The Geometry of a Transit

This large folding plate shows how a transit of Venus arises from the relative motions of the Earth and Venus around the Sun. It appears in a late edition of James Ferguson’s Astronomy Explained Upon Sir Isaac Newton’s Principles, and Made Easy to Those Who Have Not Studied Mathematics (first published in 1757).
Ferguson was renowned for the clarity of his texts and diagrams. Like his London competitor Benjamin Martin, he combined writing, lecturing and instrument-making, though he rose further than Martin to become a Fellow of the Royal Society.