Add Geometric and Astrometric positions for Sun and Moon (#135)

This is the start of the transition for `Sun` and `Moon` from formula-based to ephemeris-based.

It gives the ability to `Sun` and `Moon` to be initialized from a given `Instant` and `Ephem`.

`Planet` is renamed into `SolarSystemBody` so that `Sun` and `Moon` can inherit from it while still making sense.

Thanks to `SolarSystemBody`, `Sun` and `Moon` have Geometric and Astrometric positions, with each equatorial and ecliptic coordinates.

```rb
instant = Astronoby::Instant.from_time(Time.utc(2025, 2, 7))
ephem = Astronoby::Ephem.load("de440s.bsp")


sun = Astronoby::Sun.new(instant: instant, ephem: ephem)

sun.geometric.equatorial.right_ascension
sun.geometric.equatorial.declination
sun.geometric.ecliptic.latitude
sun.geometric.ecliptic.longitude
sun.geometric.distance
sun.astrometric.equatorial.right_ascension
sun.astrometric.equatorial.declination
sun.astrometric.ecliptic.latitude
sun.astrometric.ecliptic.longitude
sun.astrometric.distance


moon = Astronoby::Moon.new(instant: instant, ephem: ephem)

moon.geometric.equatorial.right_ascension
moon.geometric.equatorial.declination
moon.geometric.ecliptic.latitude
moon.geometric.ecliptic.longitude
moon.geometric.distance
moon.astrometric.equatorial.right_ascension
moon.astrometric.equatorial.declination
moon.astrometric.ecliptic.latitude
moon.astrometric.ecliptic.longitude
moon.astrometric.distance
```

lib/astronoby.rb

@@ -12,9 +12,9 @@
 require "astronoby/velocity"
 require "astronoby/astronomical_models/ephemeride_lunaire_parisienne"
 require "astronoby/astronomical_models/moon_phases_periodic_terms"
+require "astronoby/bodies/solar_system_body"
 require "astronoby/bodies/moon"
 require "astronoby/bodies/sun"
-require "astronoby/bodies/planet"
 require "astronoby/bodies/mercury"
 require "astronoby/bodies/venus"
 require "astronoby/bodies/earth"

lib/astronoby/bodies/earth.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Astronoby
-  class Earth < Planet
+  class Earth < SolarSystemBody
     def self.ephemeris_segments
       [
         [SOLAR_SYSTEM_BARYCENTER, EARTH_MOON_BARYCENTER],

lib/astronoby/bodies/jupiter.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Astronoby
-  class Jupiter < Planet
+  class Jupiter < SolarSystemBody
     def self.ephemeris_segments
       [[SOLAR_SYSTEM_BARYCENTER, JUPITER_BARYCENTER]]
     end

lib/astronoby/bodies/mars.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Astronoby
-  class Mars < Planet
+  class Mars < SolarSystemBody
     def self.ephemeris_segments
       [[SOLAR_SYSTEM_BARYCENTER, MARS_BARYCENTER]]
     end

lib/astronoby/bodies/mercury.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Astronoby
-  class Mercury < Planet
+  class Mercury < SolarSystemBody
     def self.ephemeris_segments
       [[SOLAR_SYSTEM_BARYCENTER, MERCURY_BARYCENTER]]
     end

lib/astronoby/bodies/moon.rb

@@ -1,10 +1,17 @@
 # frozen_string_literal: true
 
 module Astronoby
-  class Moon
+  class Moon < SolarSystemBody
     SEMIDIAMETER_VARIATION = 0.7275
     MEAN_GEOCENTRIC_DISTANCE = Astronoby::Distance.from_meters(385_000_560)
 
+    def self.ephemeris_segments
+      [
+        [SOLAR_SYSTEM_BARYCENTER, EARTH_MOON_BARYCENTER],
+        [EARTH_MOON_BARYCENTER, MOON]
+      ]
+    end
+
     # Source:
     #  Title: Astronomical Algorithms
     #  Author: Jean Meeus
@@ -18,8 +25,13 @@ def self.monthly_phase_events(year:, month:)
       Events::MoonPhases.phases_for(year: year, month: month)
     end
 
-    def initialize(time:)
+    def initialize(time: nil, instant: nil, ephem: nil)
       @time = time
+      @instant = instant
+      @ephem = ephem
+      unless @instant.nil? || @ephem.nil?
+        super(instant: instant, ephem: ephem)
+      end
     end
 
     # @return [Astronoby::Coordinates::Ecliptic] Apparent ecliptic coordinates

lib/astronoby/bodies/neptune.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Astronoby
-  class Neptune < Planet
+  class Neptune < SolarSystemBody
     def self.ephemeris_segments
       [[SOLAR_SYSTEM_BARYCENTER, NEPTUNE_BARYCENTER]]
     end

lib/astronoby/bodies/saturn.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Astronoby
-  class Saturn < Planet
+  class Saturn < SolarSystemBody
     def self.ephemeris_segments
       [[SOLAR_SYSTEM_BARYCENTER, SATURN_BARYCENTER]]
     end

lib/astronoby/bodies/planet.rb → lib/astronoby/bodies/solar_system_body.rb

@@ -1,14 +1,16 @@
 # frozen_string_literal: true
 
 module Astronoby
-  class Planet
+  class SolarSystemBody
     SOLAR_SYSTEM_BARYCENTER = 0
+    SUN = 10
     MERCURY_BARYCENTER = 1
     MERCURY = 199
     VENUS_BARYCENTER = 2
     VENUS = 299
     EARTH_MOON_BARYCENTER = 3
     EARTH = 399
+    MOON = 301
     MARS_BARYCENTER = 4
     JUPITER_BARYCENTER = 5
     SATURN_BARYCENTER = 6

lib/astronoby/bodies/sun.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Astronoby
-  class Sun
+  class Sun < SolarSystemBody
     SEMI_MAJOR_AXIS_IN_METERS = 149_598_500_000
     ANGULAR_DIAMETER = Angle.from_degrees(0.533128)
     INTERPOLATION_FACTOR = 24.07
@@ -25,6 +25,10 @@ class Sun
 
     attr_reader :time
 
+    def self.ephemeris_segments
+      [[SOLAR_SYSTEM_BARYCENTER, SUN]]
+    end
+
     # Source:
     #  Title: Astronomical Algorithms
     #  Author: Jean Meeus
@@ -70,8 +74,13 @@ def self.equation_of_time(date_or_time:)
     #  Chapter: 6 - The Sun
 
     # @param time [Time] Considered time
-    def initialize(time:)
+    def initialize(time: nil, instant: nil, ephem: nil)
       @time = time
+      @instant = instant
+      @ephem = ephem
+      unless @instant.nil? || @ephem.nil?
+        super(instant: instant, ephem: ephem)
+      end
     end
 
     def epoch

lib/astronoby/bodies/uranus.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Astronoby
-  class Uranus < Planet
+  class Uranus < SolarSystemBody
     def self.ephemeris_segments
       [[SOLAR_SYSTEM_BARYCENTER, URANUS_BARYCENTER]]
     end

lib/astronoby/bodies/venus.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Astronoby
-  class Venus < Planet
+  class Venus < SolarSystemBody
     def self.ephemeris_segments
       [[SOLAR_SYSTEM_BARYCENTER, VENUS_BARYCENTER]]
     end

lib/astronoby/reference_frame.rb

@@ -23,21 +23,27 @@ def initialize(
     end
 
     def equatorial
-      return Coordinates::Equatorial.zero if distance.zero?
+      @equatorial ||= begin
+        return Coordinates::Equatorial.zero if distance.zero?
 
-      Coordinates::Equatorial.from_position_vector(@position)
+        Coordinates::Equatorial.from_position_vector(@position)
+      end
     end
 
     def ecliptic
-      return Coordinates::Ecliptic.zero if distance.zero?
+      @ecliptic ||= begin
+        return Coordinates::Ecliptic.zero if distance.zero?
 
-      equatorial.to_ecliptic(epoch: Epoch::J2000)
+        equatorial.to_ecliptic(epoch: Epoch::J2000)
+      end
     end
 
     def distance
-      return Distance.zero if @position.zero?
+      @distance ||= begin
+        return Distance.zero if @position.zero?
 
-      Astronoby::Distance.from_meters(@position.magnitude)
+        Astronoby::Distance.from_meters(@position.magnitude)
+      end
     end
   end
 end

lib/astronoby/reference_frames/astrometric.rb

@@ -19,7 +19,7 @@ def self.build_from_geometric(
         position: corrected_position - earth_geometric.position,
         velocity: corrected_velocity - earth_geometric.velocity,
         instant: instant,
-        center_identifier: Planet::EARTH,
+        center_identifier: SolarSystemBody::EARTH,
         target_body: target_body
       )
     end

lib/astronoby/reference_frames/geometric.rb

@@ -12,7 +12,7 @@ def initialize(
         position: position,
         velocity: velocity,
         instant: instant,
-        center_identifier: Planet::SOLAR_SYSTEM_BARYCENTER,
+        center_identifier: SolarSystemBody::SOLAR_SYSTEM_BARYCENTER,
         target_body: target_body
       )
     end

spec/astronoby/bodies/moon_spec.rb

@@ -1,6 +1,163 @@
 # frozen_string_literal: true
 
 RSpec.describe Astronoby::Moon do
+  include TestEphemHelper
+
+  describe "#geometric" do
+    it "returns a Geometric position" do
+      time = Time.utc(2025, 2, 7, 12)
+      instant = Astronoby::Instant.from_time(time)
+      state = double(
+        position: Ephem::Core::Vector[1, 2, 3],
+        velocity: Ephem::Core::Vector[4, 5, 6]
+      )
+      segment = double(compute_and_differentiate: state)
+      ephem = double(:[] => segment)
+      body = described_class.new(instant: instant, ephem: ephem)
+
+      geometric = body.geometric
+
+      expect(geometric).to be_a(Astronoby::Geometric)
+      expect(geometric.position)
+        .to eq(
+          Astronoby::Vector[
+            Astronoby::Distance.from_kilometers(2),
+            Astronoby::Distance.from_kilometers(4),
+            Astronoby::Distance.from_kilometers(6)
+          ]
+        )
+      expect(geometric.velocity)
+        .to eq(
+          Astronoby::Vector[
+            Astronoby::Velocity.from_kilometers_per_day(8),
+            Astronoby::Velocity.from_kilometers_per_day(10),
+            Astronoby::Velocity.from_kilometers_per_day(12)
+          ]
+        )
+    end
+
+    it "computes the correct position" do
+      time = Time.utc(2025, 3, 1)
+      instant = Astronoby::Instant.from_time(time)
+      ephem = test_ephem
+      body = described_class.new(instant: instant, ephem: ephem)
+
+      geometric = body.geometric
+
+      expect(geometric.position.to_a.map(&:km).map(&:round))
+        .to eq([-139986171, 45092089, 19573501])
+      # IMCCE:    -139986152 45092088 19573500
+      # Skyfield: -139986173 45092085 19573500
+
+      expect(geometric.equatorial.right_ascension.str(:hms))
+        .to eq("10h 48m 34.8736s")
+      # IMCCE:    10h 48m 34.8733s
+      # Skyfield: 10h 48m 34.87s
+
+      expect(geometric.equatorial.declination.str(:dms))
+        .to eq("+7° 34′ 51.4383″")
+      # IMCCE:    +7° 34′ 51.439″
+      # Skyfield: +7° 34′ 51.4″
+
+      expect(geometric.ecliptic.latitude.str(:dms))
+        .to eq("+0° 0′ 30.2285″")
+      # IMCCE:    +0° 0′ 30.227″
+      # Skyfield: +0° 0′ 33.1″
+
+      expect(geometric.ecliptic.longitude.str(:dms))
+        .to eq("+160° 39′ 3.37″")
+      # IMCCE:    +160° 39′ 3.365″
+      # Skyfield: +161° 0′ 10.3″
+
+      expect(geometric.distance.au)
+        .to eq(0.9917671780230503)
+      # IMCCE:    0.991767054173
+      # Skyfield: 0.9917671790668138
+    end
+
+    it "computes the correct velocity" do
+      time = Time.utc(2025, 3, 1)
+      instant = Astronoby::Instant.from_time(time)
+      ephem = test_ephem
+      body = described_class.new(instant: instant, ephem: ephem)
+
+      geometric = body.geometric
+
+      expect(geometric.velocity.to_a.map(&:mps).map { _1.round(5) })
+        .to eq([-10408.41307, -24901.72774, -10687.38588])
+      # IMCCE:    -10408.41236 -24901.72816 -10687.38603
+      # Skyfield: -10408.41257 -24901.72803 -10687.38600
+    end
+  end
+
+  describe "#astrometric" do
+    it "returns an Astrometric position" do
+      time = Time.utc(2025, 2, 7, 12)
+      instant = Astronoby::Instant.from_time(time)
+      state = double(
+        position: Ephem::Core::Vector[1, 2, 3],
+        velocity: Ephem::Core::Vector[4, 5, 6]
+      )
+      segment = double(compute_and_differentiate: state)
+      ephem = double(:[] => segment)
+      body = described_class.new(instant: instant, ephem: ephem)
+
+      astrometric = body.astrometric
+
+      expect(astrometric).to be_a(Astronoby::Astrometric)
+      expect(astrometric.equatorial).to be_a(Astronoby::Coordinates::Equatorial)
+      expect(astrometric.distance).to be_a(Astronoby::Distance)
+    end
+
+    it "computes the correct position" do
+      time = Time.utc(2025, 3, 1)
+      instant = Astronoby::Instant.from_time(time)
+      ephem = test_ephem
+      body = described_class.new(instant: instant, ephem: ephem)
+
+      astrometric = body.astrometric
+
+      expect(astrometric.equatorial.right_ascension.str(:hms))
+        .to eq("23h 36m 54.8281s")
+      # IMCCE:    23h 36m 54.8384s
+      # Skyfield: 23h 36m 54.83s
+
+      expect(astrometric.equatorial.declination.str(:dms))
+        .to eq("-2° 50′ 44.532″")
+      # IMCCE:    -2° 50′ 44.459″
+      # Skyfield: -2° 50′ 44.5″
+
+      expect(astrometric.ecliptic.latitude.str(:dms))
+        .to eq("-0° 19′ 14.6203″")
+      # IMCCE:    -0° 19′ 14.614″
+      # Skyfield: -0° 19′ 14.9″
+
+      expect(astrometric.ecliptic.longitude.str(:dms))
+        .to eq("+353° 34′ 30.4661″")
+      # IMCCE:    +353° 34′ 30.636″
+      # Skyfield: +353° 55′ 37.5″
+
+      expect(astrometric.distance.au)
+        .to eq(0.002423811076386272)
+      # IMCCE:    0.002423811046
+      # Skyfield: 0.002423811056514585
+    end
+
+    it "computes the correct velocity" do
+      time = Time.utc(2025, 3, 1)
+      instant = Astronoby::Instant.from_time(time)
+      ephem = test_ephem
+      body = described_class.new(instant: instant, ephem: ephem)
+
+      astrometric = body.astrometric
+
+      expect(astrometric.velocity.to_a.map(&:mps).map { _1.round(5) })
+        .to eq([105.49678, 948.38896, 519.75459])
+      # IMCCE:    105.49594 948.38904 519.75463
+      # Skyfield: 105.49622 948.38902 519.75462
+    end
+  end
+
   describe "::monthly_phase_events" do
     it "returns an array" do
       moon_phases = described_class.monthly_phase_events(year: 2024, month: 1)

spec/astronoby/bodies/planet_spec.rb → spec/astronoby/bodies/solar_system_body_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-RSpec.describe Astronoby::Planet do
+RSpec.describe Astronoby::SolarSystemBody do
   describe "::geometric" do
     it "returns a Geometric position" do
       time = Time.utc(2025, 2, 7, 12)