Exercise:
The
Moon is at declination -14°.
What will be its hour angle at
moonrise
(when the top edge of the Moon first appears over the
horizon),
at a latitude of +56°20'?
At
moonrise, the true altitude of the Moon is a = +0°7'
(allowing
for semi-diameter, horizontal refraction and geocentric parallax).
Use
the formula
cos(H) = { sin(a) - sin(φ)
sin(δ) } / cos(φ)
cos(δ)
where φ=
+56°20' and δ = -14°.
This
gives cos(H) = 0.38,
so H = 67.8° or 292.2°
= 4h31m or 19h29m.
To
decide which,
note that the Moon is to the east of the
meridian,
so H = 19h29m.
(This
is 8 minutes later than sunrise,
when the Sun is at the same
declination.)
Back to "geocentric parallax".