Treasures of the Bodleian

These glass plates capture the 1874 transit of Venus. From the early 18th century, astronomers saw the fundamental importance of the planet’s rare passing across the surface of the sun: it offered a means of measuring the planet’s parallax, and thence its distance from Earth. The measurement – the astronomical unit – would allow the computation of all other distances in the solar system. Over 100,000 astronomical plates from the Royal Observatory at Greenwich are now at the Bodleian Libraries.