Transit of Venus on June 5th –Only 7th Time Since Invention of the Telescope

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On June 5, 2012 at 6:03 PM EDT, the planet Venus will do something it has done only seven times since the invention of the telescope: cross in front of the sun. This transit is among the rarest of planetary alignments and it has an odd cycle. Two such Venus transits always occur within eight years of each other and then there is a break of either 105 or 121 years before it happens again.

NASA's Solar Dynamics Observatory (SDO) will be watching the June 2012 transit to help calibrate its instruments as well as to learn more about Venus's atmosphere (see today's post: "Did Early Venus Harbor Life"). 


The moments when Venus first appears to cross the limb of the sun and the moments it leaves, known as ingress and egress respectively, are historically the most scientifically important aspects of the transit since comparison of Venus's journey viewed from different points on Earth provided one of the earliest ways to determine the distance between Earth and the sun. 

The SDO team can use the lightless center of Venus to help calibrate what is called the point spread function of the telescope. This function describes how much light leaks from one pixel into others around it. Since there is no light emitted from the very center of Venus as it crosses the sun, it serves as a perfect test case for an area of the image where the pixels should remain black. By measuring how much light bleeds into those pixels from the rest of the sun, the SDO team will have a better sense of how to correct for that. 

These measurements also help to understand the black drop effect – in which a tiny black spot appears to connect Venus to the limb of the sun — that bedeviled scientists' attempts to measure the exact position of Venus during transits in the 18th and 19th centuries.

Image at top of page shows Venus appearing as a black dot on the lower left edge of the sun in this image from NASA's Transition Region and Coronal Explorer (TRACE), captured during the 2004 transit.
Credit: NASA/TRACE/LMSAL

More information from SDO about the Venus Transit (and SDO footage of the transit available June 5 and June 6)

The Daily Galaxy via NASA/SDO

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