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Observational history

Venus has been known since pre-telescopic times since it the brightest of the Solar System objects after the Sun and the Moon (maximum magnitude of -4·7). It is considerably easier to observe than Mercury and is visible in daylight. I once set up my telescope on Venus before sunrise and was able to track it easily throughout the day. I have subsequently seen it in the daytime through a pair of 8X25 binoculars.

Galileo observed Venus and noted its phases. He discovered that they repeated every 292 days. He was careful how he reported this in order to avoid more conflict with the Church stating something along the lines of 'the godess of love mimics Cynthia' (Cynthia being the Moon). The discovery of the phases of Venus was significant in that it went against the established geocentric Solar System thinking of the time. Proponents of the geocentric theory argued that if Venus orbited the Sun, its brightness would be significantly different because of the gratly varying distance from the Earth. This is easily explained by the phases. As the planet travels further from Earth, the proportion of the planet that is illuminated is greater, thus compensating for the extra distance (see below).

  • At superior conjunction - full phase but small diameter of disc (10 arcsec)
  • At elongation - half phase but larger by 2·5 times (25 arcsec)
  • Near inferior conjunction - thin crescent but much larger still (60 arcsec)

Comparitive sizes of the phases of Venus.

Click on the above image for large version.

The phases observed by Galileo were confirmed by Fontana in 1645. The Italian astronomer Cassini commented on the fact that it would be difficult to determine the rotation period since there were no readily visible surface features. Despite this comment, in 1727, a map was produced, clearly showing the ocens and continents of the planet! Cassini also believed that he had discovered a satellite of Venus. This was 'rediscovered' in 1761 by a French astronomer who allegedly observed it for a month. These seem to have been genuine mistakes, possibly caused by poor optics or perhaps faint stars in the vicinity of the planet at the times of the observations.

Transits afford a good method of determining the orbit of a planet accurately. These are pretty rare in the case of Venus. One pair occured in 1761 and 1769. Famously, Captain James Cook was despatched in the ship the 'Endeavour' to observe the 1769 transit from Tahiti, in the Pacific Ocean. The location of the observatory is still called 'Venus point'. Problems with the timing were experienced so the expedition was not wholly successful. To quote Cook:

    "we very distinctly saw an atmospheric or dusky shade round the body of the planet which very much disturbed the timing of the contacts"
The black drop had been observed for the first time. The following list shows other significant moments in the observation of Venus.
Percival Lowell drew canals on a map of Venus.
Svante Arrhenius, the Swedish chemist, suggested the swampy nature of Venus.
Seth Nicholson and Charles St John argued the opposite.
Adams and Durham showed that CO2 was present in the atmosphere.
Dollfus measured small amounts of water vapour in the atmosphere of Venus from high altitude ballon flights (up to 14,000 metres). The high altitude was necessary in order to prevent interference form the water vapour in the Earth's atmosphere.
Measurement of radio wavelength emissions from Venus suggested a very hot surface. Various complex reasons were suggested, in order to preserve the 'swamp' idea. The simple explanation that the surface was indeed hot came with the Mariner 2 flyby in 1962. Also, radar was used to investigate the rotation of the planet. A slow retrograde rotation was reported by Robert Richardson. Since the probability of error was high, Schiaparelli's idea of synchronous rotation was not dislodged.
French astronomer C. Boyer measured a four day retrograde motion of the clouds by observing in UV wavelengths.
Richardsons retrograde rotation was confirmed.
The astronomer J. Strong sent up an unmanned balloon to a height of 26,000 metres. His measurements confirmed Dollfus's findings.


Venus is 95% of the diameter of the Earth. It comes closer to the Earth than any other planet, approaching to a distance of about 40 million miles. The planet appears featureless in visible light, apart from a few bright patches, notably the cusp caps. These are areas at the poles that seem to extend into the dark area. The surface is never visible, unless radar is used to penetrate the clouds. This has caused much speculation about the nature of the actual surface, both scientifically and otherwise. Seen as a twin to Earth, the fertile imaginations of Science fiction and fantasy writers have generated many powerful images of extraterrestrial life. Since the surface is permanently hidden from view by thick, white clouds, and given that Venus is closer to the Sun than the Earth, a lush, heavily vegetated tropical rainforest environment has been the favourite. The imagined inhabitants have ranged from humanoids to intelligent amphibians. Up until the 1960's, even scientists thought that life could exist on this planet. One of the first to suggest seriously that there was life on Venus was Franz Von Paula in the 19th century. A phenomenon called the "Ashen Light" had been observed. Venus shows phases, on account of it being an inferior (closer to the Sun) planet. Some observers noted that the night side sometimes appeared to be brighter than expected when at the crescent phase. It is still not clear as to whether this is a real phenomenon or a trick of the eyes. Anyway, Von Paula decided that the Ashen Light was actually due to the lighting on the surface as the inhabitants celebrated the accession of their new Emperor! The majority of serious scientists who subscribed to the life on Venus idea took the same view as the writers. They believed it to be rather like the Earth in the Carboniferous Coal Measures era, with giant fern trees and rather wet. It was discovered that there were high levels of carbon dioxide in the outer layers of the atmosphere so there would be a greenhouse effect. Further speculation suggested that this would make the temperature very high, high enough to create a dusty, desert world. The advent of space travel confirmed that the latter idea was closer to the truth.

In addition to the supposition of life on Venus, there were many ideas about the length of the day. These were based on observations of the vague shadings that were occasionaly observed. Since these would be clouds, there was a great variation in the estimates from one Earth day to several months. When it was finally discovered using radar (which penetrates the clouds and reflects back off the solid surface), the actual length of the day was 243.2 Earth days - longer than the year of Venus (224.7 Earth days). In addition, the rotation is retrograde (east to west rather than west to east like most of the bodies in the Solar System). The retrograde rotation can not be satisfactorily explained, the closest is that there was a huge impact that completely overturned the planet - not particularly believable but the best we have so far. Perhaps in a couple of hundred years, people will be amused by our naiivity in this matter as we are with the idea of Von Paula!

When the clouds of Venus are observed at Ultra Violet wavelengths, banding can be seen. These bands show variation over a period of hours and some spots persist for several days. Probably one of the most used images of Venus is the one that shows these 'V' shaped bands.

When observing the terminator closely, irregularities have been noted. This is caused by ridges and hollows in the cloud layer. It is not possible to predict the phases of Venus with 100% accuracy, particularly at the dichotomy (half phase). This was observed and noted by the astronomer Schröter in the 1790's and has been named 'Schröter's Effect' in his honour. At evening appearances, the half phase tends to be earlier than expected and vice-versa for morning appearances. The cause is not known (as far as I am aware).

Space Missions

There have been a number of space craft that have either visited or flown past Venus. The first positive information from remote sensing came from the American craft Mariner 2 in Dec 1962. This passed within 22,000 miles of the planet and the data disposed of the ocean planet ideas. The Russians were the first to achieve a controlled landing on the surface of Venus with Venera 7. The on-board equipment lasted 23 minutes or so before succumbing to the high pressures and temperatures of the Venusian atmosphere. It sent data that indicated an atmospheric pressure of 90 times that on Earth as well as temperatures of around 480deg. C. A second success for the Russians came in 1975 when they soft landed Venera 9 on Venus. This transmitted the first pictures from the surface. The rocks in the photos appear orange because of the lighting, they are in fact grey in colour. Venera 13 (landed March 1982) produced more surface pictures, confiming and refining the measurements of earlier spacecraft. Looking into the distance whilst standing on the surface of Venus would be very strange - the dense, hot atmosphere refracts (bends) the light and so the horizon appears curved rather than flat.

Probably the most significant and successful mission to Venus was the Magellan mapping project, intended to accurately map the heights of features to within 10m, by using radar. Radar had been used to map the surface of Venus prior to this mission, however, Magellan was to remain in orbit for several years in the early 1990's. The radar could map objects down to 120 feet across. Early mapping of Venus showed large areas of plains and lowlands, with much smaller areas of mountainous terrain. There were plenty of craters detected as well. Magellan took this a stage further and identified features such as individual lava flows, domes, 'pancakes' and volcanoes and the strange features called 'arachnoids'.

Click to view image of 'pancakes' Click image of 'pancakes' to view larger version

The 'arachnoids' are unique to Venus and are so called because of their resemblance to spiders webs. With the exception of 'Maxwell Montes', all of the features on Venus have to be named after women. Maxwell Montes escaped this because it had been named before the IAU (the official body that is responsible for overseeing the naming of astronomical objects and planetary features) had decreed that this should be so.


Diameter 12,104 km
Sidereal period (day length) 224.7 Earth days
Inclination of equator to orbit 177.34 degrees
Density (water = 1) 5.25
Escape Velocity 10.36 km per sec
Surface gravity (Earth = 1) 0.903

Resources I used (some are getting a bit old now!):

The linked books are linked to

Philip's Atlas of the Universe - Patrick Moore. This is an absolutely superb book, one of the best £25 investments that I have ever made.

The Planets - Peter Francis

Norton's Star Atlas

Astronomy Now magazines

Hutchinson's Splendour of the Heavens

Astronomy - Fred Hoyle

The Amateur Astronomer - Patrick Moore

Other resources (that I did not use but look potentially good!):

The Planets - David McNab, James Younger

Teach Yourself the Planets - David A. Rothery

Collins Pocket Guides: Stars and Planets - Ian Ridpath, Wil Tirion (Illustrator)