A review of radar astronomy — Part II

The determination of the axial rotational rate of Venus has long been an objective of planetary astronomers. It may be surprising to learn that we know the rotational period of all the planets in the solar system, even the most distant planet Pluto, with the exception of the planet that comes nearest to us, Venus! This anomaly fundamentally arose for two reasons: (1) Venus is shrouded with a thick, almost featureless cloud cover that prevents us from seeing its surface; and (2) Venus rotates very slowly. If the surface could be seen, we could track the motion of various markings and thus determine the rotation; or if Venus´ rotation were faster, we could observe the relative Doppler shift of the lines in its optical spectrum resulting from the difference in velocity between its approaching and receding limbs. We could then compute the period from this velocity. Unfortunately, Venus appears to rotate so slowly that the errors in the classical velocity measurements have been as great as the velocity actually measured.