or nearly twice that of Mercury, and more than three times that of the earth. Yet, through all our lifetime, we shall never be able to detect any change in its position. It requires three centuries for it to move over the starry vault a space equal to the moon's apparent diameter.

THE STARS ARE SUNS.-The vast distance at which, they are known to be, precludes the thought of their shining, like the planets or the moon, by reflecting back the light of our sun. They must be self-luminous, and are doubtless each the centre of a system of planets and satellites.

OUR SUN A STAR.-As we see only the suns of these distant systems, so their inhabitants see only the sun of ours, and that as a small star.

OUR SYSTEM ITSELF IN MOTION.-Like all the other stars, our sun is in motion. It is sweeping onward, with its retinue of worlds, 150,000,000 miles per year, toward a point in the constellation Hercules. The Pleiades are thought to be the centre around which this great movement is taking place, but the orbit is so vast and the centre so remote, that nothing definite is yet known.

THE NUMBER OF THE FIXED STARS.-As we look at the heavens on a clear night, the stars seem almost innumerable. To count them, one would think almost as interminable a task as to number the leaves on the trees. It is, therefore, somewhat startling to learn that the entire number visible to the most piercing eyesight, does not exceed 6,000, while few

can discern more than 4,000. This illusion may be easily explained, when we remember how the impression of a bright light remains upon the retina, as in the whirling of a firebrand. However, the number

of the heavens where the naked eye sees but six stars. Could we examine the same region of the sky with more powerful instruments, new constellations would doubtless be descried in the infinite depths of space.

SCINTILLATION.-The twinkling of the fixed stars is due to what is termed in Natural Philosophy "the interference of light." The air being unequally dense, warm, and moist in its various strata, transmits very irregularly the different colors of which white light is composed. Now one color prevails over the rest, and now another, so that the star appears to change color incessantly. As the purity of the air varies, the twinkling of the stars also changes, although it is always greatest near the horizon. Humboldt says that at Cumana, in South America, where the air is remarkably pure and uniform in density, the stars cease to twinkle after they have risen 15° above the horizon. This gives to the celestial vault a peculiarly calm and soft appear.

ance.

MAGNITUDE OF THE STARS.-As the telescope reveals no disk of even the nearest stars, we know nothing of their comparative size. The finest spider's web, placed at the focus of the instrument, hides the star from the eye. When the moon passes in front of a star, the occultation is instantaneous, and not gradual, as in the case of the planets. Classification depends, therefore, upon their relative brightness. The most conspicuous are termed stars

of the first magnitude. There are about twenty of these. The number of second magnitude stars in the entire heavens is about sixty-five; of the third, about 200; of the fifth, 1,100; and of the sixth,

3,200. Few persons can see any smaller stars than those of the fifth or sixth magnitude. The ordinary telescope shows faint stars down to the tenth, while the more powerful instruments reveal those as low as the twentieth magnitude.

THE CAUSE OF THE DIFFERENCE IN THE BRIGHTNESS OF THE STARS.-This may result from a difference in their distance, size, or intrinsic brightness. Whence it follows that the faintest stars may not be the most distant from the earth.

NAMES OF THE STARS.-Many of the brightest stars received proper names at an early date; as Sirius, Arcturus. The stars of each constellation are distinguished by the letters of the Greek alphabet; the brightest being usually called Alpha, the next Beta etc., the name of the constellation, in the genitive case, being put after each. Ex., a Arietis, & Lyræ.*

* This means a of Aries, B of Lyra; the genitive case in Latin being equivalent to the preposition of.

When the Greek letters are exhausted, the Roman alphabet is used in the same way. Star catalogues are issued, containing the stars arranged in the order of their Right Ascension, and numbered for convenience of reference. Argelander's Charts have 300,000 stars marked in the northern hemisphere.

THE CONSTELLATIONS.-From the earliest ages, the stars have been arranged in constellations, for the purpose of more readily distinguishing them. Some of these groups were named from their supposed resemblance to some figures, such as perching birds, pugnacious bulls, or contorted snakes, while others do honor to the memory of the classic heroes of antiquity.

"Thus monstrous forms, o'er heaven's nocturnal arch,

Seen by the sage, in pomp celestial march;