Editing Venus (section)

===Early 20th century advances===
Little more was discovered about Venus until the 20th century. Its almost featureless disc gave no hint what its surface might be like, and it was only with the development of [[astronomical spectroscopy|spectroscopic]] and [[ultraviolet]] observations that more of its secrets were revealed.

Spectroscopic observations in the 1900s gave the first clues about the Venusian rotation. [[Vesto Slipher]] tried to measure the [[Doppler shift]] of light from Venus, but found he could not detect any rotation. He surmised the planet must have a much longer [[rotation period]] than had previously been thought.<ref name=Slipher_1903/>

The first ultraviolet observations were carried out in the 1920s, when [[Frank E. Ross]] found that [[Ultraviolet photography|ultraviolet photographs]] revealed considerable detail that was absent in visible and [[infrared]] radiation. He suggested this was due to a dense, yellow lower atmosphere with high [[cirrus cloud]]s above it.<ref name=Ross_1928/>

It had been noted that Venus had no discernible [[oblate spheroid|oblateness]] in its disk, suggesting a slow rotation, and some astronomers concluded based on this that it was [[tidally locked]] like Mercury was believed to be at the time; but other researchers had detected a significant quantity of heat coming from the planet's nightside, suggesting a quick rotation (a high surface temperature was not suspected at the time), confusing the issue.<ref name=Martz_1934/> Later work in the 1950s showed the rotation was retrograde.