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.
- Stonyhurst Disk Animation , Data and Activities for Solar Learning, eo.nso.edu,
- "Daily Sunspot Drawings at the 150-Foot Solar Tower", The 150-Foot Solar Tower at Mt. Wilson Observatory, UCLA
https://en.wikipedia.org/wiki/Stonyhurst disks was the original source. Read more.