100 Generating Economic Cycles 



termine the rotation period of a planet by means of 

 spectrograms was tested by his spectroscopic approxi- 

 mation of the well known periods of rotation of Mars 

 and Jupiter. The rotation period of Mars is known to 

 be 24^ 37"^ 23'.66. Dr. Slipher determined its period 

 from spectrograms within an hour of the true time. The 

 rotation period of Jupiter is 9*" 50°^.4. Dr. Slipher found 

 the rotation period from spectrograms to be 9*^ 50*^, or 

 within a minute of the exact tune. Wlien, in an exactly 

 similar manner, he made his observation of Venus, the 

 spectrograms showed that the rotation period must 

 exceed three months, which meant that so slow a rota- 

 tion as three months was beyond the power of Dr. 

 Slipher's spectroscope to disclose.^ 



1 Lowell, The Evolution of Worlds, pp. 83-89. 



The reasoning of the next chapter will show that the theory of 

 perturbation by Venus will stand even if the period of the planet's 

 rotation on its axis is not coincident with the period of revolution about 

 the Sun. Nevertheless I wish to add opinions of experts confirmatory 

 of the statement in the text: 



(a) "Dr. W. W. Campbell, Director of the Lick Observatory . . . 

 states : ' We have given some attention to the spectrographic velocities 

 of the east and west edges of the planet Venus, but we have written 

 very little on the subject. Our results seem to point to a long period 

 of rotation; at least several days in length and possibly a period coin- 

 cident with the revolution period around the Sun.' " Popular Astron- 

 omy, March, 1922, p. 139. 



(b) "It may be remarked that when the observations are corrected 

 for the systematic displacements to the violet discussed above, the 

 difference between morning and evening series, viewed in the light of the 

 probable error of observations, is not of an order that would indicate 

 a divergence from the assumed parallax or a rate of rotation higher 

 than that found by Slipher." "On Systematic Displacement of Lines 

 in Spectra of Venus." By Charles E. St. John and Seth B. Nicholson. 

 Contribution from the Mount Wilson Observatory, No. 208, p. 9. 



(c) "By the use of Barnet 'Ultra Rapid' plates hypersensitised 

 with ammonia it has been possible to photograph Venus spectra with 

 a very narrow slit, and these are the finest plates hitherto obtained. 

 They give no evidence of an inclination of the lines due to a rotation 



