Geodesy Board Practice Exam 2026 - Free Geodesy Exam Practice Questions and Study Guide

The key idea is that actual solar time and mean clock time don’t run in perfect sync all year. Local Apparent Time is based on the Sun’s true position, while Local Mean Time uses a fictitious Sun that moves at a constant rate. The equation of time is simply their difference: Local Apparent Time minus Local Mean Time. This difference can be positive or negative and varies through the year because of the Earth’s elliptical orbit and the tilt of its axis, roughly ranging from about -14 to +16 minutes.