The Noon mark is a type of sundial that at its simplest is a vertical line on a south facing wall. When the shadow of a point passes over the line it will be midday. Noon in local standard time is defined as when the sun is overhead, however clocks and watches use mean time which varies from standard time by a few minutes each day. The difference is calculated using the equation of time

Equation of time

The equation of time is the difference between apparent solar time and mean solar time. At any given instant, this difference will be the same for every observer...

 and this can be shown on the noon mark by drawing an analemma

Analemma

In astronomy, an analemma is a curve representing the angular offset of a celestial body from its mean position on the celestial sphere as viewed from another celestial body relative to the viewing body's celestial equator...

, or using a correction table.