Lectures in Navigation Part 16

Spend rest of time in solving examples similar to the following:

1. At sea, Feb. 14th, 1919. CT 13d 21h 52m 33s. CC 1m 14s fast. In Lo. 72 49' 00" W. IE + 1' 10". HE 15 ft. Observed alt.i.tude Polaris 42 21' 30" N. Required lat.i.tude in.

2. At sea, March 31st, 1919. In Lo. 160 15' E. CT 7h 15m 19s. Observed alt.i.tude Polaris 38 18' N. IE + 3' 00". HE 17 ft. Required lat.i.tude in.

Etc.

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 s.h.i.+p 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:

(-W. Lo.) G.M.T. + (.).R.A. + (+)CP = G.S.T. --------- = L.S.T.--Star's R.A.

(+E. Lo.)

(or vice versa if Star's R.A. is greater) = Star's H.A. at s.h.i.+p (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 s.h.i.+p?

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 alt.i.tude 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 s.h.i.+p.

2. Jan. 23rd, 1919. P.M. at s.h.i.+p. CT 3h 45m 40s. Lat. by D.R. 38 44'

19" N. Lo. 121 16' 14" E. Observed alt.i.tude 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 s.h.i.+p.

a.s.sign for Night Work one or two examples similar to the above.

FRIDAY LECTURE

EXAMPLES: LAt.i.tUDE BY MERIDIAN ALt.i.tUDE OF A STAR, LAt.i.tUDE BY POLARIS, MARC ST. HILAIRE METHOD BY A STAR SIGHT

1. At sea, Dec. 5th, 1919. Observed meridian alt.i.tude Star Aldebaran 69 28' 40" S. No IE. HE 26 ft. Required lat.i.tude in.

2. At sea, Jan. 20th, 1919. CT 21d 2h 16m 48s A.M. In longitude 56 29'

46" W. Observed alt.i.tude of Star Polaris 48 44' 30" N. IE + 10' 20". HE 37 ft. Required lat.i.tude in.

3. At sea, June 4th, 1919. A.M. at s.h.i.+p. CT 10h 16m 32s. CC 5m 45s fast.

Lat. by D.R. 42 44' N, Longitude 53 13' 44" E. Observed alt.i.tude of Star Altair East of meridian, 52 19' 30". IE--14' 00". HE 56 ft.

Required line of position by Marc St. Hilaire Method and most probable position of s.h.i.+p.

Etc.

a.s.sign for Night Work the following Articles in Bowditch: 336 through 341, disregarding the formulas.

SAt.u.r.dAY LECTURE

LONGITUDE BY CHRONOMETER SIGHT OF THE SUN (TIME SIGHT)

You have now learned, first, how to get your lat.i.tude by a meridian alt.i.tude of the sun or a star and second, how to get your Line of Position and most probable fix, including both lat.i.tude and longitude, by the Marc St. Hilaire Method, using for your calculations either the sun or a star. We are now going to take up a method of getting your longitude only. This method requires as much, if not more, calculation than the Marc St. Hilaire Method. Its results, on the other hand, are far less complete, for while the Marc St. Hilaire Method will give you a fairly accurate idea of both your lat.i.tude and longitude, this method will, at best, only give you your longitude. Moreover, you can use it for accurate results only when the sun bears almost due East or West of you, for that is the best time, as you have already learned, to get a line of position running due North and South, which is nothing more than a meridian of longitude. The only reason we explain this method at all is because it is in common practice among merchantmen and may, therefore, be of a.s.sistance to you, if you go on a merchant s.h.i.+p.

Remember, however, that it belongs to Old Navigation as distinguished from New Navigation, exemplified by the Marc St. Hilaire Method. It is undoubtedly being used less and less among progressive, up-to-date navigators, and will continue to be used less as time goes on. The fact remains, however, that at present many merchantmen practice it, and so it will do you no harm to become familiar with the method, too.

This method is based on securing your longitude by a time sight or longitude by chronometer sight, meaning that at the time the sun bears as near due East or West as possible, you take a sight of it by s.e.xtant and at the same instant note the time by chronometer. With this information you proceed to work out your problem and secure your longitude according to the following formula. Put in your Note-Book:

To find your longitude by chronometer (or time) sight.

1. Take sight by s.e.xtant only when the sun bears as near as possible due East or West. At exact time of taking sight, note chronometer time.

2. Get G.M.T. from corrected chronometer time. Apply Equation of Time to get the corresponding G.A.T.

3. Correct observed alt.i.tude to get T.C.A. Also have at hand Lat. by D.

R. and Polar Distance. (Note: Secure P. D. by subtracting Dec. from 90, if Lat. and Dec. are of same name. If Lat. and Dec. are of opposite name, secure P. D. by adding Dec. to 90.)

4. Add together the T.C.A. the Lat. by D.R. and the P.D. Divide the sum by 2 and call the quotient Half Sum. From the Half Sum subtract the T.C.A. and call the answer the Difference.

5. Add together the secant of the Lat.i.tude, the cosecant of the P.D., the cosine of the Half Sum and the sine of the Difference (Table 44).

The result will be the log haversine of the S.H.A. or L.A.T. It must always be less than 10. If greater than 10, subtract 10 or 20 to bring it less than 10.

6. From Table 45, take out the corresponding S.H.A. (L.A.T.), reading from the top of the page if P.M. at s.h.i.+p, or from bottom of page if A.M.

at the s.h.i.+p.

7. Find the difference between L.A.T. and G.A.T. This difference is Lo.