The Moon that Wasn't: The Saga of Venus' Spurious Satellite

by Helge Kragh, Kurt Möller Pedersen (Assistant)

The planet Venus is the closest neighbour to the Earth and in several respects similar to our globe. It revolves around the Sun at an average distance of 0. 72 astronomical units, in an elliptical orbit of eccentricity 0. 007. The corresponding 3 numbers for the Earth are 1 and 0. 017. The mean density of Venus is 5. 2 g/cm , 3 that of the Earth 5. 5 g/cm . Venus’ acceleration of gravity at its equator is 8. 9 2 2 m/s , compared with 9. 8 m/s at the Earth. The escape velocity is 10. 4 km/s, while the corresponding ?gure of the Earth is 11. 2 km/s. Although the mass of Venus is somewhat smaller than that of the Earth – the ratio is M /M =0. 815 V E – the diameters of the two planets are almost the same. In other words, Venus is indeed a sister planet of the Earth. In earlier times, when almost nothing was known about the physical con- tions of Venus, the similarity appeared even stronger than today. Not only was Venus’ period of rotation thought to be close to that of the Earth, it was also p- sible (and indeed common) to imagine intelligent life on Venus.

211 pages, Hardcover

First published January 1, 2008

About the author

Helge Stjernholm Kragh is a Danish historian of science.
Kragh studied mathematics and physics at the University of Copenhagen before obtaining his PhD in 1981 at the University of Roskilde. He is a professor at the Centre for Science Studies of Aarhus University.

Kragh's areas of study are the history of physics from the mid-19th century onward, the history of astronomy, the history of cosmology and the history of chemistry.