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).
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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.