Telescopic Observations

Early telescopic observers were unable to see any genuine details on Venus, but on 9 January 1643 G. Riccioli recorded the Ashen Light, or faint visibility of the night side of Venus. It was formerly dismissed as a mere contrast effect, but it is now believed to be due to electrical phenomena in the planet's upper atmosphere.

Dark markings on the disk were reported by F. Fontana in 1645, but he was using a small-aperture, long-focus refractor, and there is no doubt that his 'markings' on Venus were illusory. In 1727 F. Bianchini, from Rome, went so far as to produce a map of the surface, and even gave names to

Table 5.3. Transits of Venus, 1631-2200.

Date

Mid-transit (GMT)

1631 Dec 7 05.21

(not observed) 1639 Dec 4 18.27 1761 June 6 05.19 1769 June 3 22.26 1874 Dec 9 04.07 1882 Dec 6 17.06 2004 June 8 08.21 2012 June 6 01.31 2117 Dec 11 02.52 2125 Dec 8 16.06 These are followed by transits on 2247 June 11, 2255 June 8, 2360 Dec 13 and 2368 Dec 10. Earlier transits occurred in 1032, 1040, 1153, 1275, 1283, 1396, 1518 and 1526.

the features he believed that he had recorded - such as 'the Royal Sea of King John', 'the Sea of Prince Constantine' and 'the Strait of Vasco da Gama'. Again these markings were illusory; Bianchini's telescope was of small aperture, and as the focal length was about 20 m it must have been very awkward to use.

J. H. Schroter, using better telescopes (including one made by William Herschel) observed Venus from 1779, at his observatory at Lilienthal, near Bremen. He recorded markings, which he correctly interpreted as being atmospheric, but also claimed to have seen high mountains protruding above the atmosphere.

In fact no Earth-based optical telescope will show surface details; all that can be made out are vague, impermanent, cloudy features. Neither is conventional photography more helpful, but in 1923 F. E. Ross, at Mount Wilson, took good photographs at infra-red and ultra-violet wavelengths. The infra-red pictures showed no detail, but vague features were shown in ultra-violet, indicating high-altitude cloud phenomena.

Table 5.4. Planetary conjunctions involving Venus. (a) 2000-2015.

Closest

Table 5.4. Planetary conjunctions involving Venus. (a) 2000-2015.

Closest

Planet

Date

approach (GMT)

Distance (")

Uranus

2000 Mar 4

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