So what is this transit of Venus event that is happening June 5th 2012 (or June 6th depending on where you are on Earth)? The event is called a transit because the definition of transit or to transit means to “pass across or through.” In this case Venus can be seen passing across the disk of the Sun as viewed from Earth. When viewed properly and safely (please don’t do so without proper equipment) you will see the planet Venus (a small black circle) slowly move across the Sun. Why is this such a big deal? First of all, seeing this with your own eyes really drives home that Earth is just a small part of a much larger place...our Solar System. The event also has some scientific significance. It was by witnessing the transit of Venus in 1761 that Russian scientist Mikhail Lomonosov discovered that Venus had an atmosphere similar to Earth! Also, this is an extremely rare event. In fact, it has only been possible to see from Earth 6 times since the telescope was invented (this will be the 7th)! Venus can only be seen transiting the Sun on 122 or 105 year intervals. The timeline figure below shows both past and future Venus transit events (notice how there are always 2 transits 8 years apart and then nothing for over 100 years).
So why does it take so long between transits and what’s going on? Venus is the next closest planet in from Earth in the direction of the Sun. Venus takes 224 days to go around the Sun and the Earth takes a bit longer, 365 days. In order for Earth, Venus and the Sun to form a perfect line Earth and Venus must have the same "clocking" (like when the minute and hour hands of a clock are on top of each other). In order for this to happen Earth and Venus must "synch up" in their orbits and because they orbit at different speeds this only happens every so often (about every 1.7 years). So if Earth and Venus line up every 1.6 years why the 100+ year wait? Earth and Venus do not orbit the Sun in the same plane, so even though they may be perfectly in line with each other as viewed from the top down, when viewed from the side the two planets are not lined up because their orbit planes do not coincide. The figure below shows a side view of Earth and Venus (not to scale) and you can see that when they appear to line up on the left side of the figure they are not lined up in the side view (bottom of the figure) because of the difference in their orbital planes.
However, when they are shown as lined up in the middle of the figure they are also lined up in the side view (bottom of the figure) because even though their orbits are in different planes their orbits do cross at two points in their orbits (called nodes). It is at these nodes where it is possible for Earth and Venus to be perfectly in line with each other resulting in Venus being seen from Earth as passing in front of the Sun. The reason there is a 100+ year wait between transits is that Earth and Venus not only have to line up but they must line up at one of the two nodes in their orbits where their planes cross. This timing of that occurrence is on the order of every 100+ years. So why do we get two transits within 8 years of each other? Because the alignment of the two planets repeats every 8 years and that alignment point is still close enough to a node that they are still aligned. However, the third 8 year alignment then occurs too far away from the node location so the two planets are yet again in different planes for during alignment for another 100+ years. The BBC put together an awesome video that does a great job of showing the orbital mechanics behind the event!
So where can you view this incredibly rare event? If you have the right equipment you can most likely see it from your own backyard. But even if you don't have any equipment, find a local astronomy club or museum and more than likely they will have a viewing event in your area. The final option is to watch the even unfold on the internet. Telescopes from around the world will be broadcasting live images and telescopes orbiting above the Earth will be doing the same. There is no excuse for missing this once in a lifetime opportunity. Below is a list of ways you can participate from the comfort of your home web browser or smart phone screen:
-Great NASA website that will have both live data on Tuesday as well as animated simulations so you know what it will look like from your location. The live data will be coming in from the NASA spacecraft SDO (which I have a bit of a personal attachment to). SDO is in a geosynchronous orbit and its sole purpose is to continuously view the Sun.
-Mt. Wilson Observatory live feed by Astronomer Without Borders:
-The TransitofVenus.org website is a great source of information about the event, including tips on how to view the transit safely.
-Live coverage of the transit from ground based telescopes from all over the world will be displayed at the Slooh website.
-CosmoQuest.org will be hosting a live Google+ hangout event
-There are some great smart phone apps out there to help you understand, enjoy and even share your viewing of the event with other:
iOS Venus Transit app
Android Venus Transit app
-You can also watch and participate in the transit on Twitter! People from all over the world will be tweeting about it. Do a search for the following hashtags:
I plan to view the transit with my own scope and hope to do some imaging as well using a DSLR hooked up to the scope. I'll have a post about my setup (and weather permitting) some test pictures of the Sun later this weekend.