with the speed in miles, not in angular amount, that the spectroscope is concerned. Nevertheless, when a like number of plates were tried on him, they indicated on measurement a rotation time within an hour of the true. This corresponds to half an hour on Venus. We see, therefore, that had Venus' day been anywhere in the neighborhood of twenty-four hours, Dr. Slipher's investigation would have disclosed it to within thirty-one minutes.
This result was further borne out by a similar test made by him of Jupiter. Inasmuch as the diameter of
Jupiter is twelve times that of Venus, while the rotation time is 9h 50m.4 at the equator, the precision attained on Venus should here have been about a minute. And this is what resulted. Slipher found the rotation time spectrographically 9h 50m, or in accordance with the known facts, while previous determinations with the spectroscope had somehow fallen short of it.
The care at Flagstaff with which the possibility of error was sought to be excluded in this investigation of the length of Venus' day and the concordant precision
