In 1627, Johannes Kepler had published his Rudolphine Tables and two years later he published extracts from the tables in his pamphlet De raris mirisque Anni 1631 which included an admonitio ad astronomos (warning to astronomers) concerning a transit of Mercury in 1631 and transits of Venus in 1631 and 1761. Horrocks' own observations, combined with those of his friend and correspondent William Crabtree, had convinced him that Kepler's Rudolphine tables, although more accurate than the commonly used tables produced by Philip Van Lansberg, were still in need of some correction. Kepler's tables had predicted a near-miss of a transit of Venus in 1639 but, having made his own observations of Venus for years, Horrocks predicted a transit would indeed occur.