of time. Put in your Note-Book:

There are three kinds of time:

1. Apparent or solar time, i.e., time by the sun.

2. Mean Time, i.e., clock time.

3. Sidereal Time, or time by the stars.

So far as this lecture is concerned, we will omit any mention of
sidereal time, i.e., time by the stars. We will devote this morning to
sun time, i.e., apparent time, and mean time.

Apparent or Solar Time is, as stated before, nothing more than sun time
or time by the sun. The hour angle of the center of the sun is the
measure of apparent or solar time. An apparent or solar day is the
interval of time it takes for the earth to revolve completely around on
its axis every 24 hours. It is apparent noon at the place where you are
when the center of the sun is directly on your meridian, i.e., on the
meridian of longitude which runs through the North and South poles and
also intersects your zenith. This is the most natural and the most
accurate measure of time for the navigator at sea and the unit of time
adopted by the mariner is the apparent solar day. Apparent noon is the
time when the latitude of your position can be most easily and most
exactly determined and on the latitude by observation just secured we
can get data which will be of great value to us for longitude sights
taken later in the day.

Now it would be very easy for the mariner if he could measure apparent
time directly so that his clock or other instrument would always tell
him just what the sun time was. It is impossible, however, to do this
because the earth does not revolve at a uniform rate of speed.
Consequently the sun is sometimes a little ahead and sometimes a little
behind any average time. You cannot manufacture a clock which will run
that way because the hours of a clock must be all of exactly the same
length and it must make noon at precisely 12 o'clock every day. Hence we
distinguish clock time from sun time by calling clock time, mean (or
average) time and sun time, apparent or solar time. From this
explanation you are ready to understand such expressions as Local Mean
Time, which, in untechnical language, signifies clock time at the place
where you are; Greenwich Mean Time which signifies clock time at
Greenwich; Local Apparent Time, which signifies sun time at the place
where you are; Greenwich Apparent Time, which signifies sun time at
Greenwich.

Now the difference between apparent time and mean time can be found for
any minute of the