deltat

Documentation

Overview

DeltaT: Chapter 10, Dynamical Time and Universal Time.

Functions in this package compute ΔT for various ranges of dates.

ΔT = TD - UT.

TD = "Dynamical Time", which is related to:

ET "Ephermis Time", an older term.
TDB "Barycentric Dynamical Time", very close to TD.
TDT "Terrestrial Dynamical Time", a more correct term.
TT "Terrestrial Time", a newer and more correct term.

UT = "Universal Time", which is related (if ambiguously) to GMT "Greenwich Mean Time".

The return value for all functions is ΔT in seconds.

Index

• func Interp10A(jde float64) (ΔT unit.Time)
• func Poly1800to1899(jde float64) (ΔT unit.Time)
• func Poly1800to1997(jde float64) (ΔT unit.Time)
• func Poly1900to1997(jde float64) (ΔT unit.Time)
• func Poly948to1600(year float64) (ΔT unit.Time)
• func PolyAfter2000(year float64) (ΔT unit.Time)
• func PolyBefore948(year float64) (ΔT unit.Time)

Examples

• Interp10A
• Poly1900to1997
• PolyBefore948

Functions

func Interp10A

func Interp10A(jde float64) (ΔT unit.Time)

Interp10A returns ΔT at a date, accurate from years 1620 to 2010.

Example

package main

import (
	"fmt"

	"github.com/soniakeys/meeus/v3/deltat"
	"github.com/soniakeys/meeus/v3/julian"
)

func main() {
	// Example 10.a, p. 78.
	dt := deltat.Interp10A(julian.CalendarGregorianToJD(1977, 2, 18))
	fmt.Printf("%+.1f seconds\n", dt)
}

Output:

+47.6 seconds

func Poly1800to1899

func Poly1800to1899(jde float64) (ΔT unit.Time)

Poly1800to1899 returns a polynomial approximation of ΔT valid for years 1800 to 1899.

The accuracy is within 0.9 seconds.

func Poly1800to1997

func Poly1800to1997(jde float64) (ΔT unit.Time)

Poly1800to1997 returns a polynomial approximation of ΔT valid for years 1800 to 1997.

The accuracy is within 2.3 seconds.

func Poly1900to1997

func Poly1900to1997(jde float64) (ΔT unit.Time)

Poly1900to1997 returns a polynomial approximation of ΔT valid for years 1900 to 1997.

The accuracy is within 0.9 seconds.

Example

package main

import (
	"fmt"
	"time"

	"github.com/soniakeys/meeus/v3/base"
	"github.com/soniakeys/meeus/v3/deltat"
	"github.com/soniakeys/meeus/v3/julian"
)

func main() {
	// Example 10.a, p. 78.
	jd := julian.TimeToJD(time.Date(1977, 2, 18, 3, 37, 40, 0, time.UTC))
	year := base.JDEToJulianYear(jd)
	fmt.Printf("julian year %.1f\n", year)
	fmt.Printf("%+.1f seconds\n", deltat.Poly1900to1997(jd))
}

Output:

julian year 1977.1
+47.1 seconds

func Poly948to1600

func Poly948to1600(year float64) (ΔT unit.Time)

Poly948to1600 returns a polynomial approximation of ΔT valid for calendar years 948 to 1600.

func PolyAfter2000

func PolyAfter2000(year float64) (ΔT unit.Time)

PolyAfter2000 returns a polynomial approximation of ΔT valid for calendar years after 2000.

func PolyBefore948

func PolyBefore948(year float64) (ΔT unit.Time)

PolyBefore948 returns a polynomial approximation of ΔT valid for calendar years before 948.

Example

// Example 10.b, p. 80.
ΔT := deltat.PolyBefore948(333.1)
UT := unit.TimeFromHour(6)
TD := UT + ΔT
fmt.Printf("%+.0f seconds\n", ΔT)
fmt.Printf("333 February 6 at %m TD", sexa.FmtTime(TD))

Output:

+6146 seconds
333 February 6 at 7ʰ42ᵐ TD