The treatise by Jeremiah Horrocks (1618-1641) on the transit of Venus of 1639 is an account of an important astronomical observation, as well as an analysis and commentary on the changing state and practice of astronomy during the significant period between the achievements of Johannes Kepler (1571-1630), Galileo Galilei (1564-1642) and Isaac Newton (1642-1727). This work has, in addition, the power to delight and charm us as the record of a young astronomer's encounter with a rare astronomical event and the manner in which he discovered, observed, and drew conclusions from it. Its appeal is heightened by the knowledge that a self-trained young man stole a march on all the astronomers of his day.