ASTRONOMY. 355 



stars of which, called the pointers, point to the pole star ; and Cassio- 

 peia, which lies on the opposite side of the pole from the great bear, 

 and contains the figure of a smaller dipper, by the position of which 

 the pole star may also be found.* 



3. It belongs to Physical Astronomy, to investigate the causes 

 of the celestial phenomena, which Descriptive and Practical Astro- 

 nomy make known. The key to this subject is the law of universal 

 gravitation, discovered by Newton. The planets gravitate towards 

 the sun, and towards each other ; that is, they are attracted, directly 

 as their masses, and inversely as the squares of their distances. 

 Hence, they would all fall together, and meet in their common centre 

 of gravity, did not their motion in their orbits give them a tendency 

 to recede from the centre, like a stone from a sling, in a line tangent 

 to the curve which they are describing. This tendency, called their 

 centrifugal force, and their mutual attraction, called their centripetal 

 force, especially that of the sun, counterbalance each other, and cause 

 the planets to move according to Kepler's three laws ; which were 

 the basis of Newton's discovery. 1. The planets describe ellipses, 

 having the sun in one of their foci ; 2. Their radius vectors, or lines 

 drawn from them to the sun, pass over equal areas in equal times; 

 and 3. The squares of their times of annual revolution are propor- 

 tional to the cubes of their mean distances from the sun. 



By the second law, the planets move slowest when farthest from 

 the sun ; as the radius vector, being then the longest, describes an 

 equal area by a smaller angular motion. The mutual attractions of 

 the planets cause slight irregularities in their orbits, called perturba- 

 tions : and it is the common centre of gravity of the earth and moon, 

 which describes an elliptic orbit around the sun ; the earth's centre 

 deviating slightly from this ellipse, during its annual revolution. 

 When the moon is at or near the syzigies, that is the new or full, 

 the sun's attraction, tending to separate the earth and moon, causes 

 the inequality called the evection, affecting the shape of the orbit; 

 and when the moon is about 45 from the syzigies, the sun's attrac- 

 tion affecting its angular velocity, causes the inequality called the 

 moon's variation. 



4. Under Practical Jlstronomy, is included the use of instru- 

 ments in making celestial observations, and the use of tables and 

 formulas in deducing results therefrom. The most important astrono- 

 mical instruments, are the Transit Instrument, and the Astronomical 

 Clock : but even these, we have no room to describe. The altitude 

 of a heavenly body, is its angle of elevation above the horizon : and 

 the azimuth, is the angle between a vertical circle passing through 

 the body, and the vertical plane of the meridian; which latter cuts 



* On the fifth day of August annually, at 9 o'clock in the evening, the central 

 star of the dipper, (cf Ursse Majoris), will be on the west side of the pole ; and the 

 second brightest star, (/3 Cassiopeiae), will be on the east side of it, and at nearly 

 the same altitude ; both of these stars being nearly in the equinoctial colure. At 

 the same time, the pole of the ecliptic will be directly above the pole of the heavens, 

 as shown in Plate VIII. On the fifth of February, at the same hour in the evening, 

 Cassiopeia will be to the west, and Ursa Major to the east, of the pole ; as repre- 

 sented by turning the plate halfway around : and Orion will then be a little to the 

 west of the meridian. 



