Thursday Lecture
Azimuths Of The Sun
This is a peculiar word to spell and pronounce but its definition is really very simple. Put in your Note-Book:
The azimuth of a heavenly body is the angle at the zenith of the observer formed by the observer's meridian and a line drawn to the center of the body observed. Azimuths are named from the latitude in and toward the E in the A.M. and from the latitude in and toward the W in the P.M.
All this definition means is that, no matter where you are in N latitude, for instance, if you face N, the azimuth of the sun will be the true bearing of the sun from you. The same holds true for moon, star or planet, but in this lecture we will say nothing of the star azimuths for, in some other respects, they are found somewhat differently from the sun azimuths. Put this in your Note-Book:
To find an azimuth of the sun: Note the time of taking the azimuth by chronometer. Apply chronometer correction, if any, to get the G.M.T. Convert G.M.T. into G.A.T. by applying the equation of time. Convert G.A.T. into L.A.T. by applying the longitude in time. The result is L.A.T. or S.H.A. With the correct L.A.T., latitude and declination, enter the azimuth tables to get the sun's true bearing, i.e., its azimuth. Example:
March 15th, 1919. CT 10h 4m 32s. D.R. latitude 40° 10' N, longitude 74° W. Find the TZ.
| G.M.T. | 10h 04m 32s |
| Eq. T. | - 09 10 |
| —————— | |
| G.A.T. | 9h 55m 22s |
| —————— | |
| G.A.T. | 9h 55m 22s |
| Lo. in T. | 4 56 00 (W - ) |
| —————— | |
| L.A.T. | 4h 59m 22s |
| Latitude and Declination opp. name. | |
| TZ = N 101° 30'W | |
| —————— | |
We will take up later a further use of azimuths to find the error of your compass. Right now all you have to keep in mind is what an azimuth is and how you apply the formulas already given you to get the information necessary to enter the Azimuth Tables for the sun's true bearing at any time of the astronomical day when the sun can be seen. In consulting these tables it must be remembered that if your L.A.T. or S.H.A. is, astronomically, 20h (A.M.), you must subtract 12 hours in order to bring the time within the scope of these tables which are arranged from apparent six o'clock A.M. to noon and from apparent noon to 6 P.M. respectively.
We are taking up sun azimuths today in order to get a thorough understanding of them before beginning a discussion of the Marc St. Hilaire Method which we will have tomorrow. You must get clearly in your minds just what a line of position is and how it is found. Yesterday I tried to explain what a line of position was, i.e., a line at right angles to the sun's or other celestial body's true bearing - in other words, a line at right angles to the sun's or other celestial body's azimuth. Today I tried to show you how to find your azimuth from the azimuth tables for any hour of the day. Tomorrow we will start to use azimuths in working out sights for lines of position by the Marc St. Hilaire Method.
Note to Instructor: Spend the rest of the time in finding sun azimuths in the tables by working out such examples as these:
1. April 29th, 1919. D.R. latitude 40° 40' N, Longitude 74° 55' 14" W. CT 10h 14m 24s. CC 4m 30s slow. Find TZ.
2. May 15th, 1919. D.R. latitude 19° 20' S, Longitude 40° 15' 44" E. CT 10h 44m 55s A.M. CC 3m 10s fast. Find TZ.
Note to Instructor:
If possible, give more examples to find TZ and also some examples on latitude by meridian altitude.
Assign for Night Work reading the following Articles in Bowditch: 371-372-373-374-375. Also, examples to find TZ.