t midnight and
runs through 12 hours to noon. It then commences again and runs through
12 hours to midnight. The Civil Day, then, is from midnight to midnight,
divided into two periods of 12 hours each.

The astronomical day commences at noon of the civil day of the same
date. It comprises 24 hours, reckoned from O to 24, from noon of one day
to noon of the next. Astronomical time, either apparent or mean, is the
hour angle of the true or mean sun respectively, measured to the
westward throughout its entire daily circuit.

Since the civil day begins 12 hours before the astronomical day and ends
12 hours before it, A.M. of a new civil day is P.M. of the astronomical
day preceding. For instance, 6 hours A.M., April 15th civil time is
equivalent to 18 hours April 14th, astronomical time.

Now, all astronomical calculations in which time is a necessary fact to
be known, must be expressed in astronomical time. As chronometers have
their face marked only from 0 to 12 as in the case of an ordinary watch,
it is necessary to transpose this watch or chronometer time into
astronomical time. No transposing is necessary if the time is P.M., as
you can see from the diagram that both civil and astronomical times up
to 12 P.M. are the same. But in A.M. time, such transposing is
necessary. Put in your Note-Book:

Whenever local or chronometer time is A.M., deduct 12 hours from such
time to get the correct astronomical time:

CT 15d-- 9h--10m--30s A.M.
--12
------------------------
CT 14d--21h--10m--30s
------

L.M.T. 10d-- 4h--40m--16s A.M.
--12
------------------------
L.M.T. 9d--16h--40m--16s

Now we come to a very important application of time. You will remember
that in one of the former lectures we stated that to find our latitude,
we had to find how far North or South of the equator we were, and to
find our longitude, we had to find how far East or West of the meridian
at Greenwich we were. Never mind about latitude for the present. We can
find our longitude exactly if we know our Greenwich time and our time at
ship. For instance, in the accompanying diagram:

[Illustration]

Suppose PG is the meridian at Greenwich, then anything to the west of PG
is West longitude and anything to the East of PG is East longitude. Now
suppose GPS is the H.A. of G. or G.A.T.--i.e., the distance in time G.
is from the sun. And L P S is the H.A. of the