53° 



THE POPULAR SCIENCE MONTHLY 



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Chart of the Planet Venus. 



plates were made in like manner on that planet. Now Mars is an ob- 

 ject which by reason of its smaller size is twice as difficult a test for 

 rotation in twenty-four hours as Venus. The plates too did not happen 

 to be so good. Nevertheless, on measurement they yielded a result 

 within a twenty-fourth part of what we know to be the Mars day. For 

 we know this time to within the hundredth of a second. Now in conse- 

 quence of the smaller quantity to be measured an error of 55 minutes 

 in the case of Mars corresponds to one of 31 minutes on Venus. To this 

 precision, then, the day of Venus would have been determined had it 

 been of twenty-four hours' duration. 



Another test of like character was forthcoming in Dr. Slipher's 

 spectrograms of the rotation time of Jupiter. Inasmuch as Jupiter's 

 day at the equator is 9 hours 50.4 minutes long, while Jupiter's diam- 

 eter is some twelve times that of Venus, the precision possible is here 

 thirty times as great. Thirty-one minutes' error on Venus would mean 

 about one minute for Jupiter. The spectrograms did even better than 

 this. The known speed of rotation at Jupiter's equator is 12.63 km. ; 

 Dr. Slipher's spectrograms gave 12.62 km., or within half a minute of 



