day by reference to the Nautical Almanac which we will
take up later in more detail. This difference is called the Equation of
Time.

There is one more fact to remember in regard to apparent and mean time.
It is the relation of the sun's hour angle to apparent time. In the
first place, what is a definition of the sun's HA? It is the angle at
the celestial pole between the meridian intersecting any given point and
the meridian intersecting the center of the sun. It is measured by the
arc of the celestial equator intersected between the meridian of any
point and the meridian intersecting the center of the sun.

[Illustration]

For instance, in the above diagram, suppose PG is the meridian at
Greenwich, and PS the meridian intersecting the sun. Then the angle at
the pole GPS, measured by the arc GS would be the Hour Angle of
Greenwich, or the Greenwich Hour Angle. And now you notice that this
angular measure is exactly the same as apparent time at Greenwich or
Greenwich Apparent Time, for Greenwich Apparent Time is nothing more
than the distance in time Greenwich, England, or the meridian at
Greenwich is from the sun, i.e., the time it takes the earth to revolve
from Greenwich to the sun; and that distance is exactly measured by the
Greenwich Hour Angle or the arc on the celestial equator, GS.

The same is correspondingly true of Local Apparent Time and the ship's
Hour Angle. Suppose, for instance, PL is the meridian intersecting the
place where your ship is. Then your ship's hour angle would be the angle
at the pole intersecting the meridian of your ship and the meridian of
the sun or LPS and measured by the arc LS. And you will note that this
distance is exactly the same as apparent time at the ship, for Apparent
Time at ship is nothing more than the distance in time which the ship is
from the sun. We can sum up all this information in a few simple rules,
which put in your Note-Book:

Mean Time = Clock Time.

G.M.T. = Greenwich Mean Time.

L.M.T. = Local Mean Time.

Apparent Time = Actual or Sun Time.

G.A.T. (G.H.A.) = Greenwich Apparent Time or Greenwich Hour Angle.

L.A.T. (S.H.A.) = Local Apparent Time or Ship's Hour Angle.

Difference between apparent and mean time or mean and apparent
time--Equation of Time.

Right under this in your Note-Book put the following diagram, which I
will explain:

[Illustration]

You will see from this diagram that civil time commences a