Last chance to see a Venus transit

On June 5 or 6 (depending on your location) Venus will make a rare transit across the sun. This means that we will be able to see Venus moving across the face of the sun.

Why is this interesting? For one thing, a very successful method of finding exoplanets is the ‘transit method’. This method relies on the fact that the light coming from a distant star will dim ever so slightly as one of its planets passes in front of it. Watching a known transit close to home will give astronomers more confidence in the advantages and limitations of this method. The only planets that can pass in front of the sun from our perspective are Mercury and Venus. The other planets would see Earth transit the sun, not the other way around. Therefore, there are limited opportunities to observe a confirmed transit.

For people on Earth to see a transit, the plane of Venus’s orbit has to cross that of the Earth’s. Otherwise, Venus will appear either above or below the sun in our sky. This occurs in a repeatable pattern with 12, 105-121, 12, 105-121 etc. years between each event. The last Venus transit was twelve years ago in 2004. This means that there won’t be another such event for over a century. Unless drastic changes occur in the science of longevity, you won’t have another chance to see a Venus transit.

Transits are also of historical significance. In the eighteenth century, astronomers used the timing of a transit to determine the exact distance from the Earth to the sun. The square of the time it takes a planet to orbit the sun (its period, or year) is proportional to the cube of the radius of the orbit (distance from the planet to the sun). Back then, astronomers knew the periods of Venus, and of course, of the Earth. They could use measurements of the timing of Venus’s transit as seen from different places on Earth to determine the distance from Earth to Venus, and plug that in to get the distance to the sun. In fact, timing Venus’s transit was the mission that had originally sent Captain Cook to the Fiji Islands in 1769.

Jay M Pasachoff, an astronomer at Williams College, will be among those studying the transit next week. He writes:

For the upcoming transit of Venus this June we want to get the most complete set of data possible, so that the astronomers of 2117 will think that their forebears way back in 2012 did a fine job even with their relatively primitive instruments.

I admire his combination of pride and humility as well as his faith in the future.

You can find more information about the transit of Venus here. It includes a map of when Venus will cross the sun as seen from your location and a link to download an app so you can send in your own transit observations. It also describes how to safely observe the sun.

Of course, you can always let the folks at NASA do the observing for you. They’re going to have a live ustream channel broadcasting the transit. The folks at Astronomers Without Borders are also putting out information on the transit.


Here's a clip about Venus transits: