94 KAXSIS ACADEMY OF SCIENCE. 



THE PERIODS OF ROTATION OF MERCURY AND VENUS, 



AND THE SATELLITES OF THE 



SOLAR SYSTEM. 



By E. MILLER, Kansas University, Lawrence, Kan. 



Long ago, when our moon was in a plastic condition and ttie earth's attrac- 

 tion forced great tidal waves of liquid or gaseous matter to roll over her sur- 

 face, a marvelous effect was produced. The rotation period of the moon was 

 gradually brought into coincidence with the period of her revolution. 



The principle involved in such planetary intei-action is known as the 

 theory of "tidal friction", and was first announced to the world by the Ger- 

 man philosopher, Kant, in 1754. But not until near the middle of the nine- 

 teenth century did it begin to play any important part in astronomical 

 theories. 



In this paper I propose to consider the synchronism of the rotation and 

 revolution periods of the planets Mercury and Venus, and of all the satellites 

 of the solar system, basing my argument upon recent astronomical observa- 

 tions. Up to the year 1889, no definite perioji of rotation had been assigned 

 to the planet Mercury; but in that year the Italian astronomer, Schiaparelli, 

 discovered from certain marks upon the surface of the planet that the revolu- 

 tion of Mercury around the sun and his rotation upon his axis were coin- 

 cident, each requiring a period of about 88 days. 



In the year 1666, Domenico Cassini, an Italian, estimated the period of the 

 rotation of the planet Venus at 23 hours and 21 minutes. Another observer 

 placed it at 24 days, 8 hours; Schroeter gave it a period of 23 hours and 28 

 minutes; and others still put it at 23 hours, 21 minutes, and 22 seconds. The 

 first accurate observations upon Venus, made in this case also by Schiaparelli, 

 with his splendid equipment, his unrivaled Italian skies, and his unsurpassed 

 powers of vision, have given us something reliable and definite, establishing 

 almost beyond a doubt that Venus rotates in precisely the same time that she 

 requires to perform one revolution, that is 224 days, 7 hours. 



Mercury and Venus, of all the planets, are the only ones that resemble our 

 moon in their periods of rotation and revolution, and, just as our satellite turns 

 but one face towards the earth, so in like manner do our inner neighbors turn 

 but one face towards their primary, the sun. 



"We can scarcely, indeed, realize, without some approach to dismay, the 

 physical condition of a globe turning always the same face towards the sun. 

 Over one hemisphere, the perpetual glare of unending day; on the other, 

 darkness without the hope of dawn." (Gierke.) 



Now Laplace's nebular hypothesis being accepted as fact, many millions 

 of years ago, the nebulous ring out of which the earth and moon were formed, 

 separated from the inner mass. The moon was, no doubt, a part of the ring, 

 and is, therefore, as old as the earth. Both were intensely heated liquid or 

 gaseous substances, and the smaller exposed to the greater attractive power 

 of the larger, was pulled into an orbit of revolution about the latter. The 

 earth's tidal action upon the lunar substance, would, in the course of millions 

 of years, gradually change the rotation-rate so as to make it coincide with its 

 period of revolution about the earth. A great tidal wave would rush over the 



