THE PLANET VENUS 



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Spectrogram of Jupiter above and of Venus below, showing Iron Comparison. 



the true rotation period. This means that they would have shown 

 Venus's to within 14 minutes if the conditions were as good. As 

 Jupiter's spectrograms are easier to measure than Venus's, while Mars's 

 are more difficult, Ave may take 25 minutes for the mean of the two 

 criteria. I need perhaps not tell you that no previous spectrograms of 

 Jupiter for rotation had come up to this precision. 



From these two determinations on Jupiter and Mars we may de- 

 termine the utmost period of rotation for Venus which the spectroscope 

 could disclose. This would be the period for which the probable error 

 was just equal to the quantity to be measured. From Mars we have 

 for a 24-hour period on Venus a probable error of 31 minutes. This is 

 one forty-eighth of the quantity measured on the supposition of a day's 

 period. One of 48 days, therefore, would have its probable error equal 

 to the quantity itself. From Jupiter we get in the same way 96 days. 

 Thus from two to three months would be the limit of leisureliness the 

 spectroscope could be got to note, and it was just this quantity that the 

 investigations on Venus themselves expressed. 



The spectroscope, therefore, definitely asserts that the rotation of 

 Venus does not take place in anything approaching twenty-four hours, 

 and by negativing any period up to two or three months long corrobo- 

 rates to the limit of its ability that shown by eye observation, one of 

 225 days. 



The care at Flagstaff with which the possibility of error was sought 

 to be excluded in this investigation of the length of Venus's day and 

 the concordant precision in the results are worthy of notice. For it is 

 by thus being particular and systematic that the accuracy of the de- 

 terminations made there in other lines besides this has been secured. 



Now a certain peculiarity of Venus's appearance of a totally dif- 

 ferent kind from those so far spoken of, here comes in to corroborate 

 both of the previous determinations : the perfect roundness of her figure. 

 For this very rondure has something to disclose. If Venus rotated in 

 anything like twenty-four hours her disk should be perceptibly flattened 

 at the poles, her figure becoming squat in consequence of her spin. For 

 though as rigid as steel to sudden impulses she would be like putty at 



