Module conversions 

Time scale conversions between astronomical time systems.

This module provides traits and implementations for converting between the eight major astronomical time scales: GPS, TAI, TT, TCG, TCB, TDB, UT1, and UTC.

Time Scale Overview

Scale	Full Name	Basis	Primary Use
UTC	Coordinated Universal Time	Atomic + leap seconds	Civil timekeeping
TAI	International Atomic Time	Atomic clocks	Reference for other scales
TT	Terrestrial Time	TAI + 32.184s	Geocentric ephemerides
UT1	Universal Time 1	Earth rotation	Sidereal time, telescope pointing
GPS	GPS Time	Atomic (no leap seconds)	Satellite navigation
TCG	Geocentric Coordinate Time	Relativistic (Earth center)	Precise geocentric dynamics
TCB	Barycentric Coordinate Time	Relativistic (solar system)	Solar system dynamics
TDB	Barycentric Dynamical Time	TCB rescaled	Solar system ephemerides

Fixed vs Variable Offsets

Some conversions use constant offsets:

• TAI <-> TT: Fixed 32.184 seconds
• TAI <-> GPS: Fixed 19.0 seconds
• TT <-> TCG: Secular rate 6.969290134e-10 (IAU 2000)
• TCG <-> TCB: Secular rate 1.550519768e-8 (IAU 2006)

Others require external data that changes over time:

• UTC <-> TAI: Leap second table (currently 37 seconds as of 2017)
• UT1 <-> TAI: IERS Earth Orientation Parameters (changes daily)
• UT1 <-> TT: Delta-T from historical tables or predictions
• TT <-> TDB: Location-dependent, ~1.66ms annual oscillation

Trait Pattern

Each target scale has a conversion trait:

• ToTAI: Convert to International Atomic Time
• ToTT: Convert to Terrestrial Time
• ToGPS: Convert to GPS Time
• ToUTC: Convert to Coordinated Universal Time
• ToUT1: Convert to Universal Time 1
• ToTCG: Convert to Geocentric Coordinate Time

Additional traits handle conversions requiring parameters:

• ToUT1WithOffset, ToTAIWithOffset: UT1 <-> TAI with IERS offset
• ToTTWithDeltaT, ToUT1WithDeltaT: UT1 <-> TT with historical Delta-T
• ToTDB, ToTTFromTDB: TT <-> TDB with observer location
• ToTCB, ToTCGFromTCB: TCG <-> TCB relativistic conversions

Usage

Simple fixed-offset conversions work directly:

use celestial_time::scales::{TAI, TT, GPS};
use celestial_time::scales::conversions::{ToTAI, ToTT};
use celestial_time::julian::JulianDate;

let tai = TAI::from_julian_date(JulianDate::new(2451545.0, 0.0));
let tt = tai.to_tt().unwrap();   // TAI + 32.184s

Conversions requiring external data take parameters:

use celestial_time::scales::{TAI, UT1};
use celestial_time::scales::conversions::{ToUT1WithOffset, ToTAIWithOffset};
use celestial_time::julian::JulianDate;

// UT1-TAI offset from IERS Bulletin A
let ut1_tai_offset = -37.0;  // seconds

let tai = TAI::from_julian_date(JulianDate::new(2451545.0, 0.0));
let ut1 = tai.to_ut1_with_offset(ut1_tai_offset).unwrap();
let back = ut1.to_tai_with_offset(ut1_tai_offset).unwrap();

Precision Notes

All conversions preserve precision by applying offsets to the smaller-magnitude component of the two-part Julian Date. Round-trip conversions maintain sub-nanosecond accuracy for fixed-offset scales.

Re-exports

pub use tcb_tdb::*;

pub use tcg_tcb::*;

pub use tt_tdb::*;

pub use ut1_tai::*;

pub use utc_tai::*;

pub use utc_ut1::*;

Modules

gps_tai

GPS and TAI time scale conversions.

tai_tt

Conversions between TAI, TT, and TCG time scales.

tcb_tdb

Conversions between Barycentric Coordinate Time (TCB) and Barycentric Dynamical Time (TDB).

tcg_tcb

Conversions between Geocentric Coordinate Time (TCG) and Barycentric Coordinate Time (TCB).

tt_tcg

Conversions between Terrestrial Time (TT) and Geocentric Coordinate Time (TCG).

tt_tdb

TT (Terrestrial Time) and TDB (Barycentric Dynamical Time) conversions.

ut1_tai

Conversions between UT1, TAI, and TT time scales.

utc_tai

Conversions between Coordinated Universal Time (UTC) and International Atomic Time (TAI).

utc_ut1

Conversions between Coordinated Universal Time (UTC) and Universal Time (UT1).

Traits

ToGPS

Convert a time scale to GPS Time.

ToTAI

Convert a time scale to International Atomic Time (TAI).

ToTCG

Convert a time scale to Geocentric Coordinate Time (TCG).

ToTT

Convert a time scale to Terrestrial Time (TT).

ToUT1

Convert a time scale to Universal Time 1 (UT1).

ToUTC

Convert a time scale to Coordinated Universal Time (UTC).