29. Eclipse of the Sun is an occulation of part of the sun's disc, occasioned by the interposition of the moon between the earth and the sun. On which account it is by some considered as an eclipse of the earth, since the light of the sun is hid from it by the moon, whose shadow involves a part of the earth.

Eclipses are divided, with respect to the circumstances, into total, partial, annular, central, &c.

Total eclipse is that in which the whole luminary is darkened. Partial eclipse is when only a part of the luminary is eclipsed. Annular eclipse is when the whole is eclipsed, except a ring, or annulus, which appears round the border or edge: this is peculiar to the sun. And a central eclipse is that in which the centres of the two luminaries and the earth come into the same straight line.

QUESTIONS.

What is the orbit of a planet? Of what figure are the orbits of the planets, and in what part of the figure is the sun placed ?

What is the aphelion or higher apsis of a planet's orbit? What is the perihelion or lower apsis of a planet's orbit?

When is the moon in apogee, and when is she in perigee?

What is the line of the apsides?

What is the eccentricity of the orbit of a planet? What is the radius vector of a planet, and by what law is the motion of a planet in every point of its orbit regulated?

By what law are the periodical revolutions of the planets, with respect to their several distances from the sun, regulated?

What is the true anomaly of a planet, and what is its mean anomaly?

What do you call the equation of the centre ? What is the mean place of a planet, and what is

its true place?

What are the nodes of a planet, and what do you call the line of the nodes?

What are the different aspects of the planets, and how many are there?

When is a planet's motion said to be direct, stationary, and retrograde ?

What is a digit, and what is the disc of the sun or moon?

What are the geocentric and heliocentric lati

tudes and longitudes of the planets ?

What is the occultation of a star or planet?

What is the transit of a planet ?

What is the aberration of a star ?

What is the elongation of a planet ?

What is meant by an eclipse ?

What is an eclipse of the moon, and what is the

cause of it?

What is an eclipse of the sun, and what is the cause of it?

CHAPTER III.

Of the Sun.

The Sun is a spherical body, placed nearly in the centre of the solar system, and the several planets revolve about it in different periods, and at different distances. The comets also revolve about the Sun, but in very eccentric orbits, being sometimes very near, and at others at an immense distance from him.

The Sun is the great source of light, heat, and animation to all those bodies; and to the influence of which, combined with their sidereal and diurnal revolutions, they owe the successive alternations of summer and winter, day and night.

2. The Sun is the largest body yet known in the universe; its mean diameter being 887,000 American miles, or about 112 times the mean diameter of the Earth; and its size 1,406,550 times that of the Earth; but its mass or quantity of matter is only 32,960 times greater, and its density about that of the Earth.

A body which weighs one pound at the surface of the Earth, would, if removed to the surface of the Sun, weigh 27 pounds, 14 ounces and 15 drachms, and bodies would fall there with a velocity of 334 feet 8 inches in the first second of time.

3. The apparent diameter of the Sun, as seen from the Earth, undergoes a périodical variation. It is greatest when the Earth is in its perihelion, which is about December the 31st, at which time it is 32′ 35.6"; and it is least, when the Earth is in its aphelion, which is about July the 1st, at which time it is 31' 31". Its mean apparent diameter is therefore 32′ 3.3"

The greatest equation of the Sun's centre is 1° 55′ 27.7", which diminishes at the rate of 16.9" in a century. The Sun's horizontal parallax, as determined by the transit of Venus, is 8". See the chapter on Parallax, &c. in a subsequent part of this work.

4. The Sun is surrounded with an atmosphere of great extent; its height, according to Dr. Herschel, is not less than 1843, nor greater than 2765 miles.

This atmosphere, Dr. Herschel thinks, consists of elastic fluids that are more or less lucid and transparent, and of which the lucid ones furnish all the bodies in the solar system with light; and he supposes that the density of the luminous solar clouds need not be greater than that of our

MVV

Aurora Borealis, to produce the effect with which we are acquainted.

5. The Sun is frequently obscured by spots, some of which have been observed so large as to exceed the Earth five or six times in diameter. Sometimes, though rarely, the Sun has appeared pure and without spots, for several years together.

The number, position, and magnitude of the solar spots, are very variable; they are often very numerous, and of considerable extent. Some imagine they may become so numerous as to hide the whole face of the Sun, or at least the greater part of it; and to this they ascribe what Plutarch mentions, viz.: that in the first year of the reign of Augustus, the Sun's light was so faint and obscure, that one might look steadily at it with the naked eye. To which Kepler adds, that in 1547, the Sun appeared reddish, as when viewed through a thick mist; and hence he conjectures that the spots in the sun are a kind of dark smoke, or clouds floating on his surface.

The solar spots, in general, consist of a dark space, or umbra, of an irregular form; they are almost always surrounded by a penumbra, which is enclosed in a cloud of light, more brilliant than the rest of the Sun, and in the midst of which the spots are seen to form and disappear. All this, according to La Place, indicates that at the surface of this enormous fire, vivid effervescences fake place, of which our volcanoes form but a feeble representation. But whatever be the nature of the solar spots, they have made us acquainted with a remarkable phenomenon, that is, the rotation of the Sun.

Amidst all their variations we can discover regular motions, which are exactly the same as the corresponding points of the surface of the Sun, if we suppose it to have a motion of rotation on an axis, almost perpendicular to the ecliptic, in the direction of its apparent annual motion round the earth.

6. The continued observation of these spots shows that the Sun revolves on its axis in 25 days, 10 hours; that its figure is not truly spherical, but an oblate spheroid like the earth; and that the solar equator is inclined 70 30' to the plane of the ecliptic.

For, some of these spots have made their first appearance near the eastern edge, from thence they have seemed gradually to pass over the Sun's disc to the opposite edge, then disappear; and hence, after an absence of about 14 days, they have re-appeared in their first place, and have taken the same course over again; finishing their entire circuit in 27 days, 12 hours and 20 minutes, which is hence inferred to be the period of the Sun's rotation round his axis; and therefore the periodical time to a fixed star, usually called the sidereal revolution, is 25d. 15h. 16m.; because, in 27d. 12h. 20m. of the month of May, when the observation was made, the Earth describes an angle about the Sun's centre of 26° 22', and therefore as the angular motion 360°+26° 22 or 386° 22′: 360°:: 27d. 12h. 20m.: 25d. 15h. 16m.

As the solar spots appear to move on the Sun's disc, from the eastern to the western edge, whence we may conclude the motion of the Sun, to which the other is owing, to be from west to east, or in the same direction, with respect to the order of the signs, as the diurnal rotation of the Earth. The more correct period of the Sun's rotation is now stated at 25d. 10h. as in the above article.

7. The Sun, together with the planets, moves round the common centre of gravity of the solar system, which is nearly in the centre of the Sun. This small motion of the Sun round the centre of gravity is occasioned by the various attractions of the surrounding planets.

8. Besides the two real motions of the Sun already mentioned, the Sun has also two apparent motions; that is, the diurnal motion from east to west, and his annual motion in the ecliptic; but these apparent motions arise from the real motions of the earth on its axis, and in its orbit.

Whether the Sun and stars have any proper motion of their own in the immensity of space, however small, is not absolutely certain, though some very accurate observers have intimated conjectures of this kind, and have shown that such a general motion is not improbable. Dr. Herschel