The solar zenith angle is the zenith angle of the Sun, i.e., the angle between the sun’s rays and the vertical direction. It is the complement to the solar altitude or solar elevation, which is the altitude angle or elevation angle between the Sun’s rays and a horizontal plane. Near (but usually not exactly at) solar noon, the zenith angle is at a minimum and is equal to the absolute value of latitude minus solar declination angle. This is the basis by which ancient mariners navigated the oceans. Solar zenith angle is normally used in combination with the solar azimuth angle to determine the position of the Sun as observed from a given location on the surface of the Earth.
Formula
cos θ s = sin α s = sin Φ sin δ + cos Φ cos δ cos h {\displaystyle \cos \theta _{s}=\sin \alpha _{s}=\sin \Phi \sin \delta +\cos \Phi \cos \delta \cos h}
where
θ s {\displaystyle \theta _{s}} is the solar zenith angle
α s {\displaystyle \alpha _{s}} is the solar altitude angle, α s = 90 ∘ − θ s {\displaystyle \alpha _{s}=90^{\circ }-\theta _{s}}
h {\displaystyle h} is the hour angle, in the local solar time.
δ {\displaystyle \delta } is the current declination of the Sun
Φ {\displaystyle \Phi } is the local latitude. This equation is equivalent to the formula for the angle between vectors in spherical coordinates applied to the local normal and the direction to the sun, with the adjustment that the latitude is the complement to the polar angle and that the hour angle is the difference of azimuths. At solar noon, this reads
cos θ s = sin Φ sin δ + cos Φ cos δ = cos ( Φ − δ ) {\displaystyle \cos \theta _{s}=\sin \Phi \sin \delta +\cos \Phi \cos \delta =\cos(\Phi -\delta )}
where we have used the difference identity for cosine. Therefore at solar noon, θ s = Φ − δ {\displaystyle \theta _{s}=\Phi -\delta }
Derivation of the formula using the subsolar point and vector analysis While the formula can be derived by applying the cosine law to the zenith-pole-Sun spherical triangle, the spherical trigonometry is a relatively esoteric subject. By introducing the coordinates of the subsolar point and using vector analysis, the formula can be obtained straightforward without incurring the use of spherical trigonometry. In the Earth-Centered Earth-Fixed (ECEF) geocentric Cartesian coordinate system, let ( ϕ s , λ s ) {\displaystyle (\phi _{s},\lambda _{s})} and ( ϕ o , λ o ) {\displaystyle (\phi _{o},\lambda _{o})} be the latitudes and longitudes, or coordinates, of the subsolar point and the observer's point, then the upward-pointing unit vectors at the two points, S {\displaystyle \mathbf {S} } and V o z {\displaystyle \mathbf {V} _{oz}} , are
S = cos ϕ s cos λ s i + cos ϕ s sin λ s j + sin ϕ s k , {\displaystyle \mathbf {S} =\cos \phi _{s}\cos \lambda _{s}{\mathbf {i} }+\cos \phi _{s}\sin \lambda _{s}{\mathbf {j} }+\sin \phi _{s}{\mathbf {k} },}
V o z = cos ϕ o cos λ o i + cos ϕ o sin λ o j + sin ϕ o k . {\displaystyle \mathbf {V} _{oz}=\cos \phi _{o}\cos \lambda _{o}{\mathbf {i} }+\cos \phi _{o}\sin \lambda _{o}{\mathbf {j} }+\sin \phi _{o}{\mathbf {k} }.}
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