1. The orbit of Earth and Venus around the Sun
2. The tilt of Venus' orbit along the line of nodes
This diagram illustrates the orbits of Venus (inner circle) and the Earth (outer circle) around the Sun. As Venus is closer to the Sun, it orbits the Sun much more quickly - once every 224 days.
The orbit of Venus is tilted by about 3.5 degrees with respect to the Earth's orbit. Therefore, Venus spends most of its orbit either above or below the Sun, as seen from the Earth. The orange line shows the points at which the two orbits cross and is known as the line of nodes. Since it travels much faster than the Earth, Venus passes the Earth once every 584 days or so. When this happens, the Earth, Venus and the Sun line up. From the Earth, we see Venus passing close to the Sun in the sky. This is known as a conjunction of Venus and the Sun
3. The conjuction when Venus is above or below Earth's orbit
4. Earth and Venus lined up in transit formation
Although the two planets are lined up during conjunctions, Venus is usually either above or, as shown here, below the Earth in its orbit. Therefore, we usually see Venus pass either above or below the Sun during a conjunction.
However, every 120 years or so, a pair of conjunctions occurs as Venus crosses the line of nodes and is lined up exactly with the Earth and Sun. At these times, the Sun is directly behind Venus, and we see a transit of Venus across the Sun. If we measure the exact position of the transit across the Sun's disc from many different latitudes, we can use geometry to calculate the size of the Earth's orbit in terms of the size of the Earth.
5. Seeing the transit of Venus from different hemispheres
Observers at different latitudes on the Earth will see the transit occur across different parts of the Sun. An observer in the northern hemisphere might see Venus against the southern half of the Sun. However, an observer in the south might see Venus against the northern half of the Sun. Astronomers can measure the path of Venus across the Sun from different latitudes and then use geometry to calculate the distance from the Earth to the Sun. Essentially, a transit of Venus allows us to survey our solar system and determine its size by using the whole Earth as a baseline.