Also, one of the examples of Mercator sailing to be done by both the Inspection and Logarithmic method.

SAt.u.r.dAY LECTURE

GREAT CIRCLE SAILING--THE CHRONOMETER

In Tuesday's Lecture of this week, I explained how a Great Circle track was laid down on one of the Great Circle Sailing Charts which are prepared by the Hydrographic Office.

Supposing, however, you do not have these charts on hand. There is an easy way to construct a great circle track yourself. Turn to Art. 194, page 82, in Bowditch. Here is a table with an explanation as to how to use it. Take, for instance, the same two points between which you just drew a line on the great circle track. Find the center of this line and the lat.i.tude of that point. At this point draw a line perpendicular to the course to be sailed, the other end of which must intersect the corresponding parallel of lat.i.tude given in the table. With this point as the center of a circle, sweep an arc which will intersect the point left and the point sought. This arc will be the great circle track to follow.

To find the courses to be sailed, get the difference between the course at starting and that at the middle of the circle, and find how many quarter points are contained in it. Now divide the distance from the starting point to the middle of the circle by the number of quarter points. That will give the number of miles to sail on each quarter point course. See this ill.u.s.tration:

[Ill.u.s.tration]

Difference between ENE and E = 2 pts. = 8 quarter points. Say distance is 1600 miles measured by dividers or secured by Mercator Sailing Method. Divide 1600 by 8 = 200. Every 200 miles you should change your course 1/4 point East.

_The Chronometer_

The chronometer is nothing more than a very finely regulated clock. With it we ascertain Greenwich Mean Time, i.e., the mean time at Greenwich Observatory, England. Just what the words "Greenwich Mean Time" signify, will be explained in more detail later on. What you should remember here is that practically every method of finding your exact position at sea is dependent upon knowing Greenwich Mean Time, and the only way to find it is by means of the chronometer.

It is essential to keep the chronometer as quiet as possible. For that reason, when you take an observation you will probably note the time by your watch. Just before taking the observation, you will compare your watch with the chronometer to notice the exact difference between the two. When you take your observation, note the watch time, apply the difference between the chronometer and watch, and the result will be the CT.

For instance, suppose the chronometer read 3h 25m 10s, and your watch, at the same instant, read 1h 10m 5s. C--W would be:

3h -- 25m -- 10s -- 1 -- 10 -- 05 ---------------- 2h -- 15m -- 05s

Now suppose you took an observation which, according to your watch, was at 2h 10m 05s. What would be the corresponding C T? It would be

WT 2h -- 10m -- 05s C -- W 2 -- 15 -- 05 -------------------- CT 4h -- 25m -- 10s

If the chronometer time is less than the W T add 12 hours to the C T, so that it will always be the larger and so that the amount to be added to W T will always be +. For instance, CT 1h--25m--45s, WT 4h--13m--25s, what is the C-W?

CT 13h--25m--45s WT 4 --13 --25 ---------------- C--W 9h--12m--20s

Now, suppose an observation was taken at 6h 13m 25s according to watch time. What would be the corresponding CT?

WT 6h--13m--25s C--W 9 --12 --20 ---------------- 15h--25m--45s --12 ---------------- CT 3h--25m--45s

Put in your Note-Book: CT = WT + C - W.

If, in finding C-W, C is less than W, add 12 hours to C, subtracting same after CT is secured.

Example No. 1:

CT 3h--25m--10s WT 1 --10 --05 ---------------- C--W 2h--15m--05s

WT 2h--10m--05s + C--W 2 --15 --05 ---------------- CT 4h--25m--10s

Example No. 2:

CT 1h--25m--45s WT 4h--13m--25s

(+12 hrs.) CT 13h--25m--45s WT 4 --13 --25 ---------------- C-W 9h--12m--20s

WT 6h--13m--25s + C-W 9 --12 --20 ---------------- 15h--25m--45s (-12 hrs.) 12 ---------------- CT 3h--25m--45s

There is one more very important fact to know about the chronometer. It is physically impossible to keep it absolutely accurate over a long period of time. Instead of continually fussing with its adjustment and hands, the daily rate of error is ascertained, and from this the exact time for any given day. It is an invariable practice among good mariners to _leave the chronometer alone_. When you are in port, you can find out from a time ball or from some chronometer maker what your error is. With this in mind, you can apply the new correction from day to day. Here is an example (Put in your Note-Book):

On June 1st, CT 7h--20m--15s, CC 2m--40s fast. On June 16th, (same CT) CC 1m--30s fast. What was the corresponding G.M.T. on June 10th?

June 1st 2m--40s fast 16th 1m--30s fast ---------------- 1m--10s 60 -- 60 10 -- 15) 70s (4.6 sec. Daily Rate of error losing

June 1st-10th, 9 days times 4.6 sec. = 41 sec. losing June 1st 2m--40s fast June 10th 41s losing --------- June 10th 1m--59s fast

CT 7h--20m--15s CC -- 1 --59 ------------ G.M.T. 7h--18m--16s on June 10th

If CC is fast, subtract from CT If CC is slow, add to CT

WEEK III--CELESTIAL NAVIGATION

TUESDAY LECTURE

CELESTIAL CO-ORDINATES, EQUINOCTIAL SYSTEM, ETC.

We have already discussed the way in which the earth is divided so as to aid us in finding our position at sea, i.e., with an equator, parallels of lat.i.tude, meridians of longitude starting at the Greenwich meridian, etc. We now take up the way in which the celestial sphere is correspondingly divided and also simple explanations of some of the more important terms used in Celestial Navigation.

As you stand on any point of the earth and look up, the heavenly bodies appear as though they were situated upon the surface of a vast hollow sphere, of which your eye is the center. Of course this apparent concave vault has no existence and we cannot accurately measure the distance of the heavenly bodies from us or from each other. We can, however, measure the direction of some of these bodies and that information is of tremendous value to us in helping us to fix our position.

Now we could use our eye as the center of the celestial sphere but more accurate than that is to use the center of the earth. Suppose we do use the center of the earth as the place from which to observe these celestial bodies and, in imagination, transfer our eye there. Then we will find projected on the celestial sphere not only the heavenly bodies but the imaginary points and circles of the earth's surface. Parallels of lat.i.tude, meridians of longitude, the equator, etc., will have the same imaginary position on the celestial sphere that they have on the earth. Your actual position on the earth will be projected in a point called your zenith, i.e., the point directly overhead.

[Ill.u.s.tration]

From this we get the definition that the Zenith of an observer on the earth's surface is the point in the celestial sphere directly overhead.

It would be a simple matter to fix your position if your position never changed. But it is always changing with relation to these celestial bodies. First, the earth is revolving on its own axis. Second, the earth is moving in an elliptic track around the sun, and third, certain celestial bodies themselves are moving in a track of their own. The changes produced by the daily rotation of the earth on its axis are different for observers at different points on the earth and, therefore, depend upon the lat.i.tude and longitude of the observer. But the changes arising from the earth's motion in its...o...b..t and the motion of various celestial bodies in their orbits, are true no matter on what point of the earth you happen to be. These changes, therefore, in their relation to the center of the earth, may be accurately gauged at any instant. To this end the facts necessary for any calculation have been collected and are available in the Nautical Almanac, which we will take up in more detail later.

Now with these facts in mind, let us explain in simple words the meaning of some of the terms you will have to become acquainted with in Celestial Navigation.