Thursday Lecture
Latitude by ex-meridian Altitude of the Sun
You have learned that when you calculate your latitude from a meridian altitude 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 altitude, i.e., an altitude 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 altitude cannot be secured. There is a way to calculate your latitude, even though the altitude you secure is taken by sextant some minutes before or after noon. This is called latitude by an ex-meridian altitude. 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 Passage."
3. With your D.R. latitude, declination and Time Interval from Meridian Passage, enter Table 26 to get the proper amount of Variation of Altitude in one minute from meridian passage.
4. With the Time Interval from Meridian Passage and the Variation, enter Table 27 to get the total amount of Variation of Altitude.
5. Add this total amount of Variation to the true observed altitude taken before or after noon, and the result is the corrected altitude.
6. Then proceed to get your latitude according to the rules already given you for latitude by meridian altitude.
Example: At sea, Jan. 23rd, 1919. CT 4h 22m 14s. CC 1m 10s fast. Longitude 66° 04' W. Latitude by D.R. 19° 16' 00" N. Circle with line under 50° 51' 00" S. HE 49 ft. IE - 1' 30". Required latitude 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 Passage.
Dec. 19° 34' 48" S Table 26 = 2.8 Variation
Lat. 19° 16' 00" N For 1 min. 0 altitude.
Time Interval from Meridian Passage 15m 02s - 2.8" Variation for 1 minute
| (Table 27) | 2" = 7' 30" | |||
| .8 = 3 00 | ||||
| ————— | ||||
| 10' 30" + | ||||
| IE | - 1' 30" | Circle with line under | 50° 51' 00" | |
| HE | + 8 42 | + 7 12 | ||
| ———— | ————— | |||
| Corr. | + 7' 12" | Circle with line through | 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 |
Assign for work in class room and Night Work, examples similar to the following:
1. At sea, July 11th, 1919. Latitude by D.R. 50° 01' 00" N. Longitude 40° 05' 16" W. Observed ex-meridian altitude Circle with line under 61° 45' 30" S. HE 15 ft. IE - 4' 10". CT (corrected) 2h 38m 00s. Required latitude in.
2. At sea, June 6th, 1919. Latitude by D. R. 49° 21' N, Longitude 18° 18' W. Observed ex-meridian altitude Circle with line under 61° 30' 22" S. HE 42 ft. CT 1h 06m 18s. CC - 1m 14s. IE 0' 30" off the arc. Required latitude in of ship.
If any time is left, work similar examples by Marc St. Hilaire Method.