A rare form of auxiliary dial which indicates the difference between solar and mean or average time. Most early long case clocks were sold together with a sundial which was an essential adjunct to the clock in order to set it to time. However, a simple conversion from solar time to mean time had to be first worked out. Solar time, according to the season of the year, could be almost 16 1/2 minutes fast or slow of mean time. This difference was shown automatically by the equation dial, though a cheaper substitute was available in the form of a printed equation table, which could be stuck on the inside of the door of the clock for easy reference.