Astronomy Principles and Practice Fourth Edition.pdf

422 Practical projects
point was chosen where the altitude was 29 ◦ 22·′34 at 10 h 05·m0UT.
Thus, the observed zenith distance: (OZD) = 60 ◦ 37·′66.
By taking the values of RA⊙, δ ⊙ from the AA and extrapolating, at the time of observation
(10 h 05 m ), the solar position is given by
RA⊙ = 00 h 18 m 34·s38
δ ⊙ =+2 ◦ 00 ′ 35·′′ 8
Sun’s hour angle:
HA⊙ = LST − RA⊙
and
LST = GAST − λ D .
From the AA,at00 h (UT), GAST = 12 h 11 m 01·s99
Approx longitude (4 ◦ 22 ′ ) = 17 m 28 s
µ LST at 00 h (UT) 11 h 53 m 34·s0
Time of observation 10 h 05 m 00·s0
Conversion of 10 h 05 m to sidereal time 1 m 39·s4
µ LST at 10 h 05 m 22 h 00 m 13·s4
RA⊙ 00 h 18 m 34·s4
HA⊙ −2 18 m 21·s0
HA⊙ = H =−34 ◦ 35 ′ 15 ′′ =−34 ◦ 35·′25
Calculation of intercept: The calculated zenith distance, z, is obtained by application of
equation (24.1), namely
cos z = sin φ sin δ ⊙ + cos φ cos δ ⊙ cos H
or
cos z = sin(55 ◦ 53·′7) sin(2 ◦ 00·′60)
+ cos(55 ◦ 53·′7) cos(2 ◦ 00·′60) cos(−34 ◦ 35·′25)
= sin(55·◦895) sin(2·◦0099) + cos(55·◦895) cos(2·◦0099) cos(−34·◦5875)
giving, on reduction, z = 60 ◦ 38·′11.
µ Calculated Zenith Distance (CZD) 60 ◦ 38·′11
(OZD) 60 ◦ 37·′66
Intercept 0·′45
As CZD > OZD, the intercept is marked off in the direction ‘Towards’ the Sun.
The azimuth of the Sun can be determined by application of equation (24.2), namely
sin z cos A = sin δ ⊙ cos φ D − cos δ ⊙ sin φ D cos H.
For the previous example, A = 139 ◦ 23 ′ EofN.ThevalueofA can then be checked by using
cos δ ⊙ sin H =−sin z sin A.