Page 203 - NAVAL SCIENCE 3 TEXTBOOK

~IAR I NE NAVIGATI ON 209

MORNING (A.M.) AFTERNOON (P.M,) universe, with the Sun, as well ilS a\l the other celestial bodies, mov-
0100 1 A.M. 1300 I P,M, ing around the Earth.
0200 2 A,;\1. 1400 2 p,M. The Sun thus appears to make one complete 360-degree rev-
0300 3 A.;\1. 1500 3 1'.;\1. olution around the Earth during each twenty-four-hour day.
0400 4 A,M. 1600 4 P.M. Actually, as we will see below, on any specific day during the year
0500 SA,M, 1700 5 P.;\1. it will usually take a few minutes more or less than twenty-four
0600 6 A,;\1. 1800 6 P,;\1. hours for the Sun to complete its journey. But on the average over
0700 7 A,~1. 1900 7 P,~1. a year, we can say that it takes exactly twenty-four hours.
0800 8 A,~1. 2000 8 P,M, Now, because the Sun goes 360 degrees around the Earth in
0900 9 A,~l. 2100 9 P.M. twenty-four hours on the average, we can say that there is a definite
1000 lOA.M. 2200 10 P.M. relationship between arc as measured in an east-west direction on
1100 11 A.~I. 2300 II P.M. the surface of the Earth (which we saw in the first chapter in this
1200 12 p.~I .lnoon 240010000 12 ,ul./ midnight unit is longitude) and time. If we divide 360 degrees of arc around
the Earth, or longitude. by twenty-four hours. we see that it takes
\·"hile 0000 and 2400 are exactly the same time. it is common prac-
the SUI1 one hour to go 15 degrees of arc, or longitude. And, since it
tice to start each day at 0001 and end it at 2400.
travels IS degrees in one hour, the Sun must go 1 degree (60 min-
utes of arc) in four minutes (IllS x 60 minutes = four minutes).
Ship's Bell T ime Thus, I degree of longitude can be thought of as being equivalent
to four minutes of time. This relationship is of basic importance
Another custom on board ship is to mark the passage of time by
both in navigation and in keeping time, as we will see.
bells. Before timepieces such as watches or chronometers were com-
mon, time on board ship was reckoned by a so-called hOllrglass,
which ran out its sand from one end to the other every thirty min- K inds of T ime
utes. The glass would then be turned over to start measuring another
The Sun is the most convenient reference for reckoning time. Time
thirty minutes, and a bell would be struck so all hands would know
measured by the Sun is solar tillle. Solar time, or SUl1time, is based
a half hour had passed. It was struck once at the end of the first half-
on the apparent motion of the Sun around the Earth.
hour of each four-hour watch, twice at the end of the second, and so
Apparent time. Time based on the apparent position of the Sun
on, until eight beUs were struck at the end of the fourth hour. After
from our position is called nppnrellt time. If the Sun is directly over
eight bells were struck, the series started over again.
the meridian we are on, we say that it is noon, local apparent time.
The practice still continues on board some Navy ships, in spite
\·"hen it is directly O\'er the meridian 180 degrees away from ours,
of the use of clocks and watches. The bells are rung in pairs; that
it is midnight local apparent time.
is, if there are two or more bells to be rung, the)' are rung closer
If the Earth stood still ill space, and the Sun orbited in a circle
together than the odd bell. For example, five bells would sound like
around it, all the days reckoned by apparent time would be exactly
"ding-ding, ding-ding, ding." An odd number of bells marks half
the same length. But the Earth travels around the Sun in an ellipti-
past the hour, and an even number marks an hour. \·"hen used,
cal orbit (like a race track); the Earth's axis is inclined with respect
bells are rung only from reveille to taps, but not during divine ser-
to the plane of its orbit around the Sun; and the Earth's speed along
vices or when fog requires that the bell be used as a fog signal.
its orbit varies. Therefore, the time required for a complete rota-
There are also commercially available ship's clocks ill gift shops
tion of the Earth on its axis relative to the Sun- or in other words,
and the like that have chimes that can be set to go through the bell
the length of a solar day- varies continually according to the posi~
sequence described above.
tion of the Earth ill its orbit.
Mean time, Time keeping would be connlsing if some days had
Time and Arc more, and some fewer, minutes because of the Earth's revolution.
To eliminate this confusion, an average solar time is used; this is
From ancient times to the present, people have reckoned time
called mea1l solar tillie, It is calculated from the Illotion around
according to the travel of the SUll once around the Earth each day,
the Earth of an imaginaq' or men/l 51111, which always makes the
Of course, since the time of the medieval astronomer Copernicus
360~degree circuit in exactly twenty-four hours. So if your wi1tch
in the sLxteenth century, we have known that it is reaUy the Earth's
says it is 1200 local mean time (UvIT), the lIIeml SUIl is over your
rotation that makes the Sun seem to move. But for navigation, and
meridian, not the actual Sun.
to make it easier for us to understand how time works, it is often
In November of each year the achIaI Sun is about 161h min-
helpful to imagine the Earth as standing still at the center of the
utes ahead of the meal1 SUI1, and by Februar)' it has 1"lIen behind b),
some 14 minutes. The difference between apparent time and mean