Prepare v0.0.2

This adds documentation for the first proper version of the gem.

Author: rhannequin

CHANGELOG.md

@@ -1,5 +1,34 @@
 # Changelog
 
+## 0.0.2 - 2026-07-14
+
+The first functional release. It ships the numeric core and the two value
+objects the rest of the library is built on: `Instant` and `Duration`. The
+scale conversions are not here yet, so an instant is built directly from a
+TAI Julian Date for now.
+
+### Features
+
+- Add `Numeric::TwoPartFloat`, a number kept as a high and a low `Float` for
+  about twice the precision of one, with Shewchuk error-free arithmetic
+- Add `Numeric::Exact`, a value kept as an exact `Rational`, with no rounding
+- Add the precision contract: every value carries a precision, `:standard` or
+  `:exact`, set when it is built and never changed. Mixing the two promotes
+  the result to `:exact` instead of dropping to `:standard`
+- Add `Horologium.configure` for the set-once default precision, and
+  `Horologium.with_precision` for a scoped, per-fiber override
+- Add `Instant`, a frozen point on the TAI timeline, built with
+  `Instant.from_tai_julian_date`
+- Add `Duration`, a frozen span in SI seconds, built with `Duration.seconds`,
+  `Duration.days`, and `Duration.nanoseconds`
+- Add instant and duration arithmetic: shift an instant by a duration, and
+  subtract two instants to measure the duration between them
+- Add `Instant#equal_within?` for comparison inside a tolerance
+- Guard against meaningless operations: adding two instants raises
+  `DimensionalError`
+
+**Full Changelog**: https://github.com/rhannequin/horologium/compare/v0.0.1...v0.0.2
+
 ## 0.0.1 - 2026-07-06
 
 - Gem creation

README.md

@@ -7,20 +7,128 @@ Horologium is a Ruby library dedicated to **scientific time**: the time scales
 intervals, and rigorous conversions between scales that astronomy and physics
 require.
 
+Ruby already has `Time`, `Date`, `DateTime`, and `ActiveSupport` for civil time:
+time zones, calendars, human formatting. None of them knows the difference
+between UTC and a continuous scale, the TAI, TT, and TDB scales an ephemeris
+needs, or a Julian Date kept precise to the nanosecond. That is the gap
+Horologium fills.
+
+## Content
+
+- [Installation](#installation)
+- [Usage](#usage)
+- [Precision](#precision)
+- [Status](#status)
+- [Development](#development)
+- [Contributing](#contributing)
+- [License](#license)
+- [Code of Conduct](#code-of-conduct)
+
 ## Installation
 
-Install the gem and add it to the application's Gemfile by running:
+Install the gem and add it to the application's Gemfile by executing:
+
+    $ bundle add horologium
+
+If [Bundler] is not being used to manage dependencies, install the gem by
+executing:
+
+    $ gem install horologium
+
+## Usage
+
+An `Instant` is a single point on the timeline, kept internally as a TAI Julian
+Date. A `Duration` is an amount of time in SI seconds, with no date and no scale
+attached. You shift an instant by a duration, and you subtract two instants to
+get the duration between them.
+
+```rb
+require "horologium"
+
+instant = Horologium::Instant.from_tai_julian_date(2_460_000.5)
+
+later = instant + Horologium::Duration.days(1)
+later == Horologium::Instant.from_tai_julian_date(2_460_001.5)  # => true
+instant < later                                                 # => true
+
+b = Horologium::Instant.from_tai_julian_date(2_460_001.5)
+b - instant == Horologium::Duration.days(1)                     # => true
+```
+
+A `Duration` counts SI seconds, so `Duration.days(1)` is always 86,400 SI
+seconds. Because of leap seconds a civil day can be a second longer or shorter,
+so a duration and a calendar day are different things.
+
+```rb
+Horologium::Duration.days(1) == Horologium::Duration.seconds(86_400)  # => true
+Horologium::Duration.nanoseconds(1_000_000_000) ==
+  Horologium::Duration.seconds(1)                                     # => true
+```
+
+Adding a duration to an instant makes sense, but adding two instants together
+does not, so it raises an error.
 
-```sh
-bundle add horologium
+```rb
+instant + instant  # => raises Horologium::DimensionalError
 ```
 
-Or install it directly:
+Exact equality is rarely what scientific code wants, so you can compare within a
+tolerance:
 
-```sh
-gem install horologium
+```rb
+a = Horologium::Instant.from_tai_julian_date(2_460_000.5)
+near = a + Horologium::Duration.nanoseconds(1)
+
+a.equal_within?(near, Horologium::Duration.nanoseconds(2))  # => true
+```
+
+## Precision
+
+A modern Julian Date is around 2.46 million. A single `Float` spends most of its
+digits on that large number and has only tens of microseconds left for the
+fraction of a day. That is too coarse for scientific time. Horologium stores an
+instant across two `Float`s whose sum is the Julian Date, so the second one
+starts where the first runs out of digits. This is the representation [ERFA]
+uses, it keeps the precision below a nanosecond for any date, and it does so
+with ordinary floating-point arithmetic.
+
+Every value carries one of two precisions, fixed when it is built:
+
+- `:standard`, the default, keeps the value as a two-part float. It is fast and
+  stays within a few nanoseconds of the true value.
+- `:exact` keeps the value as a `Rational`, with no rounding. The test suite
+  uses it to check that `:standard` stays within its stated precision.
+
+Set the default once at boot:
+
+```rb
+Horologium.configure do |c|
+  c.default_precision = :exact
+end
 ```
 
+Choose it for a single value, or for a scoped block:
+
+```rb
+Horologium::Instant.from_tai_julian_date(2_460_000.5, precision: :exact)
+
+Horologium.with_precision(:exact) do
+  # instants and durations built here default to :exact
+end
+```
+
+Exactness is contagious. An operation between two `:standard` values stays
+`:standard`. Mixing a `:standard` and an `:exact` value gives an `:exact`
+result, so precision is not quietly lost. `:exact` guarantees the arithmetic
+Horologium performs. It cannot bring back precision that an input already lost
+when it was built.
+
+## Status
+
+This library is in early development, before its first public release. The
+public API is not stable, so new versions will probably introduce breaking
+changes until a 1.0 release. Changes are documented in the [CHANGELOG].
+
 ## Development
 
 After checking out the repo, run `bin/setup` to install dependencies. Then, run
@@ -53,6 +161,9 @@ The gem is available as open source under the terms of the [MIT License].
 Everyone interacting in the Horologium project's codebases, issue trackers, chat
 rooms and mailing lists is expected to follow the [code of conduct].
 
+[Bundler]: https://bundler.io
+[ERFA]: https://github.com/liberfa/erfa
+[CHANGELOG]: https://github.com/rhannequin/horologium/blob/main/CHANGELOG.md
 [rubygems.org]: https://rubygems.org
 [MIT License]: https://opensource.org/licenses/MIT
 [code of conduct]: https://github.com/rhannequin/horologium/blob/main/CODE_OF_CONDUCT.md