In the early 17th century it was generally accepted that the size of a planet was related to its distance from the Sun. The Lancashire astronomer Jeremiah Horrocks (1619-42) took this a stage further. For the planets known at the time he suggested that the planet-Sun distance was about 15 000 times the planetary radius. This was equivalent to saying that all the planetary spheres subtended an angle of 28 arcsec at the Sun. This so called ``Horrocks's Law'' was confirmed by his Sunday 24 November 1639 (old style) measurement of the angular diameter of the transiting Venus. It leads to a solar parallax of 14 arcsec, a value considered rather small at the time, but refreshingly close to the modern value of about 8.798 arcsec.