Stonyhurst disks explained

A Stonyhurst disk is a transparent circular grid with lines of longitude and latitude that can overlay a solar image to reference the positions of sunspots. This overlay system was originally created at the Stonyhurst College observatory.^{[1] [2]}

Notes and References

1. http://eo.nso.edu/dasl/Lessons/Stony/index.htm Stonyhurst Disk Animation
2. http://www.astro.ucla.edu/~obs/150_draw.html "Daily Sunspot Drawings at the 150-Foot Solar Tower"