Spectroscopic observations of winds on Venus: I. Technique and data reduction

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Methods of measurement and reduction of high-dispersion photographic spectra of Venus are presented. Preliminary results are consistent with slow direct or no rotation at the level sample, and disagree strongly with a 4-day retrograde rotation. A serious systematic error, which affects much published work, is due to blending of solar lines in the sky with those reflected from the planet. This always tends to produce a spurious retrograde 'rotation'. Only data obtained in a dark sky, or daytime observations from which the sky lines have been accurately subtracted, can be relied upon. All such data give low wind speeds.