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■with respect to each other, yet they change their situationvery much with regard to us, some appearing rising, otherssetting, some describing large circles, others small; somegoing over our heads, others just rising above the horizon,and then disappearing. Some stars neither rise nor set,but seem to turn round one immoveable point, near whichis placed a single star, called the north pole, or polar star.This star appears more or less elevated above the horizon,according to the part of the earth from whence it is viewed.As we go north it gradually rises, and were we to reach thenorth pole of the earth, the polar star would appear directlyover our heads; and we should see only the stars of thenorthern hemisphere, none of which would ever set; theywould all appear to move round in circles parallel to thehorizon ; whence this is called a parallel sphere.
Should a spectator go south, he would see the polar stargradually sink, till he reached the equator, when it appearsin the horizon. There may be seen the stars of both hemi-spheres ; all of them rising and setting perpendicularly tothe horizon, so that, like the sun, they are exactly twelvehours above the horizon, and twelve hours below it; and thewhole heavens resemble a large concave sphere bespangledwith stars, moving round two immoveable points, which,like the ends of an axis, remain fixed in the horizon; hencethis is called a right sphere: —to have a correct ideaof the starry firmament, a spectator should be placed in thissituation.
When we cross the equator and go southward, the pole-star gradually disappears, and then as we progressively ad-vance, the stars next to it. The south pole proportionallyrises, and the stars around it, which are all some distancefrom the pole; so that no one of them has the name of thepolar star, as in the north.
In every other part of the earth, except at the equatorand at the poles, the stars seem to rise and set obliquely,with respect to the horizon, whence it is called an OBLIQUESPHERE.
The fixed stars have been divided by astronomers intocertain parcels, called constellations; and the numberof visible stars in each, with their position and magnitude,are ascertained.
The stars, from their different apparent size, are dividedinto six classes, called magnitudes; those, the largest andbrightest, are called stars of the first magnitude; those nextin size and splendour, of the second, and so on, till we cometo the sixth, and even seventh magnitude. Beyond those