While the relative sizes of the planetary orbits were known to Copernicus already in the 15th century, it was not until the 18th century that the actual size of the AU was determined to 5 per cent accuracy. This was the result of work of Edmund Halley, who suggested viewing a transit of Venus across the face of the Sun from two widely separated latitudes on Earth. Even in the 18th century it was not possible simply to view Venus in the night sky simultaneously from two widely separated latitudes, for reliable transportable clocks were yet to be developed. In the simulation the effect of parallax is exaggerated by a factor of ten in order to make the concept involved in this difficult measurement clear. In fact, the maximum effect amounts to only 1 minute of arc, if the two observers are located at the north and south poles. Actually, the measurements were made in 1769 at a wide variety of points, from the northern reaches of Scandinavia to as far south as Tahiti, which lies to the southeast of Australia.