Venus is the second closest planet to the Sun and the Earth follows as the third one. At certain times Venus passes between the Sun and the Earth, and it can be seen as a little black dot moving across the face of the Sun. This event is called a transit of Venus.

Nearly all the planets of the Solar System move around the Sun in the same plane. Only Pluto’s orbit is strongly tilted with respect to that of the other planets. However, the planes in which the Earth and Venus move are slightly tilted with respect to each other. A transit of Venus happens only when Venus is located between the Sun and the Earth and finds itself in the same plane as the Earth . This condition is fulfilled each 8 to 120 years!

You can construct a simple model of the transit of Venus with a paper plate. Paint the Sun in the centre of the plate, Venus with smaller dots next to the Sun and the Earth (outer dots) using the colours shown below:

Cut two circular slits in the paper plate just outside the Venus dots to incline the planet's orbit.

Though the orbit of Venus is inclined 3.4 degrees with respect to that of the Earth, we will exaggerate the angle for clarity. Push the right half of the paper plate down, below the Earth's orbital plane, and the left half up, above the Earth's orbital plane. The hinges represent the two nodes , where the planes of the orbits of Venus and the Earth cross each other. Only at this position ( red dot), can a transit of Venus take place. At the positions marked with yellow and green , Venus finds itself above the Earth plane, at the orange and blue ones, beneath it.