Satel

Bulk the Distance from Periodic Revolu-Synodic Revolu- Greatest distance tion. from the Earth.

lites. Diameter. Earth Jupiter.

being 1.

tion.

TABLE IV. SATELLITES OF SATURN.

Satellites. Distance from Saturn. Periodic Revolution. Synodic Revolution|

Satellites. Distance from Herschel. Periodic Revolution. Synodic Revolution.

Those stars, which, when seen by the naked eye, or through telescopes, keep constantly in the same situation with respect to each other, are called fixed stars. They are easily distinguished from the planets by their twinkling. They appear of various magnitudes. This may arise from their different sizes, or distances, or both. Astronomers have distinguished them, from their apparent magnitudes, into six classes. The first contains those of the largest apparent size, the second those which appear next in bigness; and so on to the sixth, which includes all those that can just be seen without telescopes. Those, which can be seen only by the help of the telescope, are called telescopic stars.

The stars in the preceding table are so numerous, that it would be impossible to furnish names for them all and retain those names in the memory. To remedy this inconvenience the ancients distributed them into constellations, to which they gave the names of birds, beasts, fishes, &c. from an imaginary resemblance between the forms of the constellations, and of those animals. The stars of each constellation are numbered, according to their magnitude, by the letters of the Greek alphabet. a is the largest, ß the second, the third, &c. This division of the heavens was very ancient; for some of the constellations are mentioned by Homer and Hesiod, by Amos and Job.

The whole number of the constellations is 90. Of these 48 are ancient, and 42 modern; 33 north of the Zodiac, 12 in the Zodiac, and 45 south of it. Those stars, which have never been arranged into constellations, are called unformed stars. Those, whose distance from the nearest pole is less than the latitude of the place, never set below the horizon, and are called circumpolar stars. The circles, which they appear to describe in consequence of the earth's rotation, are called circles of perpetual apparition. Those stars, whose distance from the farthest pole, is less than the latitude of the place, never rise above the horizon. They also receive the same name; and the circles, which they appear to describe, are called circles of perpetual occultation.

The real number of the fixed stars cannot be ascertained. Before the invention of the telescope, it was not supposed to surpass 3000. But since that event it has been found, that to the greater perfection that instrument is brought, the greater, in a very high proportion, is the number of the stars, which may be observed. Galileo found 80 stars in the belt of Orion's sword. De Rheita counted 188 in the Pleiades, and more than 2000 in the constellation of Orion, of which only 78 are visible to the naked eye. The fixed stars, as seen through a telescope, are found to be be collected in clusters. When a small magnifying power is used, these clusters appear like small light clouds, and hence have been called nebula. Dr. Herschel has given a catalogue of more than 2000 nebulæ, which he has discovered. When these nebulæ are examined with a telescope of great magnifying power, they are found to consist of immense multitudes of stars. Dr. Herschel is of opinion, that the starry heaven is replete with these nebulæ : that each nebula is a distinct and separate system of stars; and that each star is the sun or centre of its own system of planets. That

bright, irregular zone, which we call the Milky Way, he has very carefully examined, and concludes that it is the particular nebula to which our sun belongs. In examining it, in the space of a quarter of an hour, he has seen the astonishing number of 116,000 stars pass through the field of view of a telescope of only 15′ aperture; and, in 41 minutes, he saw 258,000 stars pass through the field of his telescope. It is probable that each nebula in the heavens is as extensive, and as well furnished with stars, as the milky way; that many nebulæ, within the reach of the telescope, have not yet been discovered; and that very many more lie beyond its reach, in the remote regions of the universe. If this be true, the number of 75,000,000, which La Lande assigned, as the whole number of the fixed stars, will be seen to fall far short of the truth.

The distance of the fixed stars, however, is so great, that their number will, probably, never be calculated with certainty. The diameter of the earth's orbit is 190 millions of miles. Of course, when the eye is placed at one end of this diameter, it is so much nearer given stars, than when at the opposite end. Yet this immense distance makes no apparent difference in the size of any of them, nor any difference in their relative situations. The distance of the nearest fixed star is estimated to be more than 5,000,000,000,000 miles from us, a distance which a cannon ball, moving at the rate of 480 miles an hour, would not pass over in less than 1,180,000 years. Astronomers generally, however, have calculated the distance of the nearest fixed star, at 400,000 times the diameter of the earth's orbit.

The real magnitudes of the fixed stars are not known. In astronomical calculations they are generally supposed to be equal to that of the sun.

With regard to their nature we can make nearer approaches to certainty. We know that they shine by their own light; 1st, because reflected light is too feeble to shine at all to such an immense distance, and much more with the bright lustre of the fixed stars; 2dly, because if they borrowed their light from any large luminous body which was near them, that body would itself be visible. They resemble the sun in several other particulars. Many of them are observed to revolve on an axis; to have spots on their surface, and changeable spots, too, like those of the sun. Hence they are very fairly concluded to be suns, each one a centre of light, and warmth, and motion for its own system of planets.

THE GLOBES, AND THEIR USE.

A globe is a round body, whose surface is every where equally remote from the centre. But by the globes, sometimes called artificial globes, is here meant two spherical bodies, whose convex surfaces are supposed to give a true representation of the earth and the apparent heavens. One of these is called the terrestrial, the other the celestial globe. On the convex surface of the terrestrial globe, all the parts of the earth and sea are delineated in their relative size, form, and situation.

On the surface of the celestial globe, the images of the several constellations and the unformed stars are delineated; and the relative magnitude and position, which the stars are observed to have in the heavens, are carefully preserved.

In order to render these globes more useful, they are fitted up with certain appurtenances, whereby a great variety of useful problems are solved in a very easy and expeditious manner.

The brazen meridian is that ring in which the globe hangs on its axis, represented by two wires passing through its poles. The circle is divided into four quarters of 90 degrees each; in one semicircle the divisions begin at each pole, and end at 90 degrees of the equator, where they meet. In the other semicircles, the divisions begin at the equator, and proceed thence toward each pole, where they end at 90 degrees. The graduated side of this brazen circle serves as a meridian for any point on the surface of the earth, the globe being turued about till that point come under it.

The hour circle is a small circle of brass, divided into 24 hours, the quarters and half quarters. It is fixed on the brazen meridian, with its centre over the north pole; to the axis is fixed an index, that points out the divisions of the hour circles as the globe is turned round its axis. Sometimes the hour circle, with its divisions, is described or marked about the north pole on the surface of the globe, and is made to pass under the index. In some of Adams's globes, the equator is used as an hour circle, over which is placed a semicircular wire, carrying two indices, one on the east side of the brazen meridian, and the other on the west.

The horizon is represented by the upper surface of the wooden circular frame encompassing the globe about its middle. On this wooden frame there is a kind of perpetual calendar, contained in several concentric circles. The inner one is divided into four quarters of 90 degrees each; the next circle is divided into the 12 months, with the days in each according to the new style; the next contains the 12 equal signs of the zodiac or ecliptic, each being divided into 30 degrees; the next the 12 months and days according to the old style; and there is another circle, containing the 32 points of the compass, with their halves and quarters. Although these circles are on most horizons, yet they are not always placed in the same order.

The quadrant of altitude is a thin slip of brass, one edge of which is graduated into ninety degrees and their quarters, equal to those of the meridian. To one end of this is fixed a brass nut and screw, by which it is put on and fastened to the meridian; if it be fixed in the zenith, or pole of the horizon, then the graduated edge rep. resents a vertical circle passing through any point of the horizon, to which it is directed.

Beside these, there are several circles, described on the surfaces of both globes; as the equator, ecliptic, circles of longitude and right ascension, the tropics, polar circles, parallels of latitude and declination, on the celestial globe; and on the terrestrial, the equator, ecliptic, tropics, polar circles, parallels of latitude, hour circles

or meridians, to every 15 degrees; and on some globes, the spiral rhumbs flowing from several centres, called flies.

In using the globes, keep the graduated side of the meridian towards you, unless the problem require a different position. With -respect to the terrestrial, we are to suppose ourselves situated at a point on its surface; with respect to the celestial, at its centre. The motion of the former represents the real diurnal motion of the earth; that of the latter, the apparent diurnal motion of the heav

ens.

The following PROBLEMS, as being most useful and entertaining, are selected from a great variety of others, which are easily solved with a terrestrial globe, fitted up with the aforesaid appurte

nances.

I. The latitude of a place being given, to rectify the globe for that place.

Let it be required to rectify the globe for the latitude of Boston, 42 degrees 23 minutes north.

Elevate the north pole, till the horizon cut the brazen meridian in 42 23, and the globe is then rectified for the latitude of Boston. Bring Boston to the meridian, and you will find it in the zenith, or directly on the top of the globe. And so of any other place.

II. To find the latitude and longitude of a place on the globe.

Bring the given place under that half of the graduated brazen meridian, where the degrees begin at the equator, and under the graduated side of it; then the degree of the meridian over it shows the latitude; and the degree of the equator, under the meridian, shows the longitude.

Thus Boston will be found to lie in about 42 23 north latitude, and 71 west longitude from Greenwich.

III. To find the sun's place in the ecliptic.

Look the day of the month on the horizon, and opposite to it, you will find the sign and degree the sun is in that day. Thus on the 25th of March, the sun's place is 4 degrees in Aries. Then look for that sign and degree in the ecliptic line marked on the globe, and you will find the sun's place; there fix on a small black patch, so is it prepared for the solution of the following problems.

Note. The earth's place is always in the sign and degree opposite to the sun; thus, when the sun is 4 degrees in Aries, the earth is 44 degrees in Libra; and so of any other.

IV. To find the sun's declination, that is, its distance from the equinoctial line, either northward or southward.

Bring its place to the meridian; observe what degree of the meridian lies over it, and that is the declination. If the sun lie on the north side of the line, the declination is north, but if on the south side the declination is south.