PLANET

, literally a wanderer, or a wandering star, in opposition to a star, properly so called, which remains sixed. It is a celestial body, revolving around the sun, or some other planet, as a centre, or at least as a focus, and with a moderate degree of excentricity, so that it never is so much farther from the sun at one time than at another, but that it can be seen as well from one part of its orbit as another; as distinguished from the comets, which on the farthest part of their trajectory go osf to such vast distances, as to remain a long time invisible.

The Planets are usually distinguished into Primary and Secondary.

Primary Planets, called also simply Planets, are those which move round the sun, as their centre, or focus of their orbit. Such as Mercury, Venus, the Earth, Mars, Jupiter, Saturn, the Georgian or Herschel, and perhaps others. And the

Secondary Planets, are such as move round some primary one, as their centre, in the same manner as the primary ones do about the sun. Such as the moon, which moves round the earth, as a secondary; and the three, Jupiter, Saturn, and Georgian, have each several secondary Planets, or moons, moving round them.

Till very lately the number of the primary Planets was esteemed only six, which it was thought constituted the whole number of them in the solar system; viz, Mercury, Venus, the Earth, Mars, Jupiter, and Saturn; all of which it appears were known to the astronomers of all ages, who never dreamt of an increase to their number. But a seventh has been lately discovered, by Dr. Herschel, viz, on March the 13th, 1781, lying beyond all the rest, and now called the Georgian, or Herschel: and possibly others may still remain undiscovered to this day.

The primary Planets are again distinguished into Superior and Inferior.

The Superior Planets are those that are above the earth, or farther from the sun than the earth is; as, Mars, Jupiter, Saturn, and the Georgian or Herschel. And

The Inferior Planets are those that are below the earth, or that are nearer the sun than the earth is; which are Venus and Mercury.

The Planets were represented by the same characters as the chemists use to represent their metals by, on account of some supposed analogy between those celestial and the subterraneous bodies. Thus,

Mercury, the messenger of the Gods, represented by <*>, the same as that metal, imitating a man with wings on his head and feet, is a small bright planet, with a light tinct of blue, the sun's constant attendant, from whose side it never departs above 28°, and by that means is usually hid in his splendor. It performs its course around him in about 3 months.

Venus, the goddess of love, marked <*>, from the figure of a woman, the same as denotes copper, from a slight tinge of that colour, or verging to a light straw colour. She is a very bright Planet, revolving next above Mercury, and never appears above 48 degrees from the sun, finishing her course about him in about seven months. When this Planet goes before the sun, or is a morning star, it has been called Phosphorus, and also Lucifer; and when following him, or when it shines in the evening as an evening star, it is called Hesperus.

Tellus, the Earth, next above Venus, is denoted by <*>, and performs its course about the sun in the space of a year.

Mars, the god of war, characterized <*>, a man holding out a spear, the same as iron, is a ruddy fierycoloured Planet, and finishes his course about the sun in about 2 years.

Jupiter, the chief god, or thunderer, marked <*>, to represent the thunderbolts, denoting the same as tin, from his pure white brightness. This Planet is next above Mars, and completes its course round the sun in about 12 years.

Saturn, the father of the Gods, is expressed by <*>, to imitate an old man supporting himself with a staff, and is the same as denotes lead, from his feeble light and dusky colour. He revolves next above Jupiter, and performs his course in about 30 years.

Lastly, the Georgian, or Herschel, is denoted by <*>, the initial of his name, with a cross for the christian Planet, or that discovered by the christians. This is the highest, or outermost, of the known Planets, and revolves around the sun in the space of about 90 years.

From these descriptions a person may easily distinguish all the Planets, except the last, which requires the aid of a telescope. For if after sun-set he sees a Planet nearer the east than the west, he may conclude it is neither Venus nor Mercury; and he may determine whether it is Saturn, Jupiter, or Mars, by the colour, light, and magnitude: by which also he may distinguish between Venus and Mercury.

It is probable that all the Planets are dark opake bodies, similar to the earth, and for the following reasons.

1. Because, in Mercury, Venus, and Mars, only that part of the disk is found to shine which is illuminated by the sun; and again, Venus and Mercury, when between the sun and the <*>arth, appear like maculæ or dark spots on the sun's face: from which it is evident, that those three Planets are opake bodies, illuminated by the borrowed light of the sun. And the same appears of Jupiter, from his being void of light in that part to which the shadow of his satellites reaches as well as in that part turned from the sun: and that his satellites are opake, and reslect the sun's light, like the moon, is abundantly shewn. Moreover, since Saturn, with his ring and satellites, and als<*> Herschel, with his satellites, only yield a faint light, considerably fainter than that of the rest of the Planets, and than that of the fixed stars, though these be vastly more remote; it is past a doubt that these Planets too, with their attendants, are opake bodies.

2. Since the sun's light is not transmitted through Mercury or Venus, when placed against him, it is plain they are dense opake bodies; which is likewise evident of Jupiter, from his hiding the satellites in his shadow; and therefore, by analogy, the same may be concluded of Saturn and Herschel.

3. From the variable spots of Venus, Mars, and Jupiter, it is evident that these Planets have a changeable atmosphere; which sort of atmosphere may, by a like argument, be inferred of the satellites of Jupiter; and| therefore, by similitude, the same may be concluded of the other Planets.

4. In like manner, from the mountains observed in the moon and Venus, the same may be supposed in the other Planets.

5. Lastly, since all these Planets are opake bodies, shining with the sun's borrowed light, are furnished with mountains, and are encompassed with a changeable atmosphere; they consequently have waters, seas &c, as well as dry land, and are bodies like the moon, and therefore like the earth. And heuce, it seems also probable, that the other Planets have their animal inhabitants, as well as our earth has.

Though all the primary Planets revolve about the sun, their orbits are not circles, but ellipses, having the sun in one of the foci. This circumstance was first found out by Kepler, from the observations of Tycho Brahe: before that, all astronomers took the planetary orbits for eccentric circles.

The Planes of these orbits do all intersect in the sun; and the line in which the plane of each orbit cuts that of the earth, is called the Line of the nodes; and the two points in which the orbits themselves touch that plane, are the Nodes; also the angle in which each plane cuts that of the ecliptic, is called the Inclination of the plane or orbit.—The distance between the centre of the sun, and the centre of each orbit, is called the excentricity of the Planet, or of its orbit.

The motions of the primary Planets are very simple, and tolerably uniform, as being compounded only of a projectile motion, forward in a right line, which is a tangent to the orbit, and a gravitation towards the sun at the centre. Besides, being at such vast distances from each other, the effects of their mutual gravitation towards one another are in a considerable degree, though not altogether, insensible; for the action of Jupiter upon Saturn, for ex. is found to be 1/204 of the action of the sun upon Saturn, by comparing the matter of Jupiter with that of the sun, and the square of the distance of each from Saturn. So that the elliptic orbit of Saturn will be found more just, if its focus be supposed not in the centre of the sun, but in the common centre of gravity of the sun and Jupiter, or rather in the common centre of gravity of the sun and all the Planets below Saturn. And in like manner, the elliptic orbit of any other Planet will be found more accurate, by supposing its focus to be in the common centre of gravity of the sun and all the Planets that are below it. But the matter is far otherwise, in respect of the secondary Planets: for every one of these, though it chiefly gravitates towards its respective primary one, as its centre, yet at equal distances from the sun, it is also attracted towards him with an equally accelerated gravity, as the primary one is towards him; but at a greater distance with less, and at a nearer distance with greater: from which double tendency towards the sun, and towards their own primary Planets, it happens, that the motion of the satellites, or secondary Planets, comes to be very much compounded, and affected with various inequalities.

The motions even of the primary Planets, in their elliptic orbits, are not equable, because the sun is not in their centre, but their focus. Hence they move; sometimes faster, and sometimes slower, as they are nearer to or farther from the sun; but yet these irregularities are all certain, and follow according to an immutable law. Thus, the ellipsis PEA &c representing the orbit of a Planet, and the focus S the sun's place: the axis of the ellipse AP, is the line of the apses; the point A, the higher apsis or aphelion; P the lower apsis or perihelion; CS the eccentricity; and ES the Planet's mean distance from the sun. Now the motion of the Planet in its perihelion P is swiftest, but in its aphelion A it is slowest; and at E the motion as well as the distance is a mean, being there such as would describe the whole orbit in the same time it is really described in. And the law by which the motion in every point is regulated, is this, that a line or radius drawn from the centre of the sun to the centre of the Planet, and thus carried along with an angular motion, does always describe an elliptic area proportional to the time; that is, the trilineal area ASB, is to the area ASG, as the time the Planet is in moving over AB, to the time it is in moving over AG. This law was first found out by Kepler, from observations; and has since been accounted for and demonstrated by Sir Isaac Newton, from the general laws of attraction and projectile motion.

As to the periods and velocities of the Planets, or the times in which they perform their courses, they are found to have a wonderful harmony with their distances from the sun, and with one another: the nearer each Planet being to the sun, the quicker still is its motion, and its period the shorter, according to this general and regular law; viz, that the squares of their periodical times are as the cubes of their mean distances from the sun or focus of their orbits. The knowledge of this law we owe also to the sagacity of Kepler, who found that it obtained in all the primary Planets; as astronomers have since found it also to hold good in the secondary ones. Kepler indeed deduced this law merely from observation, by a comparison of the several distances of the Planets with their periods or times: the glory of investigating it from physical principles is due to Sir Isaac Newton, who has demonstrated that, in the present state of nature, such a law was inevitable.

The phenomena of the Planets are, their Conjunctions, Oppositions, Elongations, Stations, Retrogradations, Phases, and Eclipses; for which see the respective articles.

For a view of the comparative magnitudes of the Planets; and for a view of their several distances, &c; see the articles Orbit and Solar System, as also Plate xxi, fig. 1.

The following Table contains a synopsis of the distances, magnitudes, periods, &c, of the several Planets, according to the latest observations and improve- ments.|

A Planet's motion, or distance from its apogee, is called the mean anomaly of the Planet, and is measured by the<*> area it describes in the given time: when the Planet arrives at the middle of its orbit, or the point E, the area or time is called the true anomaly. When the Planet's motion is reckoned from the first point of Aries, it is called its motion in longitude; which is either mean or true; viz, mean, which is such as it would have were it to move uniformly in a circle; and true, which is that with which the Planet actually describes its orbit, and is measured by the are of the ecliptic it describes. And hence may be found the Planet's place in its orbit for any given time after it has left the aphelion: for suppose the area of the ellipsis be so divided by the line SG, that the whole elliptic area may have the same proportion to the part ASG, as the whole periodical time in which the Planet describes its whole orbit, has to the given time; then will G be the Planet's place in its orbit sought.