A supervisor for the time zone lookup Astro uses to report rise and set times in the local time zone of a location.

When :tz_world is a dependency and nothing else in the application starts its backend, add this supervisor to the application's supervision tree:

children = [
  Astro.Supervisor
]

Supervisor.start_link(children, strategy: :one_for_one)

An application can instead add TzWorld.Backend.SpatialIndex to its own tree, as the README describes.

Supervising Astro yourself

The :astro application loads the JPL ephemeris as it starts. An application that prefers to start Astro alongside its own processes can mark the dependency runtime: false, so that the :astro application is not started, and list this supervisor in its own tree, which then loads the ephemeris before it starts:

# mix.exs
def deps do
  [
    {:astro, "~> 2.9", runtime: false}
  ]
end

# lib/my_app/application.ex
children = [
  Astro.Supervisor,
  MyApp.Worker
]

List it before any process that asks Astro for a position as it starts. A release must then include Astro's code, as runtime: false leaves it out of the application's own:

# mix.exs
def project do
  [
    releases: [my_app: [applications: [astro: :load]]]
  ]
end

Where the :astro application is started as well, the ephemeris it has loaded is used and not loaded again.