Sun answers to ecliptic and horizontal coordinates

README.md

@@ -36,17 +36,13 @@ longitude = Astronoby::Angle.as_degrees(2.3522)
 
 sun = Astronoby::Sun.new(epoch: epoch)
 
-horizontal_coordinates = sun
-  .coordinates
-  .to_equatorial(epoch: epoch)
-  .to_horizontal(
-    time: time,
-    latitude: latitude,
-    longitude: longitude
-  )
+horizontal_coordinates = sun.horizontal_coordinates(
+  latitude: latitude,
+  longitude: longitude
+)
 
 horizontal_coordinates.altitude.value.to_f
-# => 27.50236511886638
+# => 27.50236513017543
 
 horizontal_coordinates.altitude.to_dms.format
 # => "+27° 30′ 8.5144″"

lib/astronoby/bodies/sun.rb

@@ -12,7 +12,7 @@ def initialize(epoch:)
       @epoch = epoch
     end
 
-    def coordinates
+    def ecliptic_coordinates
       Coordinates::Ecliptic.new(
         latitude: Angle.as_degrees(0),
         longitude: Angle.as_degrees(
@@ -21,6 +21,14 @@ def coordinates
       )
     end
 
+    def horizontal_coordinates(latitude:, longitude:)
+      time = Astronoby::Epoch.to_utc(@epoch)
+
+      ecliptic_coordinates
+        .to_equatorial(epoch: @epoch)
+        .to_horizontal(time: time, latitude: latitude, longitude: longitude)
+    end
+
     private
 
     def mean_anomaly

lib/astronoby/epoch.rb

@@ -11,8 +11,14 @@ class Epoch
     DEFAULT_EPOCH = J2000
     DAYS_PER_JULIAN_CENTURY = 36525.0
 
+    JULIAN_DAY_NUMBER_OFFSET = 0.5
+
     def self.from_time(time)
       time.to_datetime.ajd
     end
+
+    def self.to_utc(epoch)
+      DateTime.jd(epoch + JULIAN_DAY_NUMBER_OFFSET).to_time.utc
+    end
   end
 end

spec/astronoby/bodies/sun_spec.rb

@@ -1,11 +1,11 @@
 # frozen_string_literal: true
 
 RSpec.describe Astronoby::Sun do
-  describe "#coordinates" do
+  describe "#ecliptic_coordinates" do
     it "returns ecliptic coordinates" do
       epoch = Astronoby::Epoch::DEFAULT_EPOCH
 
-      coordinates = described_class.new(epoch: epoch).coordinates
+      coordinates = described_class.new(epoch: epoch).ecliptic_coordinates
 
       expect(coordinates).to be_a(Astronoby::Coordinates::Ecliptic)
     end
@@ -19,23 +19,11 @@
       time = Time.new(2015, 2, 5, 12, 0, 0, "-05:00")
       epoch = Astronoby::Epoch.from_time(time)
 
-      ecliptic_coordinates = described_class.new(epoch: epoch).coordinates
-      equatorial_coordinates = ecliptic_coordinates.to_equatorial(epoch: epoch)
-      horizontal_coordinates = equatorial_coordinates.to_horizontal(
-        time: time,
-        latitude: Astronoby::Angle.as_degrees(38),
-        longitude: Astronoby::Angle.as_degrees(-78)
-      )
+      ecliptic_coordinates = described_class.new(epoch: epoch).ecliptic_coordinates
 
       expect(ecliptic_coordinates.longitude.to_degrees.value.to_f).to(
         eq(316.5726713406947)
       )
-      expect(horizontal_coordinates.altitude.to_dms.format).to(
-        eq("+35° 47′ 12.6437″")
-      )
-      expect(horizontal_coordinates.azimuth.to_dms.format).to(
-        eq("+172° 17′ 2.7299″")
-      )
     end
 
     # Source:
@@ -47,30 +35,11 @@
       time = Time.new(2000, 8, 9, 12, 0, 0, "-04:00")
       epoch = Astronoby::Epoch.from_time(time)
 
-      ecliptic_coordinates = described_class.new(epoch: epoch).coordinates
-      equatorial_coordinates = ecliptic_coordinates.to_equatorial(epoch: epoch)
-      horizontal_coordinates = equatorial_coordinates.to_horizontal(
-        time: time,
-        latitude: Astronoby::Angle.as_degrees(30),
-        longitude: Astronoby::Angle.as_degrees(-95)
-      )
+      ecliptic_coordinates = described_class.new(epoch: epoch).ecliptic_coordinates
 
       expect(ecliptic_coordinates.longitude.to_degrees.value.to_f).to(
         eq(137.36484079770828)
       )
-      expect(equatorial_coordinates.right_ascension.to_hours.value).to(
-        eq(9.32079647116818)
-      )
-      expect(equatorial_coordinates.declination.to_degrees.value).to(
-        eq(15.63002864434695)
-      )
-      # TODO: very far from the expected value
-      expect(horizontal_coordinates.altitude.to_dms.format).to(
-        eq("+53° 44′ 26.5428″")
-      )
-      expect(horizontal_coordinates.azimuth.to_dms.format).to(
-        eq("+105° 8′ 12.9488″")
-      )
     end
 
     # Source:
@@ -82,29 +51,11 @@
       time = Time.new(2015, 5, 6, 14, 30, 0, "-04:00")
       epoch = Astronoby::Epoch.from_time(time)
 
-      ecliptic_coordinates = described_class.new(epoch: epoch).coordinates
-      equatorial_coordinates = ecliptic_coordinates.to_equatorial(epoch: epoch)
-      horizontal_coordinates = equatorial_coordinates.to_horizontal(
-        time: time,
-        latitude: Astronoby::Angle.as_degrees(-20),
-        longitude: Astronoby::Angle.as_degrees(-30)
-      )
+      ecliptic_coordinates = described_class.new(epoch: epoch).ecliptic_coordinates
 
       expect(ecliptic_coordinates.longitude.to_degrees.value).to(
         eq(45.92185191446102)
       )
-      expect(equatorial_coordinates.right_ascension.to_hours.value).to(
-        eq(2.89706471944391)
-      )
-      expect(equatorial_coordinates.declination.to_degrees.value).to(
-        eq(16.60289828746774)
-      )
-      expect(horizontal_coordinates.altitude.to_dms.format).to(
-        eq("+13° 34′ 17.4238″")
-      )
-      expect(horizontal_coordinates.azimuth.to_dms.format).to(
-        eq("+293° 37′ 12.5232″")
-      )
     end
 
     # Source:
@@ -116,7 +67,7 @@
       time = Time.utc(1988, 7, 27, 0, 0, 0)
       epoch = Astronoby::Epoch.from_time(time)
 
-      ecliptic_coordinates = described_class.new(epoch: epoch).coordinates
+      ecliptic_coordinates = described_class.new(epoch: epoch).ecliptic_coordinates
       equatorial_coordinates = ecliptic_coordinates.to_equatorial(epoch: epoch)
 
       expect(equatorial_coordinates.right_ascension.to_hours.to_hms.format).to(
@@ -136,7 +87,7 @@
       time = Time.utc(1992, 10, 13, 0, 0, 0)
       epoch = Astronoby::Epoch.from_time(time)
 
-      ecliptic_coordinates = described_class.new(epoch: epoch).coordinates
+      ecliptic_coordinates = described_class.new(epoch: epoch).ecliptic_coordinates
       equatorial_coordinates = ecliptic_coordinates.to_equatorial(epoch: epoch)
 
       expect(equatorial_coordinates.right_ascension.to_hours.to_hms.format).to(
@@ -147,4 +98,89 @@
       )
     end
   end
+
+  describe "#horizontal_coordinates" do
+    it "returns horizontal coordinates" do
+      epoch = Astronoby::Epoch::DEFAULT_EPOCH
+
+      coordinates = described_class
+        .new(epoch: epoch)
+        .horizontal_coordinates(
+          latitude: Astronoby::Angle.as_degrees(-20),
+          longitude: Astronoby::Angle.as_degrees(-30)
+        )
+
+      expect(coordinates).to be_a(Astronoby::Coordinates::Horizontal)
+    end
+
+    # Source:
+    #  Title: Celestial Calculations
+    #  Author: J. L. Lawrence
+    #  Edition: MIT Press
+    #  Chapter: 6 - The Sun
+    it "computes the horizontal coordinates for the epoch" do
+      time = Time.new(2015, 2, 5, 12, 0, 0, "-05:00")
+      epoch = Astronoby::Epoch.from_time(time)
+      sun = described_class.new(epoch: epoch)
+
+      horizontal_coordinates = sun.horizontal_coordinates(
+        latitude: Astronoby::Angle.as_degrees(38),
+        longitude: Astronoby::Angle.as_degrees(-78)
+      )
+
+      expect(horizontal_coordinates.altitude.to_dms.format).to(
+        eq("+35° 47′ 12.6437″")
+      )
+      expect(horizontal_coordinates.azimuth.to_dms.format).to(
+        eq("+172° 17′ 2.7301″")
+      )
+    end
+
+    # Source:
+    #  Title: Celestial Calculations
+    #  Author: J. L. Lawrence
+    #  Edition: MIT Press
+    #  Chapter: 6 - The Sun
+    it "computes the coordinates for a given epoch" do
+      time = Time.new(2000, 8, 9, 12, 0, 0, "-04:00")
+      epoch = Astronoby::Epoch.from_time(time)
+      sun = described_class.new(epoch: epoch)
+
+      horizontal_coordinates = sun.horizontal_coordinates(
+        latitude: Astronoby::Angle.as_degrees(30),
+        longitude: Astronoby::Angle.as_degrees(-95)
+      )
+
+      # TODO: very far from the expected value
+      expect(horizontal_coordinates.altitude.to_dms.format).to(
+        eq("+53° 44′ 26.5426″")
+      )
+      expect(horizontal_coordinates.azimuth.to_dms.format).to(
+        eq("+105° 8′ 12.9486″")
+      )
+    end
+
+    # Source:
+    #  Title: Celestial Calculations
+    #  Author: J. L. Lawrence
+    #  Edition: MIT Press
+    #  Chapter: 6 - The Sun
+    it "computes the coordinates for a given epoch" do
+      time = Time.new(2015, 5, 6, 14, 30, 0, "-04:00")
+      epoch = Astronoby::Epoch.from_time(time)
+      sun = described_class.new(epoch: epoch)
+
+      horizontal_coordinates = sun.horizontal_coordinates(
+        latitude: Astronoby::Angle.as_degrees(-20),
+        longitude: Astronoby::Angle.as_degrees(-30)
+      )
+
+      expect(horizontal_coordinates.altitude.to_dms.format).to(
+        eq("+13° 34′ 17.4237″")
+      )
+      expect(horizontal_coordinates.azimuth.to_dms.format).to(
+        eq("+293° 37′ 12.5231″")
+      )
+    end
+  end
 end

spec/astronoby/epoch_spec.rb

@@ -15,5 +15,97 @@
 
       expect(described_class.from_time(time)).to eq(2459348.5)
     end
+
+    # Source:
+    #  Title: Celestial Calculations
+    #  Author: J. L. Lawrence
+    #  Edition: MIT Press
+    #  Chapter: 3 - Time Conversions
+    it "returns the Julian day number associated with the time" do
+      time = Time.utc(2010, 11, 1, 0, 0, 0)
+
+      expect(described_class.from_time(time)).to eq(2455501.5)
+    end
+
+    # Source:
+    #  Title: Celestial Calculations
+    #  Author: J. L. Lawrence
+    #  Edition: MIT Press
+    #  Chapter: 3 - Time Conversions
+    it "returns the Julian day number associated with the time" do
+      time = Time.utc(2015, 5, 10, 6, 0, 0)
+
+      expect(described_class.from_time(time)).to eq(2457152.75)
+    end
+
+    # Source:
+    #  Title: Celestial Calculations
+    #  Author: J. L. Lawrence
+    #  Edition: MIT Press
+    #  Chapter: 3 - Time Conversions
+    it "returns the Julian day number associated with the time" do
+      time = Time.utc(2015, 5, 10, 18, 0, 0)
+
+      expect(described_class.from_time(time)).to eq(2457153.25)
+    end
+  end
+
+  describe "::to_utc" do
+    # Source: https://quasar.as.utexas.edu/BillInfo/JulianDatesG.html
+    it "returns the UTC Time object corresponding to a given Julian day number" do
+      epoch = 2299160.5
+
+      time = described_class.to_utc(epoch)
+
+      expect(time).to eq Time.utc(1582, 10, 15, 0, 0, 0)
+    end
+
+    # Source: https://github.com/observerly/polaris
+    it "returns the UTC Time object corresponding to a given Julian day number" do
+      epoch = 2459348.5
+
+      time = described_class.to_utc(epoch)
+
+      expect(time).to eq Time.utc(2021, 5, 14, 0, 0, 0)
+    end
+
+    # Source:
+    #  Title: Celestial Calculations
+    #  Author: J. L. Lawrence
+    #  Edition: MIT Press
+    #  Chapter: 3 - Time Conversions
+    it "returns the UTC Time object corresponding to a given Julian day number" do
+      epoch = 2369915.5
+
+      time = described_class.to_utc(epoch)
+
+      expect(time).to eq Time.utc(1776, 7, 4, 0, 0, 0)
+    end
+
+    # Source:
+    #  Title: Celestial Calculations
+    #  Author: J. L. Lawrence
+    #  Edition: MIT Press
+    #  Chapter: 3 - Time Conversions
+    it "returns the UTC Time object corresponding to a given Julian day number" do
+      epoch = 2455323
+
+      time = described_class.to_utc(epoch)
+
+      expect(time).to eq Time.utc(2010, 5, 6, 12, 0, 0)
+    end
+
+    # Source:
+    #  Title: Celestial Calculations
+    #  Author: J. L. Lawrence
+    #  Edition: MIT Press
+    #  Chapter: 3 - Time Conversions
+    it "returns the UTC Time object corresponding to a given Julian day number" do
+      epoch = 2456019.37
+
+      time = described_class.to_utc(epoch).round
+
+      expect(time).to eq Time.utc(2012, 4, 1, 20, 52, 48)
+    end
   end
 end