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Apparent and Mean Solar Time, Continued
Figure 5-l Difference in the time of an apparent day.
Mean Solar
Time
To remedy the situation created by apparent solar time, mean solar time
was introduced. Mean solar time is based on a fictional Sun that is
considered to move at a constant rate of 360° in 24 hours along the
celestial meridian. One mean day is 24 hours in length, each hour
consisting of 60 minutes, and each minute consisting of 60 seconds.
Mean solar time and apparent solar time are nearly equal, but mean solar
time is the time used in everyday life (fig. 5-1). It is the time kept by our
ship’s chronometers and clocks, even our own wristwatch. It is also the
time used in various almanacs that we use for tabulating the positions of
celestial bodies.
Equation of
Time
The difference between the apparent day and the mean day is never more
than a minute. This difference is cumulative and amounts to as much as a
quarter-hour at certain times of the year (fig. 5-2 ). The difference
between mean and apparent time at any instant is called the equation of
time.
Figure 5-2. Difference of mean time and solar time.
5-4
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