Solar timeSolar time is based on the idea that, when the sun reaches its highest point in the sky, it is noon. Apparent solar time is based on the apparent solar day, which is the interval between two successive returns of the sun to the local meridian. Solar time can be measured by a sundial.
The length of a solar day varies throughout the year. This is because the Earth's orbit is an ellipse, and not a circle, and the Earth moves faster when it is nearest the Sun and slower when it is farthest from the sun. (see Kepler's laws of planetary motion) Because of this, apparent solar days are shorter in March and September than they are in June or December. (The amount of daylight also varies because of the 23.5º tilt of the Earth's axis. (see Tropical year)
Mean solar time is based on a fictional mean sun which travels at a constant rate throughout the year. The length of a mean solar day is a constant 24 hours throughout the year although, as noted above, the amount of daylight varies.
The difference between apparent solar time and mean solar time, which is sometimes as great as 15 minutes, is called the equation of time.