The formulae
and derivations are given
Image 2.
Nota bene: we count south latitude as negative.
From the general formulae:
To determine declinations
from positions NOT on the meridian.
The you need to use spherical trigonometry
and the necessary formulae
and derivations are quite complex.
However, nowadays the computer
using spherical trigonometry
easily converts
horizontal coordinates
to equatorial coordinates
(and
vice versa)
and measurements at transits
of the meridian are
much less important than they were.
where:
    AN/S = (±)N/S(L - δ) + 90°
    δ = L +(±)N/S(90° - AN/S)
    L = δ +(±)N/S( AN/S - 90°)
-δ_(above/below)= ±[(90° - (-L)] -δ_(below/above)= ∓[(90° - (-L)] δ_(below/above)= ±[(90° - (-L)] δ_(±)= ±[(90° - (-L)] .
where again southern latitudes are counted as negative and the +/- case is lower/upper limit for declinations below/above the horizon.
f_c = 1 - cos(L) : fraction of the sky that is circumpolar f_s = (1/2)*(1 - cos(L)) : fraction of the sky that is circumpolar and above/below the horizon f_n = cos(L) : fraction of the sky that is non-circumpolar
which follow from the integral
[1/(4π)]*2*2*π* ∫μ1dμ = 1 - μ = 1 - cos(90-(90-|L|)) = 1 - cos(L) .
The above formulae for the fractions of sky have been used to construct Table: The Circumpolar Sky given below.
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Table: The Circumpolar Sky
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Latitude N/S f_c f_s f_n f_n exact comment
(degrees)
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0 0.0000 0.0000 1.0000 1 on equator
30 0.1340 0.0670 0.8660 sqrt(3)/2
36 0.1910 0.0955 0.8090 ... in Las Vegas
60 0.5000 0.2500 0.5000 1/2
90 1.0000 0.5000 0.0000 0 at North Pole
or South Pole
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Reference: CAC-8--9
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