Astronomy Principles and Practice Fourth Edition.pdf

412 Practical projects
Figure 24.6. The Sun’s azimuth relative to a fixed reference object.
giving, on reduction
tan A ⊙ =−0·644 501.
Hence, A ⊙ = 147·◦198 = 147 ◦ 12 ′ E of N, to the nearest minute of arc.
Now the corresponding reading on the theodolite is 222 ◦ 12 ′ and the reading for the reference
object is 108 ◦ 27 ′ , the difference being 13 ◦ 45 ′ . Thus, the reference object has an azimuth which is
13 ◦ 45 ′ less than that of the Sun at the time of the observation, giving it a fixed azimuth of 133 ◦ 27 ′ E
of N. The situation is summarized in figure 24.6.
24.3.3 Sextant observations
This exercise involves the use of a sextant to make measurements of the Sun’s altitude over a short
period of time in order to determine the position of the observer. The times of these observations are
noted using a Greenwich mean time chronometer of known error and rate.
The basic principles behind the finding of an observer’s position on the Earth’s surface are first of
all described briefly.
(i) The position circle. In figure 24.7, it is seen that the latitude φ and longitude λ of a point, L (the
sub-solar point), on the Earth’s surface directly below the Sun, S, are related to the Sun’s Greenwich
hour angle (GHA⊙) by the relations
φ = δ ⊙
λ W = GHA⊙ (GHA⊙ ≤ 12 h )
or
λ E = 24 h − GHA⊙ (GHA⊙ ≥ 12 h ).