Apparent & Mean Sun

4.2 - Understand the role of the Sun in determining Apparent Solar Time (AST)

4.3 - Understand the role of the Mean Sun in determining Mean Solar Time (MST) and Local Mean Time (LMT)

Sun time, or apparent solar time, is the time the Sun shows in the day. Normally we can make a sundial to tell this.

Mean solar time is the time on our watches that humans have invented as Sun time is an unreliable way to tell the time.

Because of the Earth's elliptical orbit and the axis of Earth, apparent solar time as told on a sundial can be inaccurate to as much as 16 minutes.

The noon sun will appear at it's highest in the sky as viewed from Greenwich, London at midday. This is both AST and MST. The same sun won't appear at it's highest in Bristol, 2° to the west, until over 8 minutes later according to our watches (MST).


  • Apparent = 'Real Sun time'
  • Mean = ' Human time'

Caused by...

  • Earth's elliptical orbit
  • Axis of Earth

Why does society follow a mean sun time system rather than an apparent one?