WEEK VI—NAVIGATION
Tuesday Lecture
Longitude by Chronometer Sight of a Star
In getting your longitude by a time sight of a star, you proceed somewhat differently from the method used when observing the sun. What you wish to get first is G.S.T., i.e., the distance in time Greenwich is from the First Point of Aries. If you can then get the distance the ship is from the First Point of Aries, the difference between the two will be the longitude in, marked East or West according as to which is greater. By looking at the diagram furnished you when we were talking of Sidereal Time, all this becomes perfectly clear. The full rule for finding longitude by a star is as follows, which put in your Note-Book:
Correct your CT to get your G.M.T. From the G.M.T. get the G.S.T. From the observed altitude of the star, obtain the star's H.A. at the ship in the same way L.A.T. is secured in case of the sun. To or from the R.A. of the star add, if West of your meridian, subtract if East of your meridian, the star's H.A. at the ship, just obtained. The result is the R.A. of the ship's meridian or L.S.T.
Find the difference between G.S.T. and L.S.T. and the result is the longitude, marked East or West according as to whether G.S.T. is less or greater than L.S.T. Note: Always take the star's H.A. from the top of the page of Table 45.
Dec. 2, 1919. A.M. Observed altitude Star Sirius 2O° 05' 20", West of meridian. CT 11h 45m 29s P.M. CC 1m 28s slow. IE - 1' 20". HE 21 ft. Latitude by D. R. 38° 57' N. Required longitude in.
| CT | 11h 45m 29s | |||
| CC | + 1 28 | |||
| ————————— | ||||
| G.M.T. | 11h 46m 57s | |||
| Circle enclosing a dotRA | 16 37 10.3 | |||
| Circle enclosing a crossCP | 1 56.1 | |||
| ————————— | ||||
| G.S.T. | 28h 26m 03.4s | IE | -1' 20" | |
| - 24 | HE | -7 08 | ||
| ————————— | ——— | |||
| G.S.T. | 4h 26m 03.4s | Corr. | -8' 28" | |
| Obs. Alt. | 20° 05' 20 | |||
| Corr. | -8 28 | |||
| ————— | ||||
| T.C.A. | 19° 56' 52" | |||
| Lat. | 38 57 | sec. | .10919 | |
| P.D. | 106 36 24 | cosec. | .01849 + 1 | |
| ————— |
| 2 ) | 165° 30' 16" | |||
| —————— | ||||
| ½ S | 82° 45' 08" | cos. | 9.10106 - 13 | |
| T.C.A. | 19 56 52 | |||
| —————— | ||||
| Diff. | 62° 48' 16" | sin. | 9.94911 + 2 | |
| ———— | ||||
| 9.17785 | ||||
| - 11 | ||||
| ———— | ||||
| log. hav. Star's H.A. at ship | 9.17774 | |||
| Star's H.A. | 3h 02m 40s |
| Star's R.A. | 6 41 39 |
| ——————— | |
| L.S.T. | 9h 44m 19s |
| G.S.T. | 4 27 01 |
| ——————— | |
| Lo. in T. | 5h 17m 18s E |
| Longitude in | 79° 19' 30" E |
Assign for Night Work or work in the class room examples similar to the following:
1. April 16, 1919, in Latitude 11° 47' S. Observed altitude of the Star Aldebaran, West of the meridian 23° 13' 20". CT 6h 58m 29s. CC 2m 27s fast. IE - 2' 00". HE 26 ft. Required longitude in.
2. Dec. 10th, 1919. Observed altitude of Star Sirius 20° 05' 40" West of meridian. CT 11h 45m 29s. CC 1m 28s slow. IE - 1' 20". HE 21 ft. D.R. latitude 38° 57' N. Required longitude in.
Note to Instructor: If any time in the period is left or for Night Work assign examples to be worked by Marc St. Hilaire Method, changing slightly the D.R. Lat. and Longitude just obtained by the Time Sight Method.