354 ACROPHYSICS. 



2. Under the head of Siderial Astronomy, we include the study 

 of the Fixed Stars ; or those luminaries which maintain continually 

 nearly the same relative position in the heavens. These stars are 

 supposed to be the suns of other systems ; each having planets re- 

 volving around it, which latter, from their fainter reflected light, are 

 to us invisible. Of the immense distance of the fixed stars, we have 

 already spoken, in introducing the present branch. They are classed 

 according to their apparent magnitudes : those of the first magnitude 

 appearing the largest, and those of the sixth magnitude being the 

 smallest, which are visible to the naked eye : but the telescopic stars 

 are also classed, down to the sixteenth magnitude. These magni- 

 tudes depend not only on their actual size, but on their distance from 

 the earth, which is doubtless very different for different stars. Some 

 of them are variable ; regularly increasing and decreasing in bright- 

 ness; and some of them are double or binary stars, one of which 

 is, in many instances, found to revolve around the other. There 

 are also nebulas ; of which the galaxy or milky way is an assem- 

 blage ; and which are shown, by the telescope, to consist of almost 

 innumerable small or very distant stars. 



The position of the fixed stars, is defined, either by their right as- 

 cension and declination, which correspond to longitude and latitude 

 on the earth ; or by their celestial latitude and longitude, which are 

 essentially different. As the plane of the earth's equator, prolonged 

 to the heavens, forms the celestial equator, so the plane of the earth's 

 orbit, extended to the heavens, defines the ecliptic ; a circle which is 

 very nearly stationary among the fixed stars. The vernal equinox, 

 or point at which the sun, appearing always in the ecliptic, crosses 

 the equator, in going to the north, is the origin of both right ascension 

 and celestial longitude, or the point from which they are both reckoned ; 

 the former on the equator, but the latter on the ecliptic. And as 

 terrestrial latitude is distance, in degrees, north or south of the equa- 

 tor, so is celestial latitude distance north or south of the ecliptic. 

 Circles of declination, are perpendicular to the equator, and corres- 

 pond to terrestrial meridians ; but circles of latitude, are perpen- 

 dicular to the ecliptic, and meet at its poles. Parallels of declina- 

 tion, are parallel to the equator; but parallels of celestial latitude, 

 are small circles parallel to the ecliptic. 



In order to designate the fixed stars, they have been grouped into 

 clusters, or constellations, which are now 128 in number ; occupy- 

 ing the whole celestial sphere. The brightest star in any constella- 

 tion is named alpha, (a), as a Arietis ; the next brightest is named 

 beta; and so on, until the Greek alphabet is exhausted; when re- 

 course is had to the Roman alphabet; and after this, to numbers. 

 The twelve constellations of the zodiac, in which the planets gene- 

 rally appear, are, Aries, the ram ; Taurus, the bull ; Gemini, the 

 twins ; Cancer, the crab ; Leo, the lion ; Virgo, the virgin ; Libra, 

 the balance ; Scorpio, the scorpion ; Sagittarius, the archer ; Ca- 

 pricornus, the goat ; Aquarius, the waterer ; and Pisces, the fishes. 

 Among the other most remarkable constellations, are Orion, known 

 by three bright stars in its belt, nearly on the equator ; Ursa Major, 

 or the great bear, in which lies the cluster called the dipper,\vfQ 



