146 



THE MINOR PLANETS. 



stand at a distance from a common terrestrial globe, and let it be made to re- 

 volve upon its axis : the spectator will see the geographical marks delineated 

 on it pass across the hemisphere which is turned toward him. They will suc- 

 cessively disappear and reappear. The same effects must, of course, be ex- $ 

 pected to be seen upon the several planets, if they have a motion of rotation 

 resembling the diurnal motion of our globe. If this species of observation be 

 attempted with respect to the planets MERCURY and VENUS, we shall immedi- 

 ately find the investigation obstructed by an unexpected difficulty. Their disks 

 present no permanent marks or characteristics. They are, it is true, diversified 

 more or less by lights and shadows, but we soon discover that these varieties 

 of feature are not of a permanent kind ; but, on the contrary, that they are 

 continually shifting and changing, like the clouds that float in an atmosphere. 

 ]t has, in fact, been ascertained, that these appearances in the inferior planets 

 are produced by clouds, with which the thick atmosphere that invest them are 

 continually loaded. These clouds are so continuous that they never permit us 

 to see the geographical character of the planets Mercury and Venus at all. 



For a long period this circumstance seemed to render futile all attempts to 

 ascertain the rotation of these planets accurately. At length, however, a cir- 

 cumstance, apparently accidental, led CASSINI and SCHROTER to the discovery 

 of the fact of the rotation of VENUS on its axis. 



This discovery was effected by observing that the points of the horns of the 

 crescent of Venus were at certain moments cut off square, and after a certain 

 time would recover their sharpness. This was found to take place nearly at 

 the same time each successive evening and morning. The cause was soon 

 ascertained. In a certain part of the surface of the planet a lofty mountain 

 flung its shadow across the region which formed a point to the horn. The 

 diurnal rotation of the planet soon carried this point into another position, so 

 that the shadow disappeared and allowed the horn of the crescent to recover its 

 sharpness. Each time that the horn became thus blunted, it was ascertained 

 that the mountain had returned to the same position, and consequently that the 

 planet must have completed one revolution on its axis. 



It is a remarkable fact, that the same circumstance was found to take place 

 in the instance of the planet MERCURY, and the result has been, that these two 

 planets have been ascertained to have a diurnal rotation; that of MERCURY 

 being completed in 24 hours, 5 minutes, 28 seconds, and that of VENUS in 23 

 hours, 21 minutes, 7 seconds. Thus it appears the alternations of day and 

 night in these planets are regulated by the same intervals as the earth. 



DIRECTION OF THE AXIS OF ROTATION. SEASONS, CLIMATES, AND ZONES. 



The position of the axis on which a planet revolves, is ascertained by ob- 

 serving the direction of the apparent motion of the permanent marks upon its 

 disk the axis being necessarily perpendicular to such motion. Since, how- 

 ever, the rotation of MERCURY and VENUS, as we have just explained, do not 

 show the apparent motion of any of these permanent marks, the circumstances 

 which led to the discovery of their rotation, did not indicate the position of the 

 axes on which they turned. It is said, however, that observations have been 

 made which justify the conclusion that the axis on which the planet VENUS 

 turns, has a position in reference to its orbit very different indeed from that of 

 the earth. Let it be remembered, that the axis of the earth leans from the 

 perpendicular through an angle of 231, in consequence of which the polar cir- 

 cles and tropics have corresponding limits. It is this arrangement which 

 divides the surface of our globe into the temperate and frigid zones ; the tem- 

 perate being those which lie between the tropics and the polar circles, in which 



