532 THE POPULAR S.CIEXCE MONTHLY 



the hands of long-continued ones. We can calculate about how much 

 that flattening would be. That of the earth is ^93, that of Mars M90? 

 and both planets rotate in approximately twenty-four hours. That of 

 Venus for a like spin would lie between the two figure, because in mass 

 and density she falls between the earth and Mars. Let us say % 75 for 

 it, which Avould be close to the truth. 



Xow Yenus on occasion offers peculiar opportunity to measure any 

 such flattening if it existed. For at times she passes in transit across 

 the sun's face. At that moment she presents an absolute absence of 

 phase and in consequence any correction due to asymmetrical illumina- 

 tion is self-eliminated. Furthermore, she then shows the largest of all 

 planetary disks, one of 60 seconds in diameter. A flattening of % 75 

 would amount therefore in her case to 0".22. Such a quantity could 

 not possibly miss of detection. For that of Mars, which is only half as 

 much and is not so well displayed, has nevertheless been measured. 

 Yet no divergence from perfect sphericity has ever been found in the 

 globe of Yenus, though diameters at all azimuths have been carefully 

 taken when she is seen silhouetted in transit against the sun. 



I may have seemed to dwell at unnecessary length upon the time 

 that Yenus takes to turn. But there is cause. The rotation time of 

 Yenus, the determination, that is, of the planet's day, is one of the 

 fundamental astronomical acquisitions of recent years. It is not a ques- 

 tion of academic accuracy merely, of a little more or a little less in 

 actual duration, but one which carries in its train a completely new 

 outlook on Yenus and sheds a valuable sidelight upon the history of our 

 whole planetary system. For upon it turns our whole knowledge of 

 the planet's physical condition. More than this, it adds something 

 which must be reckoned with in the framing of any cosmogony. 



To this we shall now proceed and if the deductions and the phe- 

 nomena which corroborate them appear almost romantically strange it 

 is in the facts themselves that the romance exists. 



In the first place such isochronism gives us a glimpse into the 

 planet's past. That the day should coincide with the year means that 

 it has been brought to this condition. For that it can always have been 

 so is mechanically highly improbable. On the other hand, there is a 

 cause continually tending to bring about such a result; tidal friction. 

 Under the immense forces at work the planetary masses behave as if 

 they were plastic. In consequence tides of the whole substance are set 

 up in them if they rotate, and these tides act as a brake upon the rota- 

 tion until they finally retard it to coincidence with the orbital revolution. 



That Yenus now turns the same face in perpetuity to the sun, lets 

 us look clown a long vista in her career, and gives us a very instant idea 

 of a phase in a planet's history : that long slow change by which a day 

 is lengthened to infinity. That Yenus should have suffered such action 

 is in keeping with theory, though it could not have been predicted in 

 the absence of facts. For Yenus falls exactly on debatable ground, on 



