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Nature volume 101, pages 192193 (09 May 1918) | Download Citation



PHOTOGRAPHS OF THE SPECTRUM OF VENUS.—In a recent report to the Royal Astronomical Society (Monthly Notices, vol. lxxviii., p. 278) Mr. J. Evershed gives a preliminary account of some photographs of the spectrum of Venus which have been obtained with the large, grating spectrograph at Kodaikanal. The primary purpose of the investigation was to ascertain whether the general shift of the lines towards the red at all points on the visible disc of the sun affects also a hemisphere turned 900 or more from the earth. If the wave-lengths in the light from Venus, after correction for the motion of the planet as a whole, are found to be identical with those from ordinary sunlight, the solar displacements cannot be attributed solely to motion of the absorbing gases; but if the Venus spectra show a smaller wave-length, a general motion of the solar vapours away from the earth may reasonably be inferred. Five good photographs, with iron arc comparisons, were obtained in October, 1917 which agree with a previous series of plates taken during February, 1917, in showing a distinctly smaller wave-length for iron lines m the spectrum of Venus as compared with the corresponding lines in the control spectrum of daylight. The results thus favour the motion interpretation of the solar shifts, involving an earth effect; but as the February plates were possibly not entirely free from pole effects in the arc, confirmatory evidence will be sought during June and July next. The trustworthiness of the plates for the purpose in view is indicated by the fact that the combined results from the east and west elongations yield a value for the solar parallax which differs only very slightly from that adopted in the Nautical Almanac. Only one plate was obtained when the planet was at half phase or less, but this is of special interest as showing a discrepant velocity, which is difficult to account for except by supposing that Venus rotates in the same direction as the earth and with the same order of velocity. It is further expected that the Venus plates will eventually decide whether the sun's gravitational field is concerned in the solar line-shifts or not.

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