from a combination of the motion of light, and of the other motions in the system. If this notion be pursued in all its bearings, it cannot be doubted that a consequence of such an alteration in the velocity of light would have been, that this science, by which our knowledge of the creation is so much extended, would scarcely as yet have existed.

ELEMENTS

OF

ASTRONOMY.

CHAPTER I.

ON THE DOCTRINE OF THE SPHERE.

1. THE imaginary concave surface in which a spectator at first conceives all the heavenly bodies placed, is an hemisphere, in the centre of the base of which he himself is situate. The base of this hemisphere is the plane by which his view of the heavens is bounded. It is called the plane of the horizon.

The numerous bodies observed on the concave surface differ in lustre, and apparently in magnitude... All of them appear to have a daily motion. Many of them emerge, as it were, from below the plane of the horizon, and, after traversing the concave surface, disappear, to rise again at the same points of the horizon as before... Others in their paths never reach the horizon, but continually move round a fixed point in the heavens.

Far the greater number of the celestial bodies preserve the same situation with respect to each other; that is, they preserve the same apparent distances from each other... These are called fixed stars.

B

The sun, besides his diurnal motion of rising and descending, seems also to have a motion on the concave surface, and in a certain space of time, called a year, to return to the same position with respect to the fixed stars.

The moon appears also, besides its diurnal motion, to have a motion among the fixed stars, and in a space of time called a month, returns nearly to the same position with respect to the

sun.

2. The spectator viewing those stars that do not set, will observe one of them nearly immoveable. This is called the Polar Star, from its vicinity to the point about which the stars that do not set appear to move. The point itself is called the North pole.-The face of the spectator being turned to this point, the stars rise on his right hand, or in the east, and set on his left hand, or in the west; and thus the apparent diurnal motion of the celestial bodies that rise and set, is from east to west. The apparent motions of the sun and moon among the fixed stars, are in a contrary direction; that is, from west to east.

Besides the sun and moon, and fixed stars, ten other celestial bodies may be noticed, which, beside their apparent diurnal motions, have apparent motions that at first seem not easily brought under any general laws. Sometimes they appear to move in the same direction among the fixed stars as the sun and moon; at other times in a contrary direction, and then are said to be retrograde. At times they appear nearly stationary. They are called planets. They have been named Mercury, Venus, Mars, Ceres, Pallas, Juno, Vesta, Jupiter, Saturn, and the Georgium Sidus. Of these, five have been noticed from the remotest antiquity. The other five, lately discovered, are only visible by the assistance of telescopes. The Georgium Sidus was discovered by Dr. Herschel in 1780. Ceres was discovered on the first day of the present century, at Palermo, in Sicily, by M. Piazzi. The other three have been discovered

since. Pallas, at Bremen, by Dr. Olbers; Juno, at Lilienthal, by M. Harding; and Vesta, by Dr. Olbers. Mercury and Venus are remarked to be never far from the sun. All but Pallas are always found to be near the annual path of the sun in the concave surface.

3. The above are a few of the phenomena which offer themselves in contemplating the heavens. But the motions are in general only apparent, and take place from a combination of a variety of different motions. The difficulty of deducing the actual circumstances of the magnitudes, and of distinguishing the true from the apparent motions of these bodies, however easy it may appear when done, is such that we ought not to be surprised that the ancients made so little progress toward the knowledge of the true system and true dimensions of the universe; nor ought we to think lightly of their efforts, and to treat them with contempt for their errors. The moderns, by the joint assistance of mechanics, optics, and mathematics, have advanced the science of astronomy to a greater degree of prefection, perhaps, than any other branch of natural knowledge.

4. For more readily explaining and referring to the phenomena of the celestial bodies, certain circles are imagined to be described on the concave surface. Distances on the concave surface are measured by arches of great circles. The present division of the circle into 360 equal parts, called degrees, of each degree into 60 equal parts, called minutes, and of each minute into 60 equal parts, called seconds, was not used till long after astronomy had attained to a considerable degree of perfection.

It is much to be regretted, that, at the revival of learning in Europe, a decimal division of the circle was not adopted, which

a The old mode of expressing the measure of an arch, was by stating its relation to the whole circumference; thus the diameter of the sun, measured on the concave surface, was said to be of a great circle.

would have greatly facilitated astronomical computations. The French have lately adopted this division, but not generally.— They divide the circle into 400 parts, each quadrant containing 100, each of these parts into 100, &c. But it is much to be doubted whether the advantages of this division will compensate for the disadvantages now attending it; which necessarily arise from the number of books in which the old division is used, and the great variety of measures of that division familiar to astronomers. Accordingly, in France the old divisions seem likely to prevail, and much inconvenience will probably result from the new divisions having been adopted in some recent very valuable workson astronomy.a

But

The circles, and arches of circles, forming parts of the instruments used in practical astronomy, are actually divided into degrees and parts of a degree, as far as the magnitude of the radius will permit, so that the divisions may not be too close together. The arches or limbs of the largest astronomical quadrants and circles are divided into intervals of 5 minutes. the measure of an angle can be obtained with great precision by the assistance of ingenious contrivances, that will be noticed hereafter. The most improved instruments are thus adapted to measure angles to seconds. In general, with the best instruments, the result of a single observation can now be depended on to a very few seconds, and in many cases to one second.

5. Let us return to the consideration of the visible concave surface of the heavens. The intersection of the plane of the horizon, with the imaginary concave surface, is a great circle,

a M. Laplace, in his great work, entitled "Mecanique celeste," uses the decimal division. In the new tables of the sun and moon, published by the Board of Longitude in France, 1806, the sexagesimal division is used. In the tables of Jupiter and Saturn, since published by them, the decimal division is used. These are among the most important astronomical publications that have ever appeared.