Introduce the TDB scale

[rhannequin]

This adds Barycentric Dynamical Time, the scale the planetary ephemerides are written in. It is the third Astronoby-critical scale, after TAI and TT, and the first one that comes from a model rather than a constant offset.

instant = Horologium::Instant.from_julian_date(2_460_796.5, scale: :tt)

instant.to(:tdb).as(:julian_date)
# => 2460796.5000000168

TDB keeps almost the rate of TT, apart from periodic relativistic terms worth at most about two milliseconds, from the Earth's motion through the Sun's gravity. Scales::TDB reads TAI in TT first, a fixed 32.184 seconds, then adds the TDB - TT difference Data::BarycentricModel gives.

Data::BarycentricModel is the full Fairhead & Bretagnon (1990) series that ERFA and astropy use, ported from ERFA's dtdb.c. The value is geocentric: the part of the model that depends on where on the Earth the observer stands is a few microseconds, and is left out. It agrees with pyerfa to about a nanosecond.

The difference is a floating-point model, so unlike TAI and TT this edge is not exact, even at :exact. The model reads the date as a Float; at :exact its result is stored faithfully as a Rational, but the model itself stays a float computation. :exact keeps the arithmetic that follows it, not the model.

UTC and the civil representations come next.

[Copilot]

Pull request overview

This pull request adds support for the TDB (Barycentric Dynamical Time) time scale to Horologium, including a port of the Fairhead & Bretagnon (1990) TDB−TT series (as used by ERFA/astropy) and wiring it into the scale registry so instants can be converted to/from :tdb.

Changes:

• Add Horologium::Data::BarycentricModel implementing the Fairhead & Bretagnon series for TDB - TT.
• Add Horologium::Scales::TDB and register it as a built-in scale, including top-level requires.
• Add test coverage and RBS signatures for the new model and scale.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/test_configuration.rb	Updates expectations for registered scale lists to include :tdb.
test/scales/test_tdb.rb	Adds scale-level tests for registration, conversion behavior, and precision handling.
test/data/test_barycentric_model.rb	Adds model-level tests comparing tdb_minus_tt against reference values and bounds.
sig/horologium/scales/tdb.rbs	Adds RBS for the new Scales::TDB API surface.
sig/horologium/data/barycentric_model.rbs	Adds RBS for the new barycentric model module and helpers/constants.
lib/horologium/scales/tdb.rb	Implements the TDB scale conversion via TT plus model correction.
lib/horologium/data/barycentric_model.rb	Implements the full Fairhead & Bretagnon (1990) series ported from ERFA.
lib/horologium/configuration.rb	Registers :tdb as a built-in scale.
lib/horologium.rb	Requires the new model and scale files for library initialization.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.