Venus Transit

Venus in transit in the optical (upper image),
ultra-violet (lower left image) and
extreme ultra-violet (lower right image),
image credit: NASA/LMSAL

What is a Venus transit? Have you ever seen a solar eclipse? A solar eclipse happens when the Moon moves in between Earth and Sun and blocks out the sunlight diving bits of the Earth into shade. One was just visible from in the US and Asia. A Venus transit is in principle the same thing, just swap the Moon for Venus. So essentially, Venus moves in between us and the Sun and blocks out a bit of its light. Since Venus, although itself bigger than the Moon and nearly as big as the Earth, is so much further away from us than the Moon, it's only a little speck in front of the sun and blocks out only a little bit of light. Doesn't seem too spectacular?

Venus Transit June 5th, 2012,
image credit & copyright: Pete Marenfeld

Well, it is spectacular! First of all Venus transits are a rare occasion and don't happen about twice a year as solar eclipses do. In fact, the last Venus transit was in 2004 and the one before that in 1882. The reason for these long time spans between transits is that the orbits of Earth and Venus around the sun do not lie in the same plane (see figure below). The orbit of Venus is tilted by about 3.4 degrees. So very often Venus just misses the Sun and travels either above or below it when it passes between us and the Sun. Only 6 transits have been observed scientifically so far, the first one being in 1639. 

Illustration of Earth and Venus orbits around the sun

Illustration of transit time measurements

Second, you can do awesome stuff with a Venus transit. It allows you to calculate not only the distance between Earth and Venus, but also the distance between the Sun and Earth! Now, some math is involved, but it's really quite easy. What's needed is a record of the time when Venus enters the sun and exits the sun from two different places on Earth. With the time difference between each of those events in each place and knowing the distance between your two observation spots a simple triangulation does the trick. (You can find a detailed description here.) This method for the determination of distances is also called the parallax method.

The first to suggest using this method to get a handle on the absolute distances between objects in our solar system was astronomer Edmond Halley (yep, Halley's comet is named after him) in the 18th century. Until then astronomers only knew the relative distances between planets in our solar system, e.g. that Venus is 0.7 times as far away from the Sun as the Earth is. Even during transits today groups of people all around the world combine their data to repeat this experiment, although more for educational than scientific purposes. However, the observations of transits is one of the modern methods to detect extrasolar planets around other stars by observing the dip in brightness of the star's light when the planet passes in front of it.

Astronomers in awe at NOAO during the Venus transit June 5th/6th 2012

Hanae Inami, new CANDELS member,
takes a picture of the projected sun,
credit & copyright: Jeyhan Kartaltepe

Astronomers at NOAO watching the
Venus transit (me at the right),
credit & copyright: Jeyhan Kartaltepe

You might think that only stellar or solar astronomers are interested in observing a Venus transit. I can assure you, that most astronomers were pretty awed by the spectacle. Many of us gathered outside at the National Optical Astronomy Observatory in Tucson to watch the tiny disk of Venus move in front of the sun at a few minutes past 3 pm with sun-viewing glasses and through telescopes with special filters. In fact, there were so many of us that you can't even make out the telescopes in the pictures!

Astronomers at NOAO watching the Venus transit, credit & copyright: Jeyhan Kartaltepe

Now the all important question: Did YOU get to see the Venus transit on June 5th (June 6th in Europe)? If yes, lucky you and I hope you viewed it safely! If not, you'll now have to wait for more than 100 years (105 to be precise) before the next one happens. So until medicine advances drastically in the next few decades in prolonging your average life expectancy or cryogenics can freeze you and wake you up again in time, you won't see another one in your lifetime!

If you want more information, check out the Bad Astronomy Blog Post on the Venus transit or Karen Master's Blog post about it!