Thursday Lecture
Marc St. Hilaire Method by a Star Sight
You have already been given instructions for finding a Line of Position by the Marc St. Hilaire Method, using a sight of the sun. Today we will work out the same method by using a sight of a star. Put this in your Note-Book here and also under I(b) of the formula given you in Week IV - Friday's Lecture:
Get G.M.T. from corrected chronometer time. With your G.M.T. find the corresponding G.S.T. according to the formula already given you. With your G.S.T. apply the D.R. longitude
(- W. Lo.)
(+ L. Lo.)
to get the L.S.T. With the L.S.T. and the star's R.A. subtract the less from the greater and the result is the star's H.A. at the ship or "t." In using Sun Azimuth tables always take "t" from the P.M. column. Mark Azimuth N or S according to the lat. in and E or W, according as to whether the Star is East or West of your meridian. Then proceed as in the case of a sun sight. Formula:
G.M.T. + Circle enclosing dot.R.A. + Circle enclosing crossCP = G.S.T. (-W. Lo.) = L.S.T. - Star's R.A.
(+E. Lo.)
(or vice versa if Star's R.A. is greater) = Star's H.A. at ship (t). Then proceed as in case of sun sight.
Example:
On May 31st, 1919, in D.R. Lat. 50° N, Lo. 45° W, G.M.T. 31d 14h 33m 30s. What was Star's H.A. at ship?
| G.M.T. | 14h 33m 30s |
| .R.A. | 4 31 44.2 |
| .C.P. | 2 23 |
| —————————— | |
| G.S.T. | 19h 07m 37.2s |
| W Lo. - | 3 00 00 |
| —————————— | |
| L.S.T. | 16h 07m 37.2s |
| Star's R.A.(Spica) | 13 20 59 |
| —————————— | |
| Star's H.A. (t) | 2h 46m 38.2s |
Now let us work out some examples by this method:
1. Nov. 29th, 1919. CT 30d 2h 14m 39s A.M. CC 3m 14s fast. D.R. position Lat. 41° 14' N, Lo. 68° 46' W. Observed altitude Star Aldebaran East of meridian 50° 29' 40". HE 29 ft. Required Line of Position by Marc St. Hilaire Method and most probable position of ship.
2. Jan. 23rd, 1919. P.M. at ship. CT 3h 45m 40s. Lat. by D.R. 38° 44' 19" N. Lo. 121° 16' 14" E. Observed altitude Star Rigel 28° 59' 20" West of meridian. IE + 4' 30". HE 42 ft. Required Line of Position by Marc St. Hilaire Method and most probable position of ship.
Assign for Night Work one or two examples similar to the above.