// Section "Trigonometric functions of large angles":
//
// Meeus makes his point, but an example with integer values is a bit unfair
// when trigonometric functions inherently work on floating point numbers.
func ExamplePMod_integer() {
const large = 36000030
// The direct function call loses precition as expected.
fmt.Println("Direct: ", math.Sin(large*math.Pi/180))
// When the value is manually reduced to the integer 30, the Go constant
// evaluaton does a good job of delivering a radian value to math.Sin that
// evaluates to .5 exactly.
fmt.Println("Integer 30:", math.Sin(30*math.Pi/180))
// Math.Mod takes float64s and returns float64s. The integer constants
// here however can be represented exactly as float64s, and the returned
// result is exact as well.
fmt.Println("Math.Mod: ", math.Mod(large, 360))
// But when math.Mod is substituted into the Sin function, float64s
// are multiplied instead of the high precision constants, and the result
// comes back slightly inexact.
fmt.Println("Sin Mod: ", math.Sin(math.Mod(large, 360)*math.Pi/180))
// Use of PMod on integer constants produces results identical to above.
fmt.Println("PMod int: ", math.Sin(base.PMod(large, 360)*math.Pi/180))
// As soon as the large integer is scaled to a non-integer value though,
// precision is lost and PMod is of no help recovering at this point.
fmt.Println("PMod float:", math.Sin(base.PMod(large*math.Pi/180, 2*math.Pi)))
// Output:
// Direct: 0.49999999995724154
// Integer 30: 0.5
// Math.Mod: 30
// Sin Mod: 0.49999999999999994
// PMod int: 0.49999999999999994
// PMod float: 0.49999999997845307
}
// True returns true geometric longitude and anomaly of the sun referenced to the mean equinox of date.
//
// Argument T is the number of Julian centuries since J2000.
// See base.J2000Century.
//
// Results:
// s = true geometric longitude, ☉, in radians
// ν = true anomaly in radians
func True(T float64) (s, ν float64) {
// (25.2) p. 163
L0 := base.Horner(T, 280.46646, 36000.76983, 0.0003032) *
math.Pi / 180
M := MeanAnomaly(T)
C := (base.Horner(T, 1.914602, -0.004817, -.000014)*
math.Sin(M) +
(0.019993-.000101*T)*math.Sin(2*M) +
0.000289*math.Sin(3*M)) * math.Pi / 180
return base.PMod(L0+C, 2*math.Pi), base.PMod(M+C, 2*math.Pi)
}
// ReduceB1950ToJ2000 reduces orbital elements of a solar system body from
// equinox B1950 to J2000.
func ReduceB1950ToJ2000(eFrom, eTo *Elements) *Elements {
// (24.4) p. 161
const S = .0001139788
const C = .9999999935
W := eFrom.Node - 174.298782*math.Pi/180
si, ci := math.Sincos(eFrom.Inc)
sW, cW := math.Sincos(W)
A := si * sW
B := C*si*cW - S*ci
eTo.Inc = math.Asin(math.Hypot(A, B))
eTo.Node = base.PMod(174.997194*math.Pi/180+math.Atan2(A, B),
2*math.Pi)
eTo.Peri = base.PMod(eFrom.Peri+math.Atan2(-S*sW, C*si-S*ci*cW),
2*math.Pi)
return eTo
}
func TestNode0(t *testing.T) {
for i, j := range n0 {
if e := math.Abs(base.PMod(moon.Node(j)+1, 2*math.Pi) - 1); e > 1e-3 {
t.Error(i, e)
}
}
}
// ReduceB1950ToJ2000 reduces orbital elements of a solar system body from
// equinox B1950 in the FK4 system to equinox J2000 in the FK5 system.
func ReduceB1950FK4ToJ2000FK5(eFrom, eTo *Elements) *Elements {
const (
Lp = 4.50001688 * math.Pi / 180
L = 5.19856209 * math.Pi / 180
J = .00651966 * math.Pi / 180
)
W := L + eFrom.Node
si, ci := math.Sincos(eFrom.Inc)
sJ, cJ := math.Sincos(J)
sW, cW := math.Sincos(W)
eTo.Inc = math.Acos(ci*cJ - si*sJ*cW)
eTo.Node = base.PMod(math.Atan2(si*sW, ci*sJ+si*cJ*cW)-Lp,
2*math.Pi)
eTo.Peri = base.PMod(eFrom.Peri+math.Atan2(sJ*sW, si*cJ+ci*sJ*cW),
2*math.Pi)
return eTo
}
// Mean computes some intermediate values for a mean planetary configuration
// given a year and a row of coefficients from Table 36.A, p. 250.
func mean(y float64, a *ca) (J, M, T float64) {
// (36.1) p. 250
k := math.Floor((365.2425*y+1721060-a.A)/a.B + .5)
J = a.A + k*a.B
M = base.PMod(a.M0+k*a.M1, 360) * math.Pi / 180
T = base.J2000Century(J)
return
}
// Apparent0UT returns apparent sidereal time at Greenwich at 0h UT
// on the given JD.
//
// The result is in seconds of time and is in the range [0,86400).
func Apparent0UT(jd float64) float64 {
j0, f := math.Modf(jd + .5)
cen := (j0 - .5 - base.J2000) / 36525
s := base.Horner(cen, iau82...) + f*1.00273790935*86400
n := nutation.NutationInRA(j0) // angle (radians) of RA
ns := n * 3600 * 180 / math.Pi / 15 // convert RA to time in seconds
return base.PMod(s+ns, 86400)
}
// 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 float64) (α, δ float64) {
sdLon, cdLon := math.Sincos(l - galacticLon0)
sgδ, cgδ := math.Sincos(galacticNorth.Dec)
sb, cb := math.Sincos(b)
y := math.Atan2(sdLon, cdLon*sgδ-(sb/cb)*cgδ)
α = base.PMod(y+galacticNorth.RA, 2*math.Pi)
δ = math.Asin(sb*sgδ + cb*cgδ*cdLon)
return
}
// 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.
// If coordinates are apparent, sidereal time must be apparent as well.
//
// Results:
//
// α: right ascension
// δ: declination
func HzToEq(A, h, φ, ψ, st float64) (α, δ float64) {
sA, cA := math.Sincos(A)
sh, ch := math.Sincos(h)
sφ, cφ := math.Sincos(φ)
H := math.Atan2(sA, cA*sφ+sh/ch*cφ)
α = base.PMod(sexa.Time(st).Rad()-ψ-H, 2*math.Pi)
δ = math.Asin(sφ*sh - cφ*ch*cA)
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 := math.Sincos(g.Lon - galacticLon0)
sgδ, cgδ := math.Sincos(galacticNorth.Dec)
sb, cb := math.Sincos(g.Lat)
y := math.Atan2(sdLon, cdLon*sgδ-(sb/cb)*cgδ)
eq.RA = base.PMod(y+galacticNorth.RA, 2*math.Pi)
eq.Dec = math.Asin(sb*sgδ + cb*cgδ*cdLon)
return eq
}
// HzToEq transforms horizontal coordinates to equatorial coordinates.
//
// Sidereal time must be consistent with the equatorial coordinates.
// If coordinates are apparent, sidereal time must be apparent as well.
func (eq *Equatorial) HzToEq(hz *Horizontal, g globe.Coord, st float64) *Equatorial {
sA, cA := math.Sincos(hz.Az)
sh, ch := math.Sincos(hz.Alt)
sφ, cφ := math.Sincos(g.Lat)
H := math.Atan2(sA, cA*sφ+sh/ch*cφ)
eq.RA = base.PMod(sexa.Time(st).Rad()-g.Lon-H, 2*math.Pi)
eq.Dec = math.Asin(sφ*sh - cφ*ch*cA)
return eq
}
// Topocentric2 returns topocentric corrections including parallax.
//
// This function implements the "non-rigorous" method descripted in the text.
//
// Note that results are corrections, not corrected coordinates.
func Topocentric2(α, δ, Δ, ρsφʹ, ρcφʹ, L, jde float64) (Δα, Δδ float64) {
π := Horizontal(Δ)
θ0 := base.Time(sidereal.Apparent(jde)).Rad()
H := base.PMod(θ0-L-α, 2*math.Pi)
sH, cH := math.Sincos(H)
sδ, cδ := math.Sincos(δ)
Δα = -π * ρcφʹ * sH / cδ // (40.4) p. 280
Δδ = -π * (ρsφʹ*cδ - ρcφʹ*cH*sδ) // (40.5) p. 280
return
}
// Mean returns mean orbital elements for a planet
//
// Argument p must be a planet const as defined above, argument e is
// a result parameter. A valid non-nil pointer to an Elements struct
// must be passed in.
//
// Results are referenced to mean dynamical ecliptic and equinox of date.
//
// Semimajor axis is in AU, angular elements are in radians.
func Mean(p int, jde float64, e *Elements) {
T := base.J2000Century(jde)
c := &cMean[p]
e.Lon = base.PMod(base.Horner(T, c.L...)*math.Pi/180, 2*math.Pi)
e.Axis = base.Horner(T, c.a...)
e.Ecc = base.Horner(T, c.e...)
e.Inc = base.Horner(T, c.i...) * math.Pi / 180
e.Node = base.Horner(T, c.Ω...) * math.Pi / 180
e.Peri = base.Horner(T, c.ϖ...) * math.Pi / 180
}
func (m *moon) optical(λ, β float64) (lʹ, bʹ, A float64) {
// (53.1) p. 372
W := λ - m.Ω // (λ without nutation)
sW, cW := math.Sincos(W)
sβ, cβ := math.Sincos(β)
A = math.Atan2(sW*cβ*cI-sβ*sI, cW*cβ)
lʹ = base.PMod(A-m.F, 2*math.Pi)
bʹ = math.Asin(-sW*cβ*sI - sβ*cI)
return
}
// TrueVSOP87 returns the true geometric position of the sun as ecliptic coordinates.
//
// Result computed by full VSOP87 theory. Result is at equator and equinox
// of date in the FK5 frame. It does not include nutation or aberration.
//
// s: ecliptic longitude in radians
// β: ecliptic latitude in radians
// R: range in AU
func TrueVSOP87(e *pp.V87Planet, jde float64) (s, β, R float64) {
l, b, r := e.Position(jde)
s = l + math.Pi
// FK5 correction.
λp := base.Horner(base.J2000Century(jde),
s, -1.397*math.Pi/180, -.00031*math.Pi/180)
sλp, cλp := math.Sincos(λp)
Δβ := .03916 / 3600 * math.Pi / 180 * (cλp - sλp)
// (25.9) p. 166
return base.PMod(s-.09033/3600*math.Pi/180, 2*math.Pi), Δβ - b, r
}
// Topocentric returns topocentric positions including parallax.
//
// Arguments α, δ are geocentric right ascension and declination in radians.
// Δ is distance to the observed object in AU. ρsφʹ, ρcφʹ are parallax
// constants (see package globe.) L is geographic longitude of the observer,
// jde is time of observation.
//
// Results are observed topocentric ra and dec in radians.
func Topocentric(α, δ, Δ, ρsφʹ, ρcφʹ, L, jde float64) (αʹ, δʹ float64) {
π := Horizontal(Δ)
θ0 := base.Time(sidereal.Apparent(jde)).Rad()
H := base.PMod(θ0-L-α, 2*math.Pi)
sπ := math.Sin(π)
sH, cH := math.Sincos(H)
sδ, cδ := math.Sincos(δ)
Δα := math.Atan2(-ρcφʹ*sπ*sH, cδ-ρcφʹ*sπ*cH) // (40.2) p. 279
αʹ = α + Δα
δʹ = math.Atan2((sδ-ρsφʹ*sπ)*math.Cos(Δα), cδ-ρcφʹ*sπ*cH) // (40.3) p. 279
return
}
// Times computes UT rise, transit and set times for a celestial object on
// a day of interest.
//
// The function argurments do not actually include the day, but do include
// a number of values computed from the day.
//
// p is geographic coordinates of observer.
// ΔT is delta T.
// h0 is "standard altitude" of the body.
// Th0 is apparent sidereal time at 0h UT at Greenwich.
// α3, δ3 are slices of three right ascensions and declinations.
//
// ΔT unit is seconds. See package deltat.
//
// h0 unit is radians.
//
// Th0 must be the time on the day of interest, in seconds.
// See sidereal.Apparent0UT.
//
// α3, δ3 must be values at 0h dynamical time for the day before, the day of,
// and the day after the day of interest. Units are radians.
//
// Result units are seconds and are in the range [0,86400)
func Times(p globe.Coord, ΔT, h0, Th0 float64, α3, δ3 []float64) (mRise, mTransit, mSet float64, err error) {
mRise, mTransit, mSet, err = ApproxTimes(p, h0, Th0, α3[1], δ3[1])
if err != nil {
return
}
var d3α, d3δ *interp.Len3
d3α, err = interp.NewLen3(-86400, 86400, α3)
if err != nil {
return
}
d3δ, err = interp.NewLen3(-86400, 86400, δ3)
if err != nil {
return
}
// adjust mTransit
{
th0 := base.PMod(Th0+mTransit*360.985647/360, 86400)
α := d3α.InterpolateX(mTransit + ΔT)
H := th0 - (p.Lon+α)*43200/math.Pi
mTransit -= H
}
// adjust mRise, mSet
sLat, cLat := math.Sincos(p.Lat)
adjustRS := func(m float64) (float64, error) {
th0 := base.PMod(Th0+m*360.985647/360, 86400)
ut := m + ΔT
α := d3α.InterpolateX(ut)
δ := d3δ.InterpolateX(ut)
H := th0 - (p.Lon+α)*43200/math.Pi
sδ, cδ := math.Sincos(δ)
h := sLat*sδ + cLat*cδ*math.Cos(H)
return m + (h-h0)/cδ*cLat*math.Sin(H), nil
}
mRise, err = adjustRS(mRise)
if err != nil {
return
}
mSet, err = adjustRS(mSet)
return
}
// ApproxTimes computes approximate UT rise, transit and set times for
// a celestial object on a day of interest.
//
// The function argurments do not actually include the day, but do include
// values computed from the day.
//
// p is geographic coordinates of observer.
// h0 is "standard altitude" of the body.
// Th0 is apparent sidereal time at 0h UT at Greenwich.
// α, δ are right ascension and declination of the body.
//
// h0 unit is radians.
//
// Th0 must be the time on the day of interest, in seconds.
// See sidereal.Apparent0UT.
//
// α, δ must be values at 0h dynamical time for the day of interest.
// Units are radians.
//
// Result units are seconds and are in the range [0,86400)
func ApproxTimes(p globe.Coord, h0, Th0 float64, α, δ float64) (mRise, mTransit, mSet float64, err error) {
// Meeus works in a crazy mix of units.
// This function and Times work with seconds of time as much as possible.
// approximate local hour angle
sLat, cLat := math.Sincos(p.Lat)
sδ1, cδ1 := math.Sincos(δ)
cH0 := (math.Sin(h0) - sLat*sδ1) / (cLat * cδ1) // (15.1) p. 102
if cH0 < -1 || cH0 > 1 {
err = ErrorCircumpolar
return
}
H0 := math.Acos(cH0) * 43200 / math.Pi
// approximate transit, rise, set times.
// (15.2) p. 102.
mt := (α+p.Lon)*43200/math.Pi - Th0
mTransit = base.PMod(mt, 86400)
mRise = base.PMod(mt-H0, 86400)
mSet = base.PMod(mt+H0, 86400)
return
}
// Section "Trigonometric functions of large angles":
//
// A non integer example better illustrates that reduction to a range
// does not rescue precision.
func ExamplePMod_mars() {
// Angle W from step 9 of example 42.a, as suggested.
const W = 5492522.4593
const reduced = 2.4593
// Direct function call. It's a number. How correct is it?
fmt.Println("Direct: ", math.Sin(W*math.Pi/180))
// Manually reduced to range 0-360. This is presumably the "correct"
// answer, but note that the reduced number has a reduced number of
// significat digits. The answer cannot have any more significant digits.
fmt.Println("Reduced: ", math.Sin(reduced*math.Pi/180))
// Accordingly, PMod cannot rescue any precision, whether done on degrees
// or radians.
fmt.Println("PMod deg:", math.Sin(base.PMod(W, 360)*math.Pi/180))
fmt.Println("PMod rad:", math.Sin(base.PMod(W*math.Pi/180, 2*math.Pi)))
// Output:
// Direct: 0.04290970350270464
// Reduced: 0.04290970350923273
// PMod deg: 0.04290970351307828
// PMod rad: 0.04290970350643808
}
// Physical2 computes quantities for physical observations of Jupiter.
//
// Results are less accurate than with Physical().
//
// Results:
// DS Planetocentric declination of the Sun.
// DE Planetocentric declination of the Earth.
// ω1 Longitude of the System I central meridian of the illuminated disk,
// as seen from Earth.
// ω2 Longitude of the System II central meridian of the illuminated disk,
// as seen from Earth.
//
// All angular results in radians.
func Physical2(jde float64) (DS, DE, ω1, ω2 float64) {
d := jde - base.J2000
const p = math.Pi / 180
V := 172.74*p + .00111588*p*d
M := 357.529*p + .9856003*p*d
sV := math.Sin(V)
N := 20.02*p + .0830853*p*d + .329*p*sV
J := 66.115*p + .9025179*p*d - .329*p*sV
sM, cM := math.Sincos(M)
sN, cN := math.Sincos(N)
s2M, c2M := math.Sincos(2 * M)
s2N, c2N := math.Sincos(2 * N)
A := 1.915*p*sM + .02*p*s2M
B := 5.555*p*sN + .168*p*s2N
K := J + A - B
R := 1.00014 - .01671*cM - .00014*c2M
r := 5.20872 - .25208*cN - .00611*c2N
sK, cK := math.Sincos(K)
Δ := math.Sqrt(r*r + R*R - 2*r*R*cK)
ψ := math.Asin(R / Δ * sK)
dd := d - Δ/173
ω1 = 210.98*p + 877.8169088*p*dd + ψ - B
ω2 = 187.23*p + 870.1869088*p*dd + ψ - B
C := math.Sin(ψ / 2)
C *= C
if sK > 0 {
C = -C
}
ω1 = base.PMod(ω1+C, 2*math.Pi)
ω2 = base.PMod(ω2+C, 2*math.Pi)
λ := 34.35*p + .083091*p*d + .329*p*sV + B
DS = 3.12 * p * math.Sin(λ+42.8*p)
DE = DS - 2.22*p*math.Sin(ψ)*math.Cos(λ+22*p) -
1.3*p*(r-Δ)/Δ*math.Sin(λ-100.5*p)
return
}
// Topocentric3 returns topocentric hour angle and declination including parallax.
//
// This function implements the "alternative" method described in the text.
// The method should be similarly rigorous to that of Topocentric() and results
// should be virtually consistent.
func Topocentric3(α, δ, Δ, ρsφʹ, ρcφʹ, L, jde float64) (Hʹ, δʹ float64) {
π := Horizontal(Δ)
θ0 := base.Time(sidereal.Apparent(jde)).Rad()
H := base.PMod(θ0-L-α, 2*math.Pi)
sπ := math.Sin(π)
sH, cH := math.Sincos(H)
sδ, cδ := math.Sincos(δ)
A := cδ * sH
B := cδ*cH - ρcφʹ*sπ
C := sδ - ρsφʹ*sπ
q := math.Sqrt(A*A + B*B + C*C)
Hʹ = math.Atan2(A, B)
δʹ = math.Asin(C / q)
return
}
// E computes the "equation of time" for the given JDE.
//
// Parameter e must be a planetposition.V87Planet object for Earth obtained
// with planetposition.LoadPlanet.
//
// Result is equation of time as an hour angle in radians.
func E(jde float64, e *pp.V87Planet) float64 {
τ := base.J2000Century(jde) * .1
L0 := l0(τ)
// code duplicated from solar.ApparentEquatorialVSOP87 so that
// we can keep Δψ and cε
s, β, R := solar.TrueVSOP87(e, jde)
Δψ, Δε := nutation.Nutation(jde)
a := -20.4898 / 3600 * math.Pi / 180 / R
λ := s + Δψ + a
ε := nutation.MeanObliquity(jde) + Δε
sε, cε := math.Sincos(ε)
α, _ := coord.EclToEq(λ, β, sε, cε)
// (28.1) p. 183
E := L0 - .0057183*math.Pi/180 - α + Δψ*cε
return base.PMod(E+math.Pi, 2*math.Pi) - math.Pi
}
// PhaseAngle3 computes the phase angle of the Moon given a julian day.
//
// Less accurate than PhaseAngle functions taking coordinates.
//
// Result in radians.
func PhaseAngle3(jde float64) float64 {
T := base.J2000Century(jde)
const p = math.Pi / 180
D := base.Horner(T, 297.8501921*p, 445267.1114034*p,
-.0018819*p, p/545868, -p/113065000)
M := base.Horner(T, 357.5291092*p, 35999.0502909*p,
-.0001535*p, p/24490000)
Mʹ := base.Horner(T, 134.9633964*p, 477198.8675055*p,
.0087414*p, p/69699, -p/14712000)
return math.Pi - base.PMod(D, 2*math.Pi) +
-6.289*p*math.Sin(Mʹ) +
2.1*p*math.Sin(M) +
-1.274*p*math.Sin(2*D-Mʹ) +
-.658*p*math.Sin(2*D) +
-.214*p*math.Sin(2*Mʹ) +
-.11*p*math.Sin(D)
}
// Position returns geocentric location of the Moon.
//
// Results are referenced to mean equinox of date and do not include
// the effect of nutation.
//
// λ Geocentric longitude, in radians.
// β Geocentric latidude, in radians.
// Δ Distance between centers of the Earth and Moon, in km.
func Position(jde float64) (λ, β, Δ float64) {
T := base.J2000Century(jde)
Lʹ := base.Horner(T, 218.3164477*p, 481267.88123421*p,
-.0015786*p, p/538841, -p/65194000)
D, M, Mʹ, F := dmf(T)
A1 := 119.75*p + 131.849*p*T
A2 := 53.09*p + 479264.29*p*T
A3 := 313.45*p + 481266.484*p*T
E := base.Horner(T, 1, -.002516, -.0000074)
E2 := E * E
Σl := 3958*math.Sin(A1) + 1962*math.Sin(Lʹ-F) + 318*math.Sin(A2)
Σr := 0.
Σb := -2235*math.Sin(Lʹ) + 382*math.Sin(A3) + 175*math.Sin(A1-F) +
175*math.Sin(A1+F) + 127*math.Sin(Lʹ-Mʹ) - 115*math.Sin(Lʹ+Mʹ)
for i := range ta {
r := &ta[i]
sa, ca := math.Sincos(D*r.D + M*r.M + Mʹ*r.Mʹ + F*r.F)
switch r.M {
case 0:
Σl += r.Σl * sa
Σr += r.Σr * ca
case 1, -1:
Σl += r.Σl * sa * E
Σr += r.Σr * ca * E
case 2, -2:
Σl += r.Σl * sa * E2
Σr += r.Σr * ca * E2
}
}
for i := range tb {
r := &tb[i]
sb := math.Sin(D*r.D + M*r.M + Mʹ*r.Mʹ + F*r.F)
switch r.M {
case 0:
Σb += r.Σb * sb
case 1, -1:
Σb += r.Σb * sb * E
case 2, -2:
Σb += r.Σb * sb * E2
}
}
λ = base.PMod(Lʹ, 2*math.Pi) + Σl*1e-6*p
β = Σb * 1e-6 * p
Δ = 385000.56 + Σr*1e-3
return
}
// Position2000 returns ecliptic position of planets by full VSOP87 theory.
//
// Argument jde is the date for which positions are desired.
//
// Results are for the dynamical equinox and ecliptic J2000.
//
// L is heliocentric longitude in radians.
// B is heliocentric latitude in radians.
// R is heliocentric range in AU.
func (vt *V87Planet) Position2000(jde float64) (L, B, R float64) {
T := base.J2000Century(jde)
τ := T * .1
cf := make([]float64, 6)
sum := func(series coeff) float64 {
for x, terms := range series {
cf[x] = 0
// sum terms in reverse order to preserve accuracy
for y := len(terms) - 1; y >= 0; y-- {
term := &terms[y]
cf[x] += term.a * math.Cos(term.b+term.c*τ)
}
}
return base.Horner(τ, cf[:len(series)]...)
}
L = base.PMod(sum(vt.l), 2*math.Pi)
B = sum(vt.b)
R = sum(vt.r)
return
}
func TestTopocentric3(t *testing.T) {
// same test case as example 40.a, p. 280
α := 339.530208 * math.Pi / 180
δ := -15.771083 * math.Pi / 180
Δ := .37276
ρsφʹ := .546861
ρcφʹ := .836339
L := base.NewHourAngle(false, 7, 47, 27).Rad()
jde := julian.CalendarGregorianToJD(2003, 8, 28+(3+17./60)/24)
// reference result
αʹ, δʹ1 := parallax.Topocentric(α, δ, Δ, ρsφʹ, ρcφʹ, L, jde)
// result to test
Hʹ, δʹ3 := parallax.Topocentric3(α, δ, Δ, ρsφʹ, ρcφʹ, L, jde)
// test
θ0 := base.Time(sidereal.Apparent(jde)).Rad()
if math.Abs(base.PMod(Hʹ-(θ0-L-αʹ)+1, 2*math.Pi)-1) > 1e-15 {
t.Fatal(Hʹ, θ0-L-αʹ)
}
if math.Abs(δʹ3-δʹ1) > 1e-15 {
t.Fatal(δʹ3, δʹ1)
}
}
// Kepler3 solves Kepler's equation by binary search.
//
// Argument e is eccentricity, M is mean anomoly in radians.
//
// Result E is eccentric anomaly in radians.
func Kepler3(e, M float64) (E float64) {
// adapted from BASIC, p. 206
M = base.PMod(M, 2*math.Pi)
f := 1
if M > math.Pi {
f = -1
M = 2*math.Pi - M
}
E0 := math.Pi * .5
d := math.Pi * .25
for i := 0; i < 53; i++ {
M1 := E0 - e*math.Sin(E0)
if M-M1 < 0 {
E0 -= d
} else {
E0 += d
}
d *= .5
}
if f < 0 {
return -E0
}
return E0
}
// Ephemeris returns the apparent orientation of the sun at the given jd.
//
// Results:
// P: Position angle of the solar north pole.
// B0: Heliographic latitude of the center of the solar disk.
// L0: Heliographic longitude of the center of the solar disk.
//
// All results in radians.
func Ephemeris(jd float64, e *pp.V87Planet) (P, B0, L0 float64) {
θ := (jd - 2398220) * 2 * math.Pi / 25.38
I := 7.25 * math.Pi / 180
K := 73.6667*math.Pi/180 +
1.3958333*math.Pi/180*(jd-2396758)/base.JulianCentury
L, _, R := solar.TrueVSOP87(e, jd)
Δψ, Δε := nutation.Nutation(jd)
ε0 := nutation.MeanObliquity(jd)
ε := ε0 + Δε
λ := L - 20.4898/3600*math.Pi/180/R
λp := λ + Δψ
sλK, cλK := math.Sincos(λ - K)
sI, cI := math.Sincos(I)
tx := -math.Cos(λp) * math.Tan(ε)
ty := -cλK * math.Tan(I)
P = math.Atan(tx) + math.Atan(ty)
B0 = math.Asin(sλK * sI)
η := math.Atan2(-sλK*cI, -cλK)
L0 = base.PMod(η-θ, 2*math.Pi)
return
}
// Apparent returns apparent sidereal time at Greenwich for the given JD.
//
// Apparent is mean plus the nutation in right ascension.
//
// The result is in seconds of time and is in the range [0,86400).
func Apparent(jd float64) float64 {
s := mean(jd) // seconds of time
n := nutation.NutationInRA(jd) // angle (radians) of RA
ns := n * 3600 * 180 / math.Pi / 15 // convert RA to time in seconds
return base.PMod(s+ns, 86400)
}
// Mean0UT returns mean sidereal time at Greenwich at 0h UT on the given JD.
//
// The result is in seconds of time and is in the range [0,86400).
func Mean0UT(jd float64) float64 {
s, _ := mean0UT(jd)
return base.PMod(s, 86400)
}
