Apparent & Mean Sun4.2 - 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)
Apparent Solar Time (AST), is the time the Sun shows in the day. We can make a sundial to tell this.
Mean Solar Time (MST) is the time on our watches that humans have invented as Sun time is an unreliable way to tell the time for society. Apparent solar time as told on a sundial can be inaccurate to as much as 16 minutes. This is caused by Earth's elliptical orbit and the axis of Earth.
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).