Of the superior Planets

Mars is the fourth Planet in the System, and is the first of the superior ones; he is of a fiery red Colour, and appears thro’ the Telescope, to give a confus’d glaring Light, (not unlike the Nebuæ or cloudy Stars); in Opposition to the Sun he comes very near the Earth, and then shines almost as bright as Venus.

Mr Huygens, in the year 1656, thro’ his great Telescope of 130 Feet, observed several cloudy Spots to cross his Disk in right Lines, like Belts or Fascia; from whence he found him to have a Rotation round his Axis, in 24 Hours, 39 Minutes, and is judged to have an Atmosphere like our Earth. (See the Scheme)

The Sun appears to Mars, as much less than to us, as Venus he appearas bigger; Venus and the Earth appear to him as Mercury and Venus appear to us, but Mercury by reason of his Nearness to the Sun, never comes under his Observation.

His Axis is nearly perpendicular to the Plain of his Orbit, and the Velocity of his equatorial Surface round it, is nearly 500 Miles per Hour.

His prime geocentric Perigration, is performed in the Space of 79 Years.

Mars is alternately, either in Conjunction or Opposition to the Sun, consequently in Apogeon or Perigeon, nearly, every 390 Days; his mean heliocentric Daily Motion, is 31^m. 27^2ds. and sometimes in the Absence of Venus, he performs (but not so naturally) her Office of Morning and Evening Star.

At the End of the Year 1700,

But some give the Node and the Aphelion a Motion of 43^2ds. per Year.

His greatest horizontal Parallax, is 37^2ds. 34^3ds.

Jupiter is the fifth Planet in the System, and the second superior one to the Earth; he is of a bright silver Colour, and much bigger, both in Reality and Appearance than any of the other revolving Planets; but not so resulgent as Venus. He is observed to have several dark Spaces upon his Disk like Zones or Belts about him; and is found to have a Rotation round his Axis, in 9^h. 56^m. by which violent Motion in so large a Body, his extream equatorial Parts move with a Velocity of above 25,000 Miles per Hour. His axis is nearly perpendicular to the Plain of his Orbit, and his Polar Diameter, to that of his Equator is nearly as¹ 9 is to 10⅓.

Jupiter is so far removed from the Neighbourhood of the Sun, that he sees, (i.e. his Inhabitants) neither Mars nor the Earth, Venus nor Mercury, to him the Sun appears five times less in Diameter, than to us at the Earth, and his horizontal Prallax is about 20^2ds.

When Venus becomes heliacal, this Planet also often performs the Office of our Morning and Evening Star, according as he happens to be oriental or occidental to the Sun.

And sometimes when he is in the one Situation and Venus in the other, one of them will be the Evening and the other the Morning Star, for the same nocturnal Revolution.

His prime geocentric Perigration, is performed in the Space of 83 Years.

Every 199 Days, Jupiter is either in Opposition or Conjunction with the Sun, and his mean heliocentric Motion is 4^m. 59^2ds. per Day.

At the End of the Year 1700,

Jupiter is observed to have four Satellites or lesser Bodies attending his Globe, each performing to him the Office of Moon, like that of our Earth.

Mr Bradley makes the Diameter of the first \(\frac{1}{24}\) of the 2d \(\frac{1}{27}\) of the 3d \(\frac{1}{21}\) and of the 4th \(\frac{1}{28}\) of Jupiter’s Diameter.

Their Periods and Distances are found to be follows, viz.

From the periodic Times they are found to be, the first, 5.667, the 2d, 9.017, the 3d, 14.384, and the 4th, 25.3.

Their Revolutions round him are nearly, thus,

Their greatest Elongations from him, at a mean Distance from the Earth,

And their Velocities round him

The Radius of Jupiter’s Disk appears

And were they to drop from their Orbits,

All these little Planets, separately or together become invisible to an Eye at the Earth, in three different Situations or Positions, with respect to the Body of Jupiter; First by being eclipsed in the Shadow, secondly, in passing behind the Body of Jupiter, and thirdly, by being before him in crossing his Disk.

They all move nearly in the same Plain, inclin’d to that of Jupiter’s Motion, in an Angle of 2^deg. 55^min. and the ascending Nodes are in 11^deg. 30^min. of ♒︎, but were observed about the Year 1700, to be in 15^deg. of ♒︎. But the fourth Satellite has been observed to have something less Latitude, the Inclination of its Plain, being but about 2^deg. 40^min. and also has been seen to pass sometimes free from the Shadow, &c. as it did in the Year 1684, October, November, and December. (See Philosophical Trans. № 154)

The Immersians of his Satellites, are only visible at the Earth, when he is oriental to the Sun, i.e. from the Opposition to the Conjunction, and their Emersions when occidental, i.e. from the Conjunction to the Opposition.

The different Periods of Jupiter’s Moons, produce four kinds of Months,

To two Zones of Saturn’s Body, each 53^deg. broad, this Ring is never seen at all, by Reason of its great Parallax.

To Saturn the Sun appears ten times less in Diameter than to the Earth, and Jupiter is the only Planet (except the Sun) visible to his Inhabitants; Jupiter appears to him as Venus does to the Earth. His prime Revolution or geocentric Perigration is perform’d in 59 Years.

Every 189 Days, he is either in Opposition or Conjunction with the Sun, and his mean heliocentric Motion, is 2^min. 1^2d. per Day.

At the End of the Year 1700,

Saturn has five Satellites or Moons attending him, and they are observed to be distant from his Center nearly, as follows,

But by the periodic Times they are found to be 1.93, 2.47, 3.45, 8, and 23.35.

In English Miles,

Their periodical Revolutions round him are nearly perform’d as follows,

And their Velocities round him respectively

Their greatest Elongation from the Body of Saturn, as seen from the Earth, at a mean Distance, are

And the Radius of Saturn’s Disk,

The Least of those Planets are very near as big as our Earth, and they all move in Orbits near the Plain of Saturn’s Ring.

In some Positions of Saturn, these Satellites have their Eclipse, Transits, and Occultations, and require the same Situations as of those of ♃ to be visible.

Saturn’s Zodiac, is about 60^deg. broad, but every Satellite is not visible to all Parts of his Globe, nor at all times the Sun itself.

The various Revolutions of these Moons, produce to Saturn 5 Sorts of Months, of which his Year respectively consists, viz.

Of the Parallax of the Planets Orbits

To every superior Planet, the inferior ones being circumscrib’d within their Orbits can never come to an Opposition with the Sun, but will always be found in the same Quadrant of the Heavens with him, and can never be farther from him, than the Angle subtended by the Radius of their respective Orbits form’d by the Tangent to the Eye of the Observer.

This is called the Extent of the Sector of Vision, and is always equal to the greatest Elongation of a Planet, or the right Parallax of its Orbit.

And may always thus be found,

As any superior Planet’s Distance from the Sun,
Is to the Radius, or Sine of the Quadrant;
So is the inferior Planet’s Distance from the Sun,
To the Sine of its Elongation, or right Parallax.

Thus we find, that to an Eye at the Planet Venus,

The greatest Elongation of Mercury,

In Aphelion is 40^deg. 35^min.

And the least Semisector of Vision of the same Planet,

In Perihelion 24^deg. 57^min.

And to an Eye at the Earth, those of

And of

To an Eye at Mars.

To an Eye at Jupiter,

And to an Eye at Saturn,

1. Or as 33 Sec. 25 Thirds to 36 Sec. 25 Thirds. His mean apparent Diameter perfect, is 37 Sec. 15 Thirds.