On June 8, 2004, millions of people witnessed an event that no one still alive had ever seen: a transit of Venus. The previous transit was in 1882, but as they happen in pairs, if you missed the last one, the next one is on June 5-6, 2012. After that there isn't another for over a hundred years.

But what is a transit and why was it important?

Transits, occultations and inferior conjunctions
When a heavenly body crosses in front of another one, it’s called a transit if it doesn’t cover much of the object behind it. If it hides all (or a good deal) of the one behind, it’s an occultation.

Mercury and Venus are inferior planets to Earth. This isn't snobbery, though I think we would all prefer to live on Earth. The inferior planets orbit closer to the Sun than we do and the superior planets farther away. All of the planets orbit at different speeds, so they only line up occasionally. However when Venus (or Mercury) is between Earth and the Sun, we call it an inferior conjunction.

Ecliptic and nodes
A transit of Venus can only occur when two conditions are met. Venus must be at inferior conjunction, which happens every 584 days and Venus must be at what we call a “node”.

The planets all orbit the Sun in the same direction and nearly the same plane, which you can see here, as though there were a giant tabletop in space. (The picture is not to scale and of course, the planets don't all line up like that.) However each planet's orbit is tilted somewhat in relation to the others.

We call the plane of the Earth's orbit the ecliptic. The orbit of Venus is tilted 3.4 degrees to the ecliptic, but there are two points where the planes intersect. They are called nodes, as shown in this diagram.

First observed transits
Although people have observed the heavens for thousands of years, only seven transits of Venus have ever been seen. Why is that?

It's not impossible to see a transit of Venus without an optical aid. Wearing eclipse glasses for eye protection, I was able to see Venus in the 2004 transit. However, the viewing conditions were excellent, there were no sunspots in the way and I first saw its position in a telescope.

All of the observed transits have happened not only after the invention of the telescope, but after Johannes Kepler produced planetary tables accurate enough to predict transits.

Kepler died in 1630, but he had predicted transits of both Mercury and Venus for 1631. French astronomer Pierre Gassendi (1592-1655) was the first person to observe a transit. He saw the transit of Mercury in November, but not Venus the following month. Not only did Kepler's calculations need further refinement, but it was still night in France when the transit occurred. So it looked like 1761 would be the next chance.

The first observed transit of Venus
However in 1639, a young English astronomer in the north of England did make corrections to Kepler's calculations and saw that there would be a transit the following month. These days he'd have been able to tweet it and the whole world could have been out watching. He just managed to contact his friend William Crabtree. Horrocks and William Crabtree were the first two people to observe a transit of Venus.

Using transits to measure the Solar System
Kepler's third law of planetary motion gave a mathematical relationship between the distances of the planets from the Sun and their orbital periods, but only in terms of their distances relative to each other. If someone could measure the astronomical unit (AU), the Earth-Sun distance, the rest would follow. Yet that was a big “if”.

In 1716 Edmond Halley (1656-1742), known today for the comet named after him, suggested that a transit of Venus could be used to calculate the astronomical unit. Observers in different hemispheres could make use of parallax. Parallax is the name we give to the apparent change in position of a distant object viewed from two different places. You can see this, literally, with your own eyes. Hold a finger out in front of you and look into the distance, first closing one eye and then the other and see what happens.

A simplified diagram shows what the observers in the northern and southern hemispheres would see. Using the angle of the apparent shift, some geometry and a bit of help from Kepler, they could deduce the length of the astronomical unit.

Halley died in 1742, but a number of teams did travel to the north and the south in 1761 and in 1769. Although it turned out to be a lot harder than it seemed, they got a fair approximation of the size of the astronomical unit. But that's a story for another time.

Note: For information about seeing the June 2012 transit, I will be posting updated information on the Astronomy Forum