// 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 = unit.PMod(a.M0+k*a.M1, 360) * math.Pi / 180
T = base.J2000Century(J)
return
}
// Section "Trigonometric functions of large angles":
//
// A non integer example better illustrates that reduction to a range
// does not rescue precision.
func Example_pMod_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
// significant 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(unit.PMod(W, 360)*math.Pi/180))
fmt.Println("PMod rad:", math.Sin(unit.PMod(W*math.Pi/180, 2*math.Pi)))
// Output:
// Direct: 0.04290970350270464
// Reduced: 0.04290970350923273
// PMod deg: 0.04290970351307828
// PMod rad: 0.04290970350643808
}
// 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 Example_pMod_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 unit.PMod on integer constants produces results identical to
// above.
fmt.Println("PMod int: ", math.Sin(unit.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(unit.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
}