He refined J. Kepler’s Rudolphine Tables of planetary positions, measured the apparent diameters of the planets, and obtained a value of the solar parallax corresponding to an Earth–Sun distance of about 100 million km. In 1639 he observed a transit of Venus which he had predicted from his refinement of Kepler’s tables, measuring the planet’s diameter on a projected image. On the basis of Horrocks’s lunar theory, J. Flamsteed later constructed tables of the Moon’s motion which remained the best available until the mid-eighteenth century.