Stonyhurst disks
A Stonyhurst disk is used to determine the longitude and latitude of sunspots. The Stonyhurst disk was discovered at the observatories in Stonyhurst or Stonyhurst College The Earth doesn't orbit exactly around the Sun's equator, so through the year the center of solar pictures moves up and down a little more than 7 degrees. This angle varies sinusoidally through the year and is called B0. Earth crosses the Sun's equator about December 7th and June 7th each year.
This means that the the lines of lattitude on the sub do not appear as lines from the earth, but as elliptical arcs.
Sunspots and other features will rotate across the face of the sun along lines of latitude (unless they drift!). Stonyhurst disks are transparent overlays that match the size of whole-disk images of the sun. These disks provide a graphic means of locating sunspots on the solar disk in reference to lines of latitude and longitude. Different disk overlays are produced for different dates.