A practical illustration of this may make a clearer impression. In the early evening of April 8th, secured a sight of Regulus; the chronometer showed 8 days, 9 hours, 56 minutes, and 0 seconds (corrected), with other necessary elements given, the sight is worked as usual to find the star’s hour angle, which proves to be:
| h. | m. | s. | |||||
| H A Regulus | 1 | 59 | 47 | bearing west. | |||
| R A | 10 | 3 | 02 | RAMS | 1 h. | 6 m. | 7 s. |
| — | — | — | GMT | 9 h. | 56 m. | 0 s. | |
| L. S. T. | 12 | 02 | 49 | Accel. 9 h. | 1 m. | 29 s. | |
| G. S. T. | 11 | 03 | 45 | Accel. 56 m. | 9 s. | ||
| — | — | — | —— | —— | —— | ||
| Long. in time | 59 | 04 | G. S. T. | 11 h. | 3 m. | 45 s. | |
This being a star sight we obtain from it the sidereal time at place of observation and as the chronometer carries Greenwich mean time we seek the corresponding sidereal time by adding this and its acceleration for a sidereal interval to the right ascension of the mean sun taken from Nautical Almanac. The result is the Greenwich Sidereal Time.
It is occasionally required to find the sidereal time at the ship in which case it is only necessary to apply the longitude in time to the Greenwich Sidereal Time.
As Greenwich mean time is the most used and is the best understood it is a very convenient practice to carry G. M. T. on the navigator’s watch. It is readily converted into any other time with ease but serves more purposes as it is without conversion. A stop watch is an excellent instrument for taking time sights where great accuracy is essential. By setting it at 0 minutes a man can observe alone starting the watch as he makes contact with the horizon and when subsequently comparing with the chronometer subtract the reading of the watch to get the G. T. of observation.
The most expeditious way to convert time into arc is to multiply the hours by 15 and add the number of minutes divided by 4 to get the degrees; multiply the remaining minutes by 15 and add the seconds divided by 4 to get the minutes; multiply the remaining seconds by 15 to get the seconds in arc:
Thus:
| Long. in time | 2 hrs. | 42 m. | 23 s. |
| 30 | 30 | 45 | |
| 10 | 5 | ||
| Long. in arc | 40° | 35´ | 45´´ |
To change arc into time divide the degrees by 15 to get the hours; multiply the remainder by 4 and divide the minutes by 15 and add to get the number of minutes (m.); multiply the remainder of minutes (´) by 4 and divide the seconds (´´) by 15 carrying the division of tenths if desired, adding the result to get the seconds (s.):
Thus: