THE sublimity and beauty of the starry heavens is such a common subject of remark, that we need not stop to expatiate on it; but we wish to treat of some matters connected with the stars, which have not hitherto engaged our attention. The stars appear much more numerous at first sight than they are found to be on reckoning those which are visible: this is occasioned by the glittering and twinkling, and the apparent confusion in which they seem mingled together. They have been, as we before stated, clustered into constellations, which are known by certain names; but besides this, they are divided into classes, according to their magnitudes, the number of recognised magnitudes visible without a telescope being seven; which is continued, with the aid of powerful instruments, up to the sixteenth. It is, however, necessary to remark, (and nothing can give such MAGNITUDES OF THE STARS, FROM THE FIRST TO THE SEVENTH, AS a striking proof of the immense distance at which they are placed from the Earth,) that, although we speak of magnitudes, the stars have really no appreciable magnitude at all: they did not appear larger through Sir W. Herschel's telescope, which magnified 6000 times, than they do to the naked eye; but they appear much brighter when viewed through a telescope; sometimes too bright to be gazed upon without pain to the eye, as we are told is the case when Sirius, the brightest of the fixed stars, appears through a telescope. It is, therefore, more properly speaking, by the brightness that we compare different stars, and not by the magnitude. Every star appears through a telescope to be a mere point of light, without appreciable diameter; and this is one of the circumstances which have baffled all the attempts of astronomers to estimate their distances. The fact of the impossibility of magnifying the fixed stars, together with that of their affording no sensible parallax, has hitherto put it out of our power to do otherwise than surmise their distances. The only use of the telescope, with reference to the fixed stars, is to penetrate more and more into space, and bring out, as it were, still greater numbers of them; so that the quantity of them seems infinite: for they multiply and spring forth, as optical aid is increased. Hence two things are clear: first, that what we call magnitude is only apparent, being referred to the power of the eye and telescope; secondly, that magnitude depends upon distance, size, or actual quantity of light, each or all combined. About 50,000 stars have been catalogued; that is, their celestial latitudes and longitudes are entered, just in the same way as the position of a place on the Earth's surface. It is observable, however, that a different mode of proceeding is here adopted from that which is employed in terrestrial measurements. We have stated that the ecliptic is the path in the heavens, in which the Earth moves round the Sun, or in which the Sun appears to move round the Earth. This circle in the heavens is the place from which celestial lati tudes are measured, in the same way as the Earth's equator is the beginning of terrestrial latitude. The positions of the stars are referred to this circle by such kinds of expressions as are used with respect to places on the Earth ;—that is, a star being north or south of this line, has such or such latitude. As terrestrial longitude is measured on the circle which forms the 0, or zero of latitude; so, in like manner, is celestial longitude reckoned on the ecliptic, which is the 0, or zero of celestial latitude. But a point of commencement is necessary here, as well as on the Earth's surface; and the point chosen, is the vernal equinox, or one of the points in which the celestial equator cuts the ecliptic: -a point which, as we have said before, is called the first point of Aries. From this point then, the longitudes of the stars are reckoned; and when the latitude is previously known, we can designate the position of any star in the heavens, by this double mode of comparing its position with regard to the ecliptic. For some astronomical purposes, it is deemed more desirable to refer the position of a heavenly body to the celestial equator, instead of the ecliptic. The celestial equator is sometimes termed the equinoctial, from the circumstance, that, when the Sun appears in it, the days and nights are equal all over the world; and it is a circle supposed to be formed, as we said before, by the extension of the Earth's equator to the concave of the heavens. The distance from the celestial equator to the pole-star, is supposed, as on the terrestrial globe, to be divided into 90°; and, according as any heavenly body is situated north or south of the equinoctial, it is said to have a north or south declination. The east or west position of the heavenly body, considered with refer R ence to the equinoctial, differs from longitude as reckoned on the ecliptic, in two ways: 1st. it is called right ascension, and not longitude: 2nd. it is usually reckoned in hours, instead of in degrees. The point from which the right ascension is reckoned, is the same as with the longitude; viz., the first point of Aries: so that, if the Sun be exactly at the opposite part of the heavens from the first point of Aries, he is said to have twelve hours of right ascension; twelve hours being equivalent to 180°: such a position takes place on the 21st of September, when the Sun enters Libra, which is diametrically opposite to the sign Aries. By these two methods, the position of any heavenly body, whether Sun, Moon, Planet, Comet, or Fixed Star, can be determined, and described with as much exactness as the position of any place on the Earth's surface. The situation of any heavenly body, with respect to a particular place on the Earth, is determined by its altitude and azimuth; but its situation, in regard to the heavens in general, is determined by its declination and right ascension, or by its latitude and longitude. Besides the classification of the fixed stars into constellations, it is customary, in order to distinguish one star from another, in any one constellation, to attach letters, one to cach star, by which they shall be known. The letters of the Greek alphabet are used for this purpose; and, when they are all exhausted, numerals are employed, at least for the stars of small visible magnitudes. Thus, there is a star marked y in the constellation, called Draco, or the Dragon, near the North Pole. This star comes once every day, almost precisely into the zenith of London, or immediately overhead: this star has of late years engaged much of the attention of astronomers, with the hope of obtaining, by its means, some indications of the distance at which the stars are situated from the Earth. For this purpose, there was erected at Greenwich, about five years ago, a zenithmicrometer. A micrometer, as the word by its derivation implies, is an astronomical instrument for measuring small angular distances. The object of this instrument, which was twenty-five feet long, was to measure the angular meridian-distance from the zenith of the stary Draconis, at different times of the year, or at different parts of the Earth's orbit, which is the same thing. A variation in this angular distance at the lapse of six months, would have proved an annual parallax*:-but this the observers at Greenwich failed to discover, and the subject remains as it was before. If the observers had succeeded in detecting a parallax of only one second of a degree, it could have been proved by calculation that this, or any other of the fixed stars, could not have been distant from the Earth, less than 19 billions of miles. In order, therefore, that astronomers may know this star when spoken of, it is called y Draconis, that is, the star in the constellation of the Dragon, which is marked * The question of parallax was first studied by Hipparchus, who flourished about 150, B. C. He was first led to it by observing the different apparent positions of a tree which he saw on a plain. He also first discovered that the interval between the vernal and the autumnal equinox is seven days longer than between the autumnal and vernal, occasioned by the eccentricity of the Earth's orbit. He divided the heavens into constellations. He taught the use of longitude and latitude, afterwards employed by Ptolemy. He likewise laid the first foundations of trigonometry, and made a calculation of eclipses for 600 years. |