[rhannequin]

Adds two layers that draw the Sun and the planets where they are in the sky:

ephem = Astronoby::Ephem.load("inpop19a.bsp")

chart.render(
  Skymap::Layers::Stars.new(stars: stars),
  Skymap::Layers::Sun.new(ephem: ephem),
  Skymap::Layers::Planets.new(ephem: ephem),
  Skymap::Layers::Moon.new(ephem: ephem)
)

Like the Moon, they need an ephemeris, and they are left out when below the horizon.

• Planets draws Mercury to Neptune as a star would be, using the apparent magnitude from Astronoby, with the same sizes as the stars and a warm tint. It has a magnitude_limit: that defaults to 5 as for the stars, so Uranus (5.7) and Neptune (7.7) are out unless it is raised. Planets are not labeled yet
• Sun is a disc at 3% of the radius of the sky, the same size as the Moon, as the Sun and the Moon look about the same size from the ground
• StarSize.for takes the sizing of the stars out of the Stars layer, so both layers share it. Stars are sized exactly as before
• The ephemeris of the tests keeps the barycenters of Mercury to Neptune, and is renamed de440s_2025_2030_excerpt.bsp since it is not only about the Moon anymore. It goes from 508 KB to 710 KB, and is cut with ruby-ephem excerpt from DE440s. While at it, its comment area now follows the DAF layout (1,000 characters per record), which the previous one did not, so tools that read the comments now show it correctly
• The map of the README now shows Saturn, the only planet above the horizon on September 24, 2026 at 22:00 UTC. The Sun is below the horizon

[Copilot]

🟡 Changes recommended

Bright magnitude cutoffs reverse the planet-sizing interval, incorrectly shrinking all accepted planets to the minimum radius.

1 open issue

• Prevent negative sizing interval for bright magnitude cutoffs · New

What changed in this PR

Adds Sun and planet layers to Skymap’s existing sky-map rendering, using ephemerides to position bodies and omit those below the horizon.

Changes:

• Adds Sun discs and magnitude-sized, warm-tinted planets.
• Shares sizing logic between stars and planets without changing existing star sizing.
• Adds rendering tests and updates documentation, the sample map, and the test ephemeris reference.

File	Description
spec/​support/​astronomy_helpers.rb	Uses the renamed ephemeris fixture.
spec/​skymap/​star_size_spec.rb	Tests the shared sizing factory.
spec/​skymap/​renderer/​svg_spec.rb	Tests Sun and planet SVG output.
spec/​skymap/​layers/​sun_spec.rb	Tests Sun placement, sizing, and visibility.
spec/​skymap/​layers/​planets_spec.rb	Tests planet placement, filtering, and sizing.
sky_map.svg	Adds Saturn to the sample map.
README.md	Documents the new layers and updated fixture.
lib/​skymap/​star_size.rb	Extracts shared sizing defaults and factory.
lib/​skymap/​renderer/​svg.rb	Draws Sun and planet elements.
lib/​skymap/​renderer/​sun.rb	Defines the Sun render element.
lib/​skymap/​renderer/​planet.rb	Defines the planet render element.
lib/​skymap/​layers/​sun.rb	Projects and sizes the visible Sun.
lib/​skymap/​layers/​stars.rb	Uses the shared sizing factory.
lib/​skymap/​layers/​planets.rb	Projects, filters, and sizes planets.
lib/​skymap.rb	Loads the new layers and render elements.
CHANGELOG.md	Records the Sun and planet features.

_{Review effort: Balanced}