Saturday 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 latitude 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 latitude 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 illustration:
Miles sailed
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 ¼ 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 |