, she generally shines with so great Lustre, as to cast a Shadow upon the Earth, (if the Moon be absent).

When occidental to the Earth, she frequently performs the Office of our Morning Star, because she then rises with the Dawn, and is the last obscur’d by the Sun, in this Quality the Poets call her Lucifer: When oriental to the Earth, she as often becomes our Evening Star, as generally setting with the Close of Day, and being the first visible at the Approach of Night. In this Position she is called by the Poets Hesperus.

Milton takes notice of her superior Lustre, in describing his Evening in Paradise thus,

Now came still Ev’ning on, and Twilight grey
Had in her sober Livery all Things clad;
Silence was pleas’d: Now glow’d the Firmament
With living Saphires: Hesperus, that led
The Starry Host, rode brightest; till the Moon,
Rising in clouded Majesty, at length,
Apparent Queen, unveil’d her peerless Light,
And o’er the Dark her silver Mantle threw.

She is the most regular of all the erratic Stars, but has the greatest geocentric Latitude from the Ecliptic, it being sometimes equal to 9 Degrees.

To Venus, the Sun and the Earth are the greatest apparent Bodies in the Universe, unless she has a Moon, which indeed most Astronomers are now apt to believe she has; for Mr Cassini twice saw a small Star very near her, which he supposed to be a Satellite; first he saw it in the Year 1672, and again it appeared very distinctly thro’ a 34 Foot Telescope, August the 28th, 1686, 16^h. 15^min. P.M. (i.e. about a Quarter past four in the Morning) And since, several very able Astronomers have imagined, they have seen the same: It shew’d the same Phase as Venus did, and appeared to be distant from her, about ⅗ of her Diameter, its Magnitude about ¼ of Venus’s Diameter.

Mercury appears to Venus, as Venus does to Earth, and the Sun appears to Venus near thrise as big as to us.

Venus is conjoin’d with the Sun, every 292 Days, and her mean heliocentric Motion is 1^deg. 36^min. 7^2ds. per Day.

Her prime Revolution geocentric is perform’d in 8 Years.

Anno 1700, December 31.

In some astronomical Tables, there is a Motion given to her Nodes of 37^2ds. per Annum.

Her mean horizontal Parallax, seen from the Earth, hath been made 56^2ds. 31^3ds. but her true greatest (tho’ much wanted) is not yet known.

She is observed to turn round upon her Axis, in 23 Days 8 Minutes.

And the Obliquity of her Poles, to the Plain of the Ecliptic, is judg’d to be 85^deg.