Science Networks. Historical Studies

2 primary works

The Moon that Wasn't

by Helge Kragh

Published 1 January 2008

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 wasalso p- sible (and indeed common) to imagine intelligent life on Venus.

Varying Gravity

by Helge Kragh

Published 16 February 2016

The main focus of this book is on the interconnection of two unorthodox scientific ideas, the varying-gravity hypothesis and the expanding-earth hypothesis. As such, it provides a fascinating insight into a nearly forgotten chapter in both the history of cosmology and the history of the earth sciences.

The hypothesis that the force of gravity decreases over cosmic time was first proposed by Paul Dirac in 1937. In this book the author examines in detail the historical development of Dirac’s hypothesis and its consequences for the structure and history of the earth, the most important of which was that the earth must have been smaller in the past.