Equation clock

For the next few decades, people were still accustomed to using sundials, and wanted to be able to use clocks to find solar time.

The shaft carries a cam, which is approximately "kidney shaped" such that its radius is essentially a graph of the annual variation of the equation of time.

Cranks or pins attached to the two shafts move the two ends of a combining lever (sometimes referred to as a whippletree) sinusoidally; if the dimensions are chosen correctly, the midpoint of the rod moves in a way that simulates the equation of time.

Many equation clocks also have other features, such as displays of the phase of the moon or the times of sunrise and sunset.

Pictures and descriptions of various equation clocks, which still exist in museums, can be accessed through the External links listed below.

Most later equation clocks, made in the 18th century, directly display solar time.

Three types exist: Clocks have been constructed in which the minute markings are on a circular plate that can be turned around the same axis as the hands.

The hand is driven clockwise at constant speed by the clock mechanism, and the plate is turned by the mechanism that simulates the equation of time, rotating anticlockwise as the equation of time increases, and clockwise when it decreases.

Mean time can also be shown by a separate, stationary set of minute markings on the dial, outside the edge of the plate.

For example, during the months of December and January, when the equation of time is decreasing so a sundial runs slower than usual, the mechanism makes the pendulum effectively longer than usual, so the clock runs slower and keeps pace with sundial time.

Another disadvantage of variable pendulum clocks is that the equation of time cannot be easily displayed.

After its invention in 1720,[2] this mechanism became the standard one, and was used for much of the 18th century, until the demand for equation clocks ceased.

However, components functionally the same as those in equation clocks are still used in, for example, solar trackers, which move so as to follow the movements of the Sun in the sky.

Generally, these machines use modern technology, involving electronics and computers, instead of the mechanical devices that were used in historic equation clocks, but the function is the same.