the sun one year. The earth turns round its axis from west to east in 23h. 56m. which occasions the apparent diurnal motion of the sun and all the heavenly bodies round it from east to west in the same time, and is, of course, the cause of their rising and setting, of day and night. The axis of the earth is inclined about 23° 28' to the plane of its orbit, and keeps nearly in a direction parallel to itself, throughout its annual course, which causes the return of spring and summer, autumn and winter. Thus the diurnal motion gives us the grateful vicissitude of night and day, and the annual motion the regular succession of the seasons. The earth is attended by a satellite called the Moon, whose diameter is 2161 miles; her distance from the centre of the earth is 240,000 miles: she goes round her orbit in 27 days 8 hours; but, reckoning from change to change, in 294 days. Her orbit is inclined to the ecliptic in an angle of 5° 9', cutting it in two points diametrically opposite to each other, called her nodes. As the moon shines only by the reflected light of the sun, she must appear different when in different situations with respect to that luminary. When she is in conjunction with the sun, her dark side is turned towards the earth, which renders her invisible; this is called new moon: when she is in opposition, her light side is wholly visible from the earth; this is called full moon.

If at the time of new moon she is near to either of her nodes, she may intercept a part of the sun's light, and thus cause an eclipse of the sun; and if she is near either of her nodes at the time of full moon, she may pass into the shadow of the earth, and cause an eclipse of the moon. In a similar manner, when the moon passes between an observer on the earth and a star, it is called an Occultation of the star. The instant when the moon's limb first covers the star is called the Immersion, and the moment of its re-appearance is called the Emersion. When Mercury or Venus passes between the sun and an observer, and appears to pass over the sun's disk, it is called a Transit of Mercury or Venus. Eclipses, Occultations and Transits, are of great importance in ascertaining the longitudes of places on the earth. Eclipses of the moon furnish a convincing proof of the rotundity of the earth, since the shadow of the earth, seen upon the moon when eclipsed, is always circular. This is farther confirmed by the appearance of objects at sea; for when a ship is making towards the land, the mariners first descry the tops of steeples, trees, &c. pointing above the water; the lower parts being hid, by reason of the curvature of the earth.

The earth is not a perfect globe or sphere, but is a little flatted at the poles, being nearly of the figure of an oblate spheriod, the equatorial diameter being about 26 miles longer than the polar: but since this difference bears but a small comparison to the whole diameter, we may, for all the practical purposes of navigation, consider the earth as a perfect sphere, as will be done in the rest of this work. The natural divisions of the earth will be given hereafter.

MARS is the next planet to the earth; his diameter is 4189 miles, his distance from the sun 144 millions of miles, and his periodic revolution is performed in about 687 days. He revolves round his axis in 24 hours 40 minutes, appearing of a dusky reddish hue, and is supposed to be encompassed with a very great atmosphere.

Between Mars and Jupiter are situated the four lately discovered planets, Vesta, Juno, Pallas, and Ceres, named Asteroids by Doctor Herschel. The elements of their orbits have not been accurately ascertained, but are nearly as in the following description.

VESTA, was discovered by Doctor Olbers of Bremen, on the 29th of March, 1807. Its mean distance from the sun is about 205 millions of miles; its periodic revolution is performed in about 3 1-6 years.

JUNO, was discovered by Mr. Harding, of Lilienthal (near Bremen) on the first of September, 1804. It appears like a star of the eighth magnitude. Its distance from the sun is about 255 millions of miles; its periodic revolu tion is performed in 1582 days. The inclination of its orbit to the ecliptic is 13° 50′ and the eccentricity of the orbit† 0.25.

•The Inclination decreases at present about 50' in 100 years, by reason of the attraction of the planets en the earth. It is also affected by the Nutation given in Table XLIII. which sometimes amounts to 9. In estimating the eccentricities of the planets, their mean distance from the sun is put equal to unity

PALLAS, was also discovered by Dr. Olbers, March 28, 1802. Its diameter, according to Doctor Herschel, is only 110 miles; it appears like a star of the eighth magnitude. Its mean distance from the sun is about 266 millions of miles. Its periodic revolution is performed in 1683 days. The inclination of its orbit to the ecliptic is 34° 39' and the eccentricity of the orbit 0.2463.

CERES, was discovered by Mr. Piazzi of Palermo on the first of January, 1801. Its diameter, according to Dr. Herschel, is only 160 miles. It appears like a star of the seventh or eighth magnitude. Its distance from the sun is about 266 millions of miles, and its periodic revolution is performed in 1688 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 miles, his distance from the sun 490 millions of miles, and the time of his periodic revolution is 43324 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, 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 eclipses 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 with but 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 450, 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 it had been seen several times, 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. This 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. Olber's comet, which appeared in 1815, has a revolution of 72 years; and Encke's comet, which has been observed in several successive approaches to the perihelion, completes its revolution in the short period of 1204 days.

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 some

the causes of the Seasons of the length of Days &c.

THE ANNUAL MOTION OF THE EARTH ROUND THE SUN:

ICBLISHED ET EI M.M.BLUNT, 1826.