Stonyhurst disk for sunspots. This device is designed to allow the accurate mapping of sunspots (dark spots) on the Sun's surface. This is needed because the Earth's orbit is not aligned precisely with the Sun's equator, and so the solar lines of latitude and longitude vary through the year. Stonyhurst disks, named after the observatory in England where they were devised in the 19th century, are produced for specific dates. These observations were made on 9 August 1893, at 09:37. The ephemeris (position of the Sun in the sky) is also given (upper right). This diagram was published in volume 16 (1908) of the US magazine Popular Astronomy.