Friday, June 8, 2012

The Transit of Venus

Credit: NASA Solar Dynamics Observatory (SDO)
The transit of Venus was over on June 6 and I did not have time to finish this post while it was happening. However, this transit is rare enough that it's worth mentioning even belatedly. Indeed, another transit of Venus will not occur this century.

A few basic facts
The transit of Venus occurs when Venus passes between the Sun and the Earth and becomes visible against the solar disk. The planet  can be seen as a small black disk that moves across the sun obscuring small portions of the solar disk for a duration of a few hours.  In 2012, the transit lasted six hours and 40 minutes and was observed over two days: June 5 and June 6. The radius of Venus is a little over 6000 km. Venus is a rocky planet of about the same size as the Earth, and significantly larger than the Moon. However, Venus appears to obscure a much smaller portion of the Sun than the Moon from Earth because it's further from us. For this reason, the transit also is slower than a solar eclipse.

A bit of History
The last transit of Venus occurred 8 years ago in June 2004. These transits are predictable. They occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 year. The eight year gap is because the length of eight Earth years is almost the same as 13 years on Venus, so every eight years the planets are in roughly the same relative positions. The next transit of Venus will occur in December 2117, and the previous one was in 1882. The transits are so rare because of the inclination of the orbit of Venus relative to the Earth. The first transit was observed in 1639 and was used by Jeremiah Horrocks to correct Kepler's prediction for the orbit of Venus. Many people observed the transit of Venus in 2012 the same way Jeremiah did it so many years ago by focusing the image of the Sun through a simple telescope onto a piece of paper.

 How can scientists learn from this event today?
Scientists will use this event to test methods used to characterize extrasolar planets from transits events against what we already know about Venus, and to learn more about Venus. We can see how accurate is the prediction for the diameter of Venus relative to the diameter of the Sun from the dip in the sun's brightness. This event is also expected to improve our understanding of the atmosphere and climate of Venus, and to provide a testbed for methods of finding the atmospheric composition of extrasolar planets from transit events.

Did I see the transit of Venus in Zurich?
Not really. It was cloudy and too early in the morning. I have to confess I am not much of a practical astronomer/observer. I am more interested in the science that we can learn from events by analyzing data from instruments much more powerful than my eyes rather than in observing events myself. I do love to look at the beautiful images and videos that NASA provided for this transit.

For those who remember the transit of Venus in 2004, there was not so much talk about it then even though the transit looked similar by eye/small telescope/fancy glasses. What makes this event more dramatic for me is that we have the technology to learn a lot from it now, while we did not eight years ago. Of course, it's also the last time we'll see Venus transiting in front of the Sun in our lifetimes, and I am glad 'the world' is making the most of it.

More beautiful images and videos
If you search for the transit of Venus on youtube you will find several videos from NASA including one where they photograph it from the International Space Station.

There are also images and videos that can be downloaded directly from NASA website:

Venus and dogs
This is a topic that everyone and their dog* is writing about (quote from David Tsang, postdoctoral scholar at Caltech)**.

* some of my astrophysicist friends at Caltech have dogs
**Dave's comment was related to my suggestion of writing a paper on the two solar mass neutron star that was observed in 2010. Of course, neutron stars are not directly related to Venus, but Dave's expression was something that popped into my head while writing this post. Why? Because it's applicable to this post as well. There is a lot of information on the transit of Venus already on the web (and in books), but I wrote about it because it made me understand it better.