Add Astronoby::Duration

[rhannequin]

Durations show up in the library here and there, but until now they were handed back to the caller as raw integers of seconds. Every other physical quantity Astronoby returns already has its own value object (Angle, Distance, Velocity, AngularVelocity), so a length of time coming back as a bare Integer was the odd one out.

This introduces a new value object for handling durations: Astronoby::Duration.

It can be initialized with seconds, minutes, hours and days, while the duration is always stored in seconds, the international unit for time. A reader is available for each unit (#seconds, #minutes, #hours, #days), so the call site can ask for whatever unit reads best.

As a regular value object, it is immutable, handles comparison with other Astronoby::Duration objects, and can be used in some arithmetic operations with the + and - operators. It also has #abs, #positive?, #negative? and #zero?, which are handy for signed durations like the equation of time.

In the library, the two places that returned a length of time have been converted to this new value object, with breaking changes:

• Astronoby::EclipsePhase#duration now returns a Astronoby::Duration
• Astronoby::Sun#equation_of_time now returns a Astronoby::Duration

Both used to return an Integer of seconds, so a caller relying on that now needs .seconds to get the same value back.

I deliberately kept it to these two. The orbital, synodic and lunar periods, the Delta T offset and the various time-scale conversions are durations conceptually too, but they are internal constants and implementation details that get divided and reciprocated in the math, never handed back to a user. Wrapping them would only add ceremony in hot loops, so they stay raw numbers.

one_minute = Astronoby::Duration.from_minutes(1)
ninety_seconds = Astronoby::Duration.from_seconds(90)

one_minute < ninety_seconds
# => true

sum = one_minute + ninety_seconds
sum.seconds
# => 150

sum.minutes
# => 2.5

And how it reads at the call sites now:

eclipse.total.duration.minutes
# => 65.5

sun.equation_of_time.seconds
# => -185