sun is about 266 millions of miles, and its periodic revolution is performed in 1683 days, being at nearly the same distance from the sun as Pallas. The inclination of the orbit of Ceres to the ecliptic is 10° 37' and the eccentricity 0,097. The situations of the nodes of the two planets Ceres and Pallas, and the inclinations of their orbits, are very different from each other, so that when those planets are in the same plane, they are at a great distance from each other, notwithstanding their mean distances from the sun are nearly equal. It has been supposed by some, that these small bodies are fragments of a former planet.

JUPITER is situated still higher in the system, and is the largest of all the planets, being easily distinguished from them by his peculiar magnitude and light. His diameter is 89,170 mites, his distance from the sun 490 millions of miles, and the time of his periodic revolution is 48824 days. Though Jupiter is the largest of all the planets, yet his diurnal revolution is the swiftest, being only 9 hours and 56 minutes.

Jupiter is attended by four satellites, invisible to the naked eye; but through a telescope they make a beautiful appearance. In speaking of them, we distinguish them according to their places, into the first, the second, and so on; by the first we mean that which is nearest to the planet. The appearance of these satellites is marked in the XIIth. page of the Nautical Almanac, for some particular hour of the night; the times when they are eclipsed, by passing into the shadow of Jupiter, are also given in the Nautical Almanac; these eclipes are of considerable use in determining the longitudes of places on the earth.

Before the discovery of the planet Uranus, SATURN was reckoned the most remote planet of our system. He shines but with a pale and feeble light. His diameter is 79,042 miles, his distance from the sun 900 millions of miles, and his periodic revolution in his orbit is performed in about 29 years 167 days. This planet is surrounded with a broad flat ring, has a diurnal revolution round its axis, and is attended by seven satellites.

By some observations made by Dr. Herschel, it appeared that the largest diameter of Saturn corresponds to the latitude of 45°, but from later observations he has been induced to believe, that this irregularity is owing to an optical deception, arising from the refraction of the light in passing through the atmosphere of the ring.

URANUS, Herschel, or Georgium Sidus, is the most remote planet of our system. It was discovered in the year 1781, by Dr. Herschel; though there are many reasons to suppose it had been seen before, but had been considered as a fixed star. Its diameter is 35,109 miles, its distance from the sun 1800 millions of miles, and its periodic revolution in its orbit is performed in 83 years. Dr. Herschel has also discovered six satellites attending this planet.

The astronomy of comets is yet in its infancy. The return of one of them in the year 1758 was foretold by Dr. Halley, and it happened nearly as he predicted. He also foretold the return of another in the year 1790, but it never appeared. Probably this mistake of Dr. Halley was owing to the inaccuracy of the observations of the comet at its former appearance; for Mr. Mechain, having collected all the observations, and calculated the orbit again, found it to differ essentially from that determined by Dr. Halley. Comets move round the sun in all directions; but the planets and satellites, except one of the satellites of Uranus, move from west to east when seen from the sun; but if viewed from any other of the planets, as the earth, they would appear to revolve round it as a centre; but the sun would be the only one that moved uniformly the same way: for the other planets would sometimes appear to move from west to east, and then to stand still; then they would seem to move from east to west; and after standing some time, they would again move from west to east; and so on continually. The motion of a planet from west to east is called the direct motion, or according to the order of the signs. The contrary motion from east to west, is called retrograde. When the planet appears to stand still, it is said to be stationary.

tances of the planets and satellites, we have given the adjoining plates III. and IV. which require no explanation.

In noting the situations of the stars and planets, astronomers have been under the necessity of imagining various lines and circles on the sphere; and geographers have done the same for fixing the situation of places on the earth. The most remarkable of these are the following.

A great circle is that whose plane passes through the centre of the sphere; and a small circle is that whose plane does not pass through that centre.

A diameter of a sphere, perpendicular to any great circle, is called the axis of that circle; and the extremities of a diameter are called its poles. Hence the pole of a great circle is 90° from every point of it upon the surface of the sphere; but as the axis is perpendicular to the circle when it is perpendicular to any two radii, a point on the surface of a sphere 90° distant from any two points of a great circle will be the pole.

-All angular distances on the surface of a sphere, to an eye at the centre, are measured by arcs of great circles. Hence all triangles formed upon the surface of a sphere, for the solution of spherical problems, must be formed by the arcs of great circles.

Secondaries to a great circle are great circles which pass through its poles ; and consequently must be perpendicular to their great circles.

The axis of the earth is that diameter about which it performs its diurnal motion; and the extremities of this diameter are called the poles.

The terrestrial equator is a great circle of the earth perpendicular to its axis. Hence the axis and poles of the earth are the axis and poles of its equator. That half of the earth which lies on the side of the equator, in which Europe and the United States of America are situated, is called the northern hemisphere, and the other the southern; and the poles are respectively called the north and south poles.

The latitude of a place upon the earth's surface is its angular distance from the equator, measured upon a secondary to it. These secondaries to the equator are called meridians.

The longitude of a place on the earth's surface is an arc of the equator intercepted between the meridian passing through the place, and another, called the first meridian, passing through that place from which you begin to measure, or it is the angle formed at the pale by these two meridians. The Americans and English generally place the first meridian at London or Greenwich, the French place it at Paris, the Spaniards at Cadiz; some Geographers place it at Teneriffe, and others at other places. Throughout this work Greenwich will be reckoned as the first meridian. The longitude is counted from the first meridian, both eastward and westward, till it meets at the same meridian on the opposite point; therefore the longitude (and also the difference of longitude between any two places) can never exceed 180°.

If the plane of the terrestrial equator be produced to the sphere of the fixed stars, it marks out a circle called the celestial equator; and if the axis of the earth be produced in like manner, the points of the heavens, to which it is produced, are called poles, being the poles of the celestial equator. The star nearest to each pole is called the pole star.

Secondaries to the celestial equator are called circles of declination; of these 24, which divide the equator into equal parts, each containing 15°, are called hour circles.

Small circles parallel to the celestial equator are called parallels of declination.

The sensible horizon is that circle in the heavens whose plane touches the earth at the spectator. The rational horizon is a great circle in the heavens, passing through the earth's centre, parallel to the sensible horizon.

If the radius drawn from the centre of the earth to the place where the spectator stands be produced both ways to the heavens, the point vertical to him is called the zenith, and the point opposite, the nadir. Hence the zenith and nadir are the poles of the rational horizon.

Secondaries to the horizon are called vertical circles, because they are per

pendicular to the horizon. On these circles, therefore, the altitude of a heavenly body is measured.

The secondary common to the celestial equator, and the horizon of any place, is the celestial meridian of that place. This meridian corresponds with the terrestrial meridian of the same place, which passes through the poles of the earth, the zenith and nadir crossing the equator at right angles, and cutting the horizon in the north and south points; that point being called north which passes through the north pole, and the opposite direction is called south. The vertical circle which cuts the meridian of any place at right angles is called the prime vertical; the points where it cuts the horizon are called the east and west points, and to an observer, with his face directed towards the south, the east point will be to his left hand, and the west to his right hand. Hence the east and west points are 90° distant from the north and south. These four are called the cardinal points. The meridian of any place divides the heavens into two hemispheres lying to the east and west; that lying to the east is called the eastern hemisphere, and the other the western hemisphere. When the sun is at its greatest altitude on the meridian of any place, it is noon or mid-day.

The azimuth of an heavenly body is its distance on the horizon, when referred to it by a secondary, from the north or south points. The amplitude is its distance from the east or west points, at the time of rising or setting. The ecliptic is that great circle in the heavens which the sun appears to describe in the course of a year. The ecliptic and equator, being great circles, must bisect each other, and their angle of inclination is called the obliquity of the ecliptic; and the points where they intersect are called the equinoctial points. The times when the sun comes to these points are called the equinoxes. The ecliptic is divided into 12 equal parts, called signs ;— viz. Aries Y, Taurus 8, Gemini II, Cancer, Leo S, Virgo M, Libra Scorpio M, Sagittarius ↑, Capricornus V, Aquarius ~~~, Pisces . The order of these is according to the apparent motion of the sun. The first point of Aries coincides with one of the equinoctial points, and the first point of Libra with the other. The first six signs are called northern, lying on the north side of the equator; and the last six are called southern, lying on the south side.

The zodiac is a space extending eight degrees on each side the ecliptic, within which the motion of all the planets is contained, except the newly discovered planets.

The right ascension of a body is an arc of the equator intercepted between the first point of Aries and a circle of declination passing through the body, measured according to the order of the signs.

Right ascension of the meridian or mid-heaven, is the distance of the meridian, from the first point of Aries, and is found by adding the apparent time past noon, to the sun's right ascension.

The ascensional difference of any object is the difference between the right ascension of the object and that point of the equator which rises or sets with it.

The declination of a star or any celestial object is its angular distance from the equator, measured upon a secondary to it passing through the object.

The longitude of a star or any celestial object is an arc of the ecliptic intercepted between the first point for Aries and a secondary to the ecliptic passing through the body, measured according to the order of the signs.— If the observer be on the earth, the longitude is called the geocentric longitude; but if seen from the sun it is called the heliocentric longitude; the body in each case being referred perpendicularly to the ecliptic in a plane passing through the eye.

Nonagesimal degree of the ecliptic is its highest point at any given time, and is 90° from the points where the ecliptic intersects the horizon.

The latitude of a star or any celestial object is its angular distance from the ecliptic, measured upon a secondary to it drawn through the body.-If the body be observed from the earth, its angular distance from the ecliptic is called the geocentric latitude; but if observed from the sun it is called the heliocentric latitude. The secondary circle drawn perpendicular to the eclip