// EclToEq converts ecliptic coordinates to equatorial coordinates.
//
// λ: ecliptic longitude coordinate to transform
// β: ecliptic latitude coordinate to transform
// sε: sine of obliquity of the ecliptic
// cε: cosine of obliquity of the ecliptic
//
// Results:
// α: right ascension
// δ: declination
func EclToEq(λ, β unit.Angle, sε, cε float64) (α unit.RA, δ unit.Angle) {
sλ, cλ := λ.Sincos()
sβ, cβ := β.Sincos()
α = unit.RAFromRad(math.Atan2(sλ*cε-(sβ/cβ)*sε, cλ)) // (13.3) p. 93
δ = unit.Angle(math.Asin(sβ*cε + cβ*sε*sλ)) // (13.4) p. 93
return
}
// EclToEq converts ecliptic coordinates to equatorial coordinates.
func (eq *Equatorial) EclToEq(ecl *Ecliptic, ε *Obliquity) *Equatorial {
sβ, cβ := ecl.Lat.Sincos()
sλ, cλ := ecl.Lon.Sincos()
eq.RA = unit.RAFromRad(math.Atan2(sλ*ε.C-(sβ/cβ)*ε.S, cλ)) // (13.3) p. 93
eq.Dec = unit.Angle(math.Asin(sβ*ε.C + cβ*ε.S*sλ)) // (13.4) p. 93
return eq
}
// Position precesses equatorial coordinates from one epoch to another,
// including proper motions.
//
// If proper motions are not to be considered or are not applicable, pass 0, 0
// for mα, mδ
//
// Both eqFrom and eqTo must be non-nil, although they may point to the same
// struct. EqTo is returned for convenience.
func Position(eqFrom, eqTo *coord.Equatorial, epochFrom, epochTo float64, mα unit.HourAngle, mδ unit.Angle) *coord.Equatorial {
p := NewPrecessor(epochFrom, epochTo)
t := epochTo - epochFrom
eqTo.RA = unit.RAFromRad(eqFrom.RA.Rad() + mα.Rad()*t)
eqTo.Dec = eqFrom.Dec + mδ*unit.Angle(t)
return p.Precess(eqTo, eqTo)
}
// GalToEq converts galactic coordinates to equatorial coordinates.
//
// Resulting equatorial coordinates will be referred to the standard equinox of
// B1950.0. For subsequent conversion to other epochs, see package precess and
// utility functions in package meeus.
func GalToEq(l, b unit.Angle) (α unit.RA, δ unit.Angle) {
sdLon, cdLon := (l - galacticLon0).Sincos()
sgδ, cgδ := galacticNorth.Dec.Sincos()
sb, cb := b.Sincos()
y := math.Atan2(sdLon, cdLon*sgδ-(sb/cb)*cgδ)
α = unit.RAFromRad(y + galacticNorth.RA.Rad())
δ = unit.Angle(math.Asin(sb*sgδ + cb*cgδ*cdLon))
return
}
// GalToEq converts galactic coordinates to equatorial coordinates.
//
// Resulting equatorial coordinates will be referred to the standard equinox of
// B1950.0. For subsequent conversion to other epochs, see package precess and
// utility functions in package meeus.
func (eq *Equatorial) GalToEq(g *Galactic) *Equatorial {
sdLon, cdLon := (g.Lon - galacticLon0).Sincos()
sgδ, cgδ := galacticNorth.Dec.Sincos()
sb, cb := g.Lat.Sincos()
y := math.Atan2(sdLon, cdLon*sgδ-(sb/cb)*cgδ)
eq.RA = unit.RAFromRad(y + galacticNorth.RA.Rad())
eq.Dec = unit.Angle(math.Asin(sb*sgδ + cb*cgδ*cdLon))
return eq
}
// HzToEq transforms horizontal coordinates to equatorial coordinates.
//
// A: azimuth
// h: elevation
// φ: latitude of observer on Earth
// ψ: longitude of observer on Earth
// st: sidereal time at Greenwich at time of observation.
//
// Sidereal time must be consistent with the equatorial coordinates
// in the sense that tf coordinates are apparent, sidereal time must be
// apparent as well.
//
// Results:
//
// α: right ascension
// δ: declination
func HzToEq(A, h, φ, ψ unit.Angle, st unit.Time) (α unit.RA, δ unit.Angle) {
sA, cA := A.Sincos()
sh, ch := h.Sincos()
sφ, cφ := φ.Sincos()
H := math.Atan2(sA, cA*sφ+sh/ch*cφ)
α = unit.RAFromRad(st.Rad() - ψ.Rad() - H)
δ = unit.Angle(math.Asin(sφ*sh - cφ*ch*cA))
return
}
// HzToEq transforms horizontal coordinates to equatorial coordinates.
//
// Sidereal time st must be consistent with the equatorial coordinates
// in the sense that if coordinates are apparent, sidereal time must be
// apparent as well.
func (eq *Equatorial) HzToEq(hz *Horizontal, g globe.Coord, st unit.Time) *Equatorial {
sA, cA := hz.Az.Sincos()
sh, ch := hz.Alt.Sincos()
sφ, cφ := g.Lat.Sincos()
H := math.Atan2(sA, cA*sφ+sh/ch*cφ)
eq.RA = unit.RAFromRad(st.Rad() - g.Lon.Rad() - H)
eq.Dec = unit.Angle(math.Asin(sφ*sh - cφ*ch*cA))
return eq
}
// TrueEquatorial returns the true geometric position of the Sun as equatorial coordinates.
func TrueEquatorial(jde float64) (α unit.RA, δ unit.Angle) {
s, _ := True(base.J2000Century(jde))
ε := nutation.MeanObliquity(jde)
ss, cs := s.Sincos()
sε, cε := ε.Sincos()
// (25.6, 25.7) p. 165
α = unit.RAFromRad(math.Atan2(cε*ss, cs))
δ = unit.Angle(math.Asin(sε * ss))
return
}
// ApparentEquatorial returns the apparent position of the Sun as equatorial coordinates.
//
// α: right ascension in radians
// δ: declination in radians
func ApparentEquatorial(jde float64) (α unit.RA, δ unit.Angle) {
T := base.J2000Century(jde)
λ := ApparentLongitude(T)
ε := nutation.MeanObliquity(jde)
sλ, cλ := λ.Sincos()
// (25.8) p. 165
ε += unit.AngleFromDeg(.00256).Mul(node(T).Cos())
sε, cε := ε.Sincos()
α = unit.RAFromRad(math.Atan2(cε*sλ, cλ))
δ = unit.Angle(math.Asin(sε * sλ))
return
}
// Precess precesses coordinates eqFrom, leaving result in eqTo.
//
// The same struct may be used for eqFrom and eqTo.
// EqTo is returned for convenience.
func (p *Precessor) Precess(eqFrom, eqTo *coord.Equatorial) *coord.Equatorial {
// (21.4) p. 134
sδ, cδ := eqFrom.Dec.Sincos()
sαζ, cαζ := (eqFrom.RA + p.ζ).Sincos()
A := cδ * sαζ
B := p.cθ*cδ*cαζ - p.sθ*sδ
C := p.sθ*cδ*cαζ + p.cθ*sδ
eqTo.RA = unit.RAFromRad(math.Atan2(A, B) + p.z.Rad())
if C < base.CosSmallAngle {
eqTo.Dec = unit.Angle(math.Asin(C))
} else {
eqTo.Dec = unit.Angle(math.Acos(math.Hypot(A, B))) // near pole
}
return eqTo
}
// ProperMotion3D takes the 3D equatorial coordinates of an object
// at one epoch and computes its coordinates at a new epoch, considering
// proper motion and radial velocity.
//
// Radial distance (r) must be in parsecs, radial velocitiy (mr) in
// parsecs per year.
//
// Both eqFrom and eqTo must be non-nil, although they may point to the same
// struct. EqTo is returned for convenience.
func ProperMotion3D(eqFrom, eqTo *coord.Equatorial, epochFrom, epochTo, r, mr float64, mα unit.HourAngle, mδ unit.Angle) *coord.Equatorial {
sα, cα := eqFrom.RA.Sincos()
sδ, cδ := eqFrom.Dec.Sincos()
x := r * cδ * cα
y := r * cδ * sα
z := r * sδ
mrr := mr / r
zmδ := z * mδ.Rad()
mx := x*mrr - zmδ*cα - y*mα.Rad()
my := y*mrr - zmδ*sα + x*mα.Rad()
mz := z*mrr + r*mδ.Rad()*cδ
t := epochTo - epochFrom
xp := x + t*mx
yp := y + t*my
zp := z + t*mz
eqTo.RA = unit.RAFromRad(math.Atan2(yp, xp))
eqTo.Dec = unit.Angle(math.Atan2(zp, math.Hypot(xp, yp)))
return eqTo
}
// AstrometricJ2000 is a utility function for computing astrometric coordinates.
//
// It is used internally and only exported so that it can be used from
// multiple packages. It is not otherwise expected to be used.
//
// Argument f is a function that returns J2000 equatorial rectangular
// coodinates of a body.
//
// Results are J2000 right ascention, declination, and elongation.
func AstrometricJ2000(f func(float64) (x, y, z float64), jde float64, e *pp.V87Planet) (α unit.RA, δ, ψ unit.Angle) {
X, Y, Z := solarxyz.PositionJ2000(e, jde)
x, y, z := f(jde)
// (33.10) p. 229
ξ := X + x
η := Y + y
ζ := Z + z
Δ := math.Sqrt(ξ*ξ + η*η + ζ*ζ)
{
τ := base.LightTime(Δ)
x, y, z = f(jde - τ)
ξ = X + x
η = Y + y
ζ = Z + z
Δ = math.Sqrt(ξ*ξ + η*η + ζ*ζ)
}
α = unit.RAFromRad(math.Atan2(η, ξ))
δ = unit.Angle(math.Asin(ζ / Δ))
R0 := math.Sqrt(X*X + Y*Y + Z*Z)
ψ = unit.Angle(math.Acos((ξ*X + η*Y + ζ*Z) / R0 / Δ))
return
}
func ExampleTopocentric3() {
// same test case as example 40.a, p. 280
α := unit.RAFromDeg(339.530208)
δ := unit.AngleFromDeg(-15.771083)
Δ := .37276
ρsφʹ := .546861
ρcφʹ := .836339
L := unit.Angle(unit.NewHourAngle(' ', 7, 47, 27))
jde := julian.CalendarGregorianToJD(2003, 8, 28+
unit.NewTime(' ', 3, 17, 0).Day())
Hʹ, δʹ := parallax.Topocentric3(α, δ, Δ, ρsφʹ, ρcφʹ, L, jde)
fmt.Printf("Hʹ = %.2d\n", sexa.FmtHourAngle(Hʹ))
θ0 := sidereal.Apparent(jde)
αʹ := unit.RAFromRad(θ0.Rad() - L.Rad() - Hʹ.Rad())
// same result as example 40.a, p. 280
fmt.Printf("αʹ = %.2d\n", sexa.FmtRA(αʹ))
fmt.Printf("δʹ = %.1d\n", sexa.FmtAngle(δʹ))
// Output:
// Hʹ = -4ʰ44ᵐ50ˢ.28
// αʹ = 22ʰ38ᵐ8ˢ.54
// δʹ = -15°46′30″.0
}
