Venus

• transit of venus
• 2004 transit
• 2012 transit
• see also:
  • astronomy index and links
  • Solar System
  • for some nice pics of Venus try http://www.madpc.net/~firmament/astro/main_screen.html?../images/venus/venus_thumbnails.html&2 

Transit of Venus:

• see my pics of the 2004 transit in a collage as seen from Cairns
• this occurs when venus passes in front of the sun as viewed from earth
• the June 8, 2004, venus transit is a very rare event (no living person has ever seen one!) lasts about 6 hours and will be visible from Australia although sunset will prevent the 2nd half of it being visible. It will most certainly generate unprecedented attention from the media and the public.
• the history of the transits of Venus is the history of the quest for the determination of the Earth-Sun distance. A history where we find astronomers that, with better or worse fortune, travelled the world to observe this phenomenon with the goal of determining our distance to the Sun (eg. Capt. Cook), as Halley had proposed in the XVIIth century.
• The first such attempt to determine the Astronomical Unit was undertaken in 1761, however with little success due to the lack of experience of the observers.
• using the transit to measure the distance of earth to the sun:
  • A Venus Transit observed with sufficient accuracy from the Earth allows the determination of the distance between the Earth and the Sun. This measure is crucial for astronomers to know because all other distance determinations in the universe ultimately depend on this value. Today astronomers use other, more precise methods for the determination of this "Astronomical Unit", so the present one is above all of historical and didactic interest.
  • Many observations all over Eurasia/Africa/Australia are needed in order to get a reasonably accurate result for the Astronomical Unit. And for every participant, this is a marvellous opportunity to take part in a first-ever "public" re-enactment of a famous historic observation.
• how often do they occur?
  • Venus Transits occur approximately 4 times in 243 years, more precisely in pairs of transit events separated by about 8 years and these pairs are separated by about 120 years. The reason for these long intervals lies in the fact that the orbits of Venus and Earth do not lie in the same plane and a transit can only occur if both planets and the Sun are situated exactly on one line (the "line of nodes"). It takes Venus about 225 days and Earth about 365 days to circle the Sun. This means that when Venus returns to one of the nodes, Earth is not yet there, and vice versa. In addition, a couple of smaller, complicated effects lead to the observed long and slightly irregular intervals. Following the event in June 2004, the next Venus Transit will happen on June 6, 2012, and thereafter in December 2117 and December 2125.
  • The first Transit of Venus was observed 1639.
• relevance to discovery of exoplanets:
  • Transits of planets in front of stars other than the Sun (in astronomical terminology: "exoplanets") constitute events similar to this Venus Transit. Exoplanetary transits do represent one of the best opportunities to detect such bodies around other stars. In fact, the transit method is probably - for the next decade at least - the only method to detect Earth-sized planets (which could possibly be habitable) around stars other than the Sun. It is based of the observation of a small, regular "dimming" of the light from the central star, every time the planet passes in front of the stellar disk and blocks a little of the stellar light. Although even the largest telescopes on Earth cannot see the exoplanetary disk, the dimming effect may be observed by means of very accurate brightness measurements ("photometry"). The effect is quite small, however; in two known cases, the measured dimming was only about 1 %. During the present Venus Transit, about 0.1% of the solar light will be blocked by the planet's disk.
• "black drop effect":

  • Because Venus has an atmosphere, sunlight gets refracted around the limb of the planet just at those crucial times. Although under normal circumstances this would be a bright light effect, during transits the point where Venus makes contact with the edge of the Sun, appears to grant Venus a little extra diameter. In fact the opposite is true, most of Venus' size is slightly compromised in transit, by sunlight leaking over its limb - what we witness is every sunrise and sunset on this distant planet!

• transit of mercury:
  • As seen from Earth, the planet Mercury also passes in front of the Sun. Because Mercury circles the Sun every 88 days (as compared with 225 days for Venus) and is also much closer to the Sun, transits of Mercury are much more common than those of Venus. On average, they occur approximately once every 7 years. The most recent one occurred on May 7, 2003

2004 transit:

• see images taken by amateurs:
  • my pics in a collage
  • Lawrence's collage
  • H-alpha image
  • video of 2004 transit black drop effect

2012 transit: