Lectures in Navigation Part 18

3. April 11th, 1919. P.M. at ship. In lat.i.tude 43 16' 48" S. Observed alt.i.tude Star Spica 33 18' 20", East of meridian. CT 11h 08m 44s P.M.

IE 3' 20" on arc. CC 4m 18s slow. HE 39 ft. Required longitude in.

4. September 15th, 1919. P.M. at ship. In lat.i.tude 49 38'N. Observed alt.i.tude Star Deneb, East of meridian, 36 16' 50". IE 3' 40" off arc.

HE 40 ft. CC 6m 18s slow. CT 10h 00m 13s P.M. Declination of star is 44 59' 36" N. Required longitude in.

If any time is left, work same examples by Marc St. Hilaire Method a.s.suming a position near the one found by Time Sight.

a.s.sign for Night Work any of the above examples, to be worked either as Time Sights or by the Marc St. Hilaire Method, and also the following Arts. in Bowditch: 326-327-328-329.

THURSDAY LECTURE

LAt.i.tUDE BY EX-MERIDIAN ALt.i.tUDE OF THE SUN

You have learned that when you calculate your lat.i.tude from a meridian alt.i.tude of the sun, one of the necessary requisites is to have the sun exactly on your meridian. In fact, that is just another way of expressing meridian alt.i.tude, i.e., an alt.i.tude taken when the sun is on your meridian. Now suppose that 10 or 15 minutes _before_ noon you fear that the sun will be clouded over _at_ noon so that a meridian alt.i.tude cannot be secured. There is a way to calculate your lat.i.tude, even though the alt.i.tude you secure is taken by s.e.xtant some minutes before or after noon. This is called lat.i.tude by an ex-meridian alt.i.tude. It must be kept in mind that this method can be used accurately only within 26 minutes of noon, either before or after, and only then when you know your longitude accurately. Put in your Note-Book:

1. Get your L.A.T. (S.H.A.).

2. Subtract it from 24h 00m 00s, or vice versa, according as to whether L.A.T. is just before or just after local apparent noon. Call the result "Time Interval from Meridian Pa.s.sage."

3. With your D.R. lat.i.tude, declination and Time Interval from Meridian Pa.s.sage, enter Table 26 to get the proper amount of Variation of Alt.i.tude in one minute from meridian pa.s.sage.

4. With the Time Interval from Meridian Pa.s.sage and the Variation, enter Table 27 to get the total amount of Variation of Alt.i.tude.

5. Add this total amount of Variation to the true observed alt.i.tude taken before or after noon, and the result is the corrected alt.i.tude.

6. Then proceed to get your lat.i.tude according to the rules already given you for lat.i.tude by meridian alt.i.tude.

Example: At sea, Jan. 23rd, 1919. CT 4h 22m 14s. CC 1m 10s fast.

Longitude 66 04' W. Lat.i.tude by D.R. 19 16' 00" N. (_) 50 51' 00" S.

HE 49 ft. IE--1' 30". Required lat.i.tude in.

CT 4h - 22m - 14s CC - 1 - 10 --------------------- G.M.T. 4h - 21m - 04s Eq. T. - 11 - 50 --------------------- G.A.T. 4h - 09m - 14s Lo. in T 4 - 24 - 16 (W-) --------------------- L.A.T. 22d - 23h - 44m - 58s

24h - 00m - 00s - 23 - 44 - 58 ----------------- 15m - 02s = Time Interval from Meridian Pa.s.sage.

Dec. 19 34' 48" S Table 26 = 2.8 Variation Lat. 19 16' 00" N For 1 min. 0 alt.i.tude.

Time Interval from Meridian Pa.s.sage 15m 02s - 2.8" Variation for 1 minute (Table 27) 2" = 7' 30"

.8 = 3 00 ------------- 10' 30" +

IE - 1' 30" (_) 50 51' 00"

HE + 8 42 + 7 12 --------- ----------- Corr. + 7' 12" -(-)- 50 58' 12"

+ 10 30 ----------- 51 08' 42"

- 90 00 00 ----------- ZD 38 51' 18" N Dec. 19 34 48 S ----------- Lat. in 19 16' 30" N

a.s.sign for work in cla.s.s room and Night Work, examples similar to the following:

1. At sea, July 11th, 1919. Lat.i.tude by D.R. 50 01' 00" N. Longitude 40 05' 16" W. Observed ex-meridian alt.i.tude (_) 61 45' 30" S. HE 15 ft. IE--4' 10". CT (corrected) 2h 38m 00s. Required lat.i.tude in.

2. At sea, June 6th, 1919. Lat.i.tude by D. R. 49 21' N, Longitude 18 18' W. Observed ex-meridian alt.i.tude (_) 61 30' 22" S. HE 42 ft. CT 1h 06m 18s. CC--1m 14s. IE 0' 30" off the arc. Required lat.i.tude in of ship.

If any time is left, work similar examples by Marc St. Hilaire Method.

FRIDAY LECTURE

EXAMPLES: LAt.i.tUDE BY EX-MERIDIAN ALt.i.tUDE OF THE SUN

1. Jan. 1st, 1919. WT 11h 53m 18s A.M. C-W 5h 56m 16s. Lat.i.tude by D. R. 58 05' S. Longitude 89 00' 48" W. (_) ex-meridian 55 16' 30" N.

IE 2' 00" off the arc. CC 1m 28s fast. HE 36 ft. Required lat.i.tude in.

2. March 11th, 1919. CT 11d 9h 14m 39s A.M. Lat.i.tude by D. R.

39 20' N, Longitude 39 48' 16" E. (_) ex-meridian 46 17' 30" S.

IE 2' 00" on the arc. CC 1m 16s slow. HE 29 ft. Required lat.i.tude in.

3. April 26th, 1919. CT 26d 4h 46m 38s A.M. Lat.i.tude by D. R.

24 25' S, Longitude 107 16' 56" E. (_) ex-meridian 52 18' 50" N. IE--2'

40". CC 3m 56s slow. HE 33 ft. Required lat.i.tude in.

4. May 10, 1919. CT 2h 18m 46s A.M. Lat.i.tude by D. R. 23 54' S, Longitude 143 20' 18" E. (_) ex-meridian 48 26' 20" N. IE 3' 20" on the arc. CC 4m 18s fast. HE 41 ft. Required lat.i.tude in.

5. June 21st, 1919. CT 4h. 56m 18s. Lat.i.tude by D. R. 42 01' N, Longitude 75 00' 18" W. (_) ex-meridian 71 29' 40" S, IE--2' 30". CC 3m 04s slow. HE 28 ft. Required lat.i.tude in.

6. Dec. 18th, 1919. WT 11h 50m 18s A.M. C-W 3h 14m 18s. Lat.i.tude by D. R. 11 55' S. Longitude 48 02' 29" W. (_) ex-meridian 78 32' 30" S.

IE 3' 30" on the arc. CC 2m 44s slow. HE 35 ft. Required lat.i.tude in.

If there is any time left, give examples of lat.i.tude by meridian alt.i.tude, Marc St. Hilaire Method by sun or star sight, etc.

SAt.u.r.dAY LECTURE

FINDING THE WATCH TIME OF LOCAL APPARENT NOON

Noon at the ship is the pivotal point of the day's work at sea. It is then that the navigator must report to the commanding officer the lat.i.tude and longitude by dead reckoning, the lat.i.tude and longitude by observation, the course and distance made good, the deviation of the compa.s.s and the course and distance to destination. Apparent noon, then, is a most important time to calculate accurately, and to do so when the ship is under way, is not so easy at it first appears.

If the ship is stationary, and you know the longitude you are in, the problem is simple. Then it is merely a question of starting with L.A.T.

of 00h-00m-00s, adding or subtracting the longitude, according as to whether it is West or East, to get G.A.T.; applying the equation of time with sign reversed to get G.M.T.; applying the C. Cor. with sign reversed to get the C.T.; and applying the C-W to get the WT. If, for instance, this WT happens to be 11h-42m-31s, when the watch reads that number of hours, minutes and seconds, the sun will be on the meridian and it will be apparent noon.