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Apparent Time and Mean Solar Time
Background
Information
In this section of the course, we will discuss time in more abstract terms.
We will look at how time is measured, some basic terms and definitions
associated with time, time zones and time zone conversions, and how we
convert time to arc and arc to time.
The instrument for measuring time is a timepiece. Earth itself may be
considered as our celestial timepiece. Each complete rotation of Earth on
its axis provides a unit of time that we know as a day. Time is important
to you because of its relationship to longitude. As a Quartermaster, you
will have to understand this relationship to do your job.
The Solar Day
The two types of time we will discuss here are:
1. Apparent solar time.
2. Mean solar time.
You probably already know that the motion of the Sun and the stars
around Earth is only apparent--an illusion created by the rotation of the
Earth itself. Solar time is based upon the rotation of the Earth with
respect to the Sun.
The solar day is equal to one rotation of Earth relative to the Sun.
Apparent Time
Apparent solar time is measured upon the basis of the apparent motion of
the real Sun (the one you see rise and set every day). This is why we use
the term apparent when we measure time using the apparent Sun. When
the Sun is directly over our local meridian (directly overhead), we say that
it is noon, local apparent time. When it is directly over the meridian that
is 180° (on the opposite side of Earth) away from ours, it is midnight
local apparent time.
If Earth remained stationary in space, all the days reckoned by apparent
time would be of the same length. But Earth travels in an elliptical orbit
around the Sun, and its speed relative to the Sun varies with its position in
its orbit. Consequently, the time required for a complete revolution of
Earth on its axis, although constant as applied to points on Earth, varies
regarding Earth relative to the Sun. The length of a day measured by a
complete revolution of Earth with regard to the Sun, also varies. For this
reason it is impractical for man-made timepieces to keep apparent time;
another solution had to be figured to account for these unequal lengths of
time.
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