ROTATION OF MARS 67 



planet varied so much that no reliance could be placed 

 on the result as a means of ascertaining whether our 

 day remains the same from age to age. 



Herschel considered the planet Mars a more favour- 

 able field for experiment than Jupiter. On Mars he 

 saw spots of a different nature : " Their constant and 

 determined shape, as well as remarkable colour, show 

 them to be permanent and fastened to the body of 

 the planet. These will give the revolution of his 

 equator to a great certainty, and by a great number 

 of revolutions, to a very great exactness also." A 

 circumstance, with which Herschel was not acquainted, 

 materially helped him in his observations on Mars. 

 The atmosphere on that planet is not nearly so dense 

 as the earth's, and similar trade-wind belts to those 

 on Jupiter do not seem to exist. By these means he 

 concluded that the length of a day on Mars is a little 

 longer than our day, or 24 hrs. 39 min. 5 sec. 1 The 

 value of an accurate measure of the length of day 

 in other planets he conceived to be this : " Future 

 astronomers may be enabled to make some estimate 

 of the general equability of the rotatory motions of 

 the planets. For if in length of time they should 

 perceive some small retardation in the diurnal motion 

 of a planet, occasioned by some resistance of a very 

 subtle medium in which the heavenly bodies perhaps 

 move, or, on the other hand, if there should be found 

 an acceleration from some cause or other, they might 

 then ascribe the alteration either to the diurnal motion 

 of the earth, or to the gyration of the other planet, 

 according as circumstances, or observed phenomena, 



1 Time of rotation determined since Herschel's days, 24 hrs. 37 min. 

 227 sees. 



