This is a mathematically accurate sundial utilising the fascinating shape of the analemma. The 'mean' in Greenwich Mean Time means that each day the sun is NOT at its highest point at exactly 12 noon due to the Earth's oblique and elliptic orbit. Apparent 'midday' is averaged over the year and can vary by up to +/- ~15mins. A sundial incorporating an analemma advances and retards the shadow on the dial face depending on the time of year (by relying on the fact that the sun's height above the horizon changes over the year). Works accurately for latitudes similar to UK - easily adjustable for other latitudes by contacting me.