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Measurement of the distance of Venus using Transits

The most important scientific result obtained from observations of previous transits of Venus was a measure of the Astronomical Unit; the distance between the Earth and the Sun. Once this was accurately measured, it could allow many other distances and scales in the Universe to be calculated.

The distance was determined using triangulation. Take two observers and seperate them by a certain distance. If we place another object at a distance from these two observers we make a triangle. By knowing the distance between the two observers and measuring the angles of the triangle, it is possible to work out the distance from the two observers to the distant object.

The only problem is that the Sun is a long, long way away so the angles are very nearly 90 degrees, making it very difficult to get a precise value for the distance. This can be solved by increasing the distance between the observers and so reducing the angles. However, in the case of observers on the Earth looking at the Sun, even putting them on different continents has very little effect!

To get around this problem we make use of the fact that Venus will appear to be in different places on the Sun's disk for observers in different locations. The best way to see how this works is by holding a pencil (a finger will do if you don't have one) up in front of you and closing one eye. If you then open that eye and close the other eye, the pencil (or finger) will appear to be in a different place compared to more distant objects like a wall or hills. This effect is called parallax and in this case is due to your two eyes being in different places. Going back to our two observers, parallax will mean that when making observations at the same time they both see Venus in a slightly different place on the Sun's disk.

Knowing the distance between the observers and measuring the angle between the two directions they measure for Venus, we can then work out the distance to the planet. From other methods eighteenth century astronomers already knew the relative distances of Venus and the Earth from the Sun so with measurements made during the transits of 1761 and 1769 it was then possible to work out the distance from the Earth to the Sun.

Nowadays the distance is measured more accurately using radar, but it can still be calculated using the same methods as in the eighteenth century. You can have a go yourself using the Open University's AU calculator.