An excellent approximation assumes that the Sun revolves around a stationary Earth on the celestial sphere, which rotates every 24 hours about its celestial axis.
The celestial axis is the line connecting the celestial poles.
In a broader sense, a sundial is any device that uses the Sun's altitude or azimuth (or both) to show the time.
In addition to their time-telling function, sundials are valued as decorative objects, literary metaphors, and objects of mathematical study.
For example, it will show "noon" after the official noon has passed.
This correction may be made in the adjustment plaque, or by numbering the hour-lines with two sets of numbers. The hour lines are spaced equally about the circle, and the shadow of the gnomon (a thin cylindrical rod) rotates uniformly.In these designs, their times agree only when the plate is aligned properly.Sundials indicate the local solar time, unless corrected for some other time.Secondly, the solar time must be corrected for the longitude of the sundial relative to the longitude of the official time zone.For example, a sundial located west of Greenwich, England but within the same time-zone, shows an earlier time than the official time.A spot of light may be formed by allowing the sun's rays to pass through a small hole or by reflecting them from a small circular mirror.