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December 21: Venus Transit

     
  December 21: Venus Transit  
  Venus Transit  
 

 

Click on the above image for a link to a movie showing the Venus transit

 

 
 

On June 6th 2012, a Venus transit of the Sun took place. It was visible as a black circle crossing the solar disk. From Belgium, we could only see the transit for a short period, as it was nighttime in Belgium for the majority of the transit, and it was only visible very early in the morning. However, the PROBA2 observed the transit from the beginning to the very end from a front row seat in orbit! The event was recorded by SWAP, and can be seen by clicking on the image above, a second movie can be found here. This is an extremely rare event and we will have to wait until 2117 for the Earth, Sun and Venus to line up again. So, the next Venus transit will take place in 105 years!

Previous Venus transits were historically of great scientific importance. They were especially useful for gaining the first realistic estimates of the size of the Solar System. Observations of the 1639 transit, combined with the principle of parallax, provided fairly accurate estimates of the distance between the Sun and the Earth. Parallax is a method of estimating distance based upon two different viewpoints. The method could be applied as Venus transits occur in pairs, separated by approximately eight Earth years, and observations could be made from different points in Earths orbit.

The 2012 transit was also of scientific importance to the  PROBA2 team, as it provided an opportunity to check out their telescopes and calculations. Since Venus ‘touched’ the Sun exactly at the predicted time, they obtained re-assurance that PROBA2’s orbit is accurately known. The contrast of the images is limited by stray light in the telescope. Ideally Venus should block all the sunlight and should have appeared completely black. By measuring the amount of (undesired) light in the ‘Venus spot’, it was possible to learn a lot about the stray-light in the SWAP optical system.

For more information on PROBA2's observation campaign take a look here.