| Course - 275° |
| Change in Lo. per hr. - 14', 56s. |
| 3600s |
| -56 |
| ——— |
| 3544s, Net rate of approach of sun |
| 4h |
| 60 |
| ——— |
| 240m |
| + 24 |
| ——— |
| 264m x 60 = 15840s |
| 15840s | ||
| + 02 | ||
| ——— | ||
| 15842s, | Total time to noon. | |
| 3544) | 15842 | (4.47 hours |
| 14176 | ||
| ——— | ||
| 16660 | ||
| 14176 | ||
| ——— | ||
| 24840 | ||
| 24808 | ||
| ——— |
| Corrected time to Noon | 4h 28m 12s |
| WT of A.M. sight | 6h 53m 13s |
| ———————— | |
| WT of L.A.N. | 11h 21m 25s |
| WT of 11:30 A.M. | 10h 51m 25s |
When, therefore, the watch reads 10h 51m 25s, the deck clocks should be set to 11.30 A.M. and thirty minutes later it will be apparent noon at the ship.
In all these calculations it is taken for granted that the speed of the ship and hence the change in longitude can be gauged accurately. A check on this can be made by comparing the longitude of the A.M. sight with the D.R. longitude of the same time. Any appreciable difference between the two can be ascribed to current. Now, if a proportionate amount of current is allowed for in reckoning the speed of the ship from the time of the A.M. sight to noon, then a proper correction can be made in the net rate of approach of the sun and the corrected time to noon will be very close to the exact time of noon. Of course there will be an error in this calculation but it will be small and the result gained will be accurate enough for ordinary work.
So much for finding the watch time of Local Apparent Noon. Careful navigators carry the process further and get the watch times of 15, 10 and 5 minutes before noon, so that by the use of constants for each one of these times, an accurate check on the noon latitude can be quickly and easily secured. We have not time in this course to explain how these constants are worked out but it is well worth knowing. The information regarding it is in Bowditch Art. 325, p. 128, and Art. 405, p. 181.
A word about the watch used by the navigator should be included here. This watch should be a good one and receive as much care, in its way, as the chronometer. It should be wound at the same time every day, carefully handled and, in other respects, treated like the fine time-piece that it is.
While authorities differ on this point, the best practice seems to be not to change the navigator's watch to correspond with the apparent time of each day's noon position. The reason for this is two-fold. First, because constant moving of the hands will have an injurious effect on the works of the watch, and second, because, by not changing the watch, the C-W remains approximately the same, and thus a good check can be kept on both the watch and the chronometer as well as on the navigator's figures in reckoning the times of his various sights.