Re: How venus transit happen?

An excellent and timely question since we just had a Venus transit on 8 June, our first in 120 years! (And, as you note, another will occur on 6 June 2012. I'm already planning to be somewhere where I can see this one!)

The first step to understanding how transits occur is to understand what they are. A transit will occur when an "inferior" planet (that is, one whose orbit is inside of Earth's; typically we're referring to Venus or Mercury) gets between us and the Sun. (Planets with orbits larger than Earth's, like Mars, can never get between us and the Sun.) In order to do this, the planet first has to be on the same side of the Sun as the Earth. (More exactly, the planet needs to be in inferior conjunction.)

How often does this happen? Well, say we start with Earth and Venus at the same longitude. One Venus year later (225 Earth days), Venus will be back at that longitude. But the Earth has moved away more than half of an orbit, so now Venus has to catch up. It turns out that Venus comes into inferior conjunction every 584 days. (The length of time between inferior conjunctions is called the synodic period.)

Clearly, this isn't enough to get a transit, though. Otherwise we'd have one every year and a half or so. So what's missing? The third dimension is what. All of the planets in our solar system orbit in more or less the same plane (except Pluto). In other words, you can draw a fairly accurate model of the solar system on a flat piece of paper. However, the orbits are a bit tilted relative to each other. For example, Venus's orbit is tilted 3.4 degrees relative to Earth's. The Sun always seems to lie in Earth's orbital plane (as seen from Earth, anyway). So with it's tilted orbit, Venus usually appears to pass above or below the Sun when it reaches inferior conjunction. Only when it's near the points where its orbit crosses the plane of ours will it be able to pass across the face of the Sun for us. There's a nice discussion of this here. The result of all of this is that we get two transits about 8 years apart and then we wait over a hundred years for the next pairing.

I hope this helps!