The Equation of Time
A sundials tells Real Solar Time whereas your watch tells average or Mean Solar Time. Mean Solar Time assumes that the earth's orbit around the sun is circular, completely regular and turns on its axis exactly every 24 hours. However, in reality, the earth's orbit is an ellipse (a bit like a squashed circle ) and its axis is tilted. This means that there is a difference between the time that your watch shows and the time a sundial shows. In fact, your watch and a sundial will only agree four times a year, and during the year, your clock will be anything from 14 minutes faster to 16 minutes slower than your sundial.

The Equation of Time is used to work out the time your watch should say, or Mean Solar Time from the time a sundial says, or Real Solar Time. There are tables available you can use to work this out.