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)
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.
- 1896
- Percival Lowell drew canals on
a map of Venus.
- 1918
- Svante Arrhenius, the Swedish chemist, suggested the swampy nature of Venus.
- 1920's
- Seth Nicholson and Charles St John argued the opposite.
- 1932
- Adams and Durham showed that CO2 was present in the atmosphere.
- 1950
- 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.
- 1956
- 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.
- 1960's
- French astronomer C. Boyer measured a four day retrograde motion of the
clouds by observing in UV wavelengths.
- 1962
- Richardsons retrograde rotation was confirmed.
- 1964
- The astronomer J. Strong sent up an unmanned balloon to a height of 26,000
metres. His measurements confirmed Dollfus's findings.
Nature
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 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.
Statistics:
| 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 Amazon.co.uk
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)
