// MoonTopocentric returns observed topocentric semidiameter of the Moon.
//
// Δ is distance to Moon in AU.
// δ is declination of Moon.
// H is hour angle of Moon.
// ρsφʹ, ρcφʹ are parallax constants as returned by
// globe.Ellipsoid.ParallaxConstants, for example.
func MoonTopocentric(Δ float64, δ unit.Angle, H unit.HourAngle, ρsφʹ, ρcφʹ float64) float64 {
const k = .272481
sπ := parallax.Horizontal(Δ).Sin()
// q computed by (40.6, 40.7) p. 280, ch 40.
sδ, cδ := δ.Sincos()
sH, cH := H.Sincos()
A := cδ * sH
B := cδ*cH - ρcφʹ*sπ
C := sδ - ρsφʹ*sπ
q := math.Sqrt(A*A + B*B + C*C)
return k / q * sπ
}
// ParallacticAngle returns parallactic angle of a celestial object.
//
// φ is geographic latitude of observer.
// δ is declination of observed object.
// H is hour angle of observed object.
func ParallacticAngle(φ, δ unit.Angle, H unit.HourAngle) unit.Angle {
sδ, cδ := δ.Sincos()
sH, cH := H.Sincos()
// (14.1) p. 98
return unit.Angle(math.Atan2(sH, φ.Tan()*cδ-sδ*cH))
}