Use observer object for geocentric parallax (#79)

Before, `Astronoby::GeocentricParallax::for_equatorial_coordinates` used
to require the `latitude`, `longitude` and `elevation` of the observer.

Now, for consistency and ease of use, it requires a
`Astronoby::Observer` object instead.

lib/astronoby/bodies/moon.rb

@@ -46,9 +46,7 @@ def apparent_equatorial_coordinates
     def horizontal_coordinates(observer:)
       apparent_topocentric_equatorial_coordinates =
         Astronoby::GeocentricParallax.for_equatorial_coordinates(
-          latitude: observer.latitude,
-          longitude: observer.longitude,
-          elevation: observer.elevation,
+          observer: observer,
           time: @time,
           coordinates: apparent_equatorial_coordinates,
           distance: distance

lib/astronoby/geocentric_parallax.rb

@@ -17,9 +17,7 @@ def self.angle(distance:)
     end
 
     # Correct equatorial coordinates with the equatorial horizontal parallax
-    # @param latitude [Astronoby::Angle] Observer's latitude
-    # @param longitude [Astronoby::Angle] Observer's longitude
-    # @param elevation [Astronoby::Distance] Observer's elevation above sea level
+    # @param observer [Astronoby::Observer] Observer
     # @param time [Time] Date-time of the observation
     # @param coordinates [Astronoby::Coordinates::Equatorial]
     #   Equatorial coordinates of the observed body
@@ -28,52 +26,42 @@ def self.angle(distance:)
     # @return [Astronoby::Coordinates::Equatorial] Apparent equatorial
     #   coordinates with equatorial horizontal parallax
     def self.for_equatorial_coordinates(
-      latitude:,
-      longitude:,
-      elevation:,
+      observer:,
       time:,
       coordinates:,
       distance:
     )
       new(
-        latitude,
-        longitude,
-        elevation,
+        observer,
         time,
         coordinates,
         distance
       ).apply
     end
 
-    # @param latitude [Astronoby::Angle] Observer's latitude
-    # @param longitude [Astronoby::Angle] Observer's longitude
-    # @param elevation [Astronoby::Distance] Observer's elevation above sea level
+    # @param observer [Astronoby::Observer] Observer
     # @param time [Time] Date-time of the observation
     # @param coordinates [Astronoby::Coordinates::Equatorial] Equatorial
     #   coordinates of the observed body
     # @param distance [Astronoby::Distance] Distance of the observed body from
     #   the center of the Earth
     def initialize(
-      latitude,
-      longitude,
-      elevation,
+      observer,
       time,
       coordinates,
       distance
     )
-      @latitude = latitude
-      @longitude = longitude
-      @elevation = elevation
+      @observer = observer
       @time = time
       @coordinates = coordinates
       @distance = distance
     end
 
     def apply
-      term1 = Angle.atan(Constants::EARTH_FLATTENING_CORRECTION * @latitude.tan)
-      quantity1 = term1.cos + elevation_ratio * @latitude.cos
+      term1 = Angle.atan(Constants::EARTH_FLATTENING_CORRECTION * latitude.tan)
+      quantity1 = term1.cos + elevation_ratio * latitude.cos
       quantity2 = Constants::EARTH_FLATTENING_CORRECTION * term1.sin +
-        elevation_ratio * @latitude.sin
+        elevation_ratio * latitude.sin
 
       term1 = -quantity1 * equatorial_horizontal_parallax.sin * hour_angle.sin
       term2 = declination.cos - quantity1 * equatorial_horizontal_parallax.sin * hour_angle.cos
@@ -92,6 +80,10 @@ def apply
 
     private
 
+    def latitude
+      @observer.latitude
+    end
+
     def right_ascension
       @coordinates.right_ascension
     end
@@ -101,12 +93,14 @@ def declination
     end
 
     def hour_angle
-      @_hour_angle ||=
-        @coordinates.compute_hour_angle(time: @time, longitude: @longitude)
+      @_hour_angle ||= @coordinates.compute_hour_angle(
+        time: @time,
+        longitude: @observer.longitude
+      )
     end
 
     def elevation_ratio
-      @elevation.meters / Constants::EARTH_EQUATORIAL_RADIUS_IN_METERS.to_f
+      @observer.elevation.meters / Constants::EARTH_EQUATORIAL_RADIUS_IN_METERS.to_f
     end
 
     def equatorial_horizontal_parallax

spec/astronoby/geocentric_parallax_spec.rb

@@ -40,9 +40,11 @@
 
   describe "::for_equatorial_coordinates" do
     it "returns equatorial coordinates" do
-      elevation = Astronoby::Distance.zero
-      latitude = Astronoby::Angle.zero
-      longitude = Astronoby::Angle.zero
+      observer = Astronoby::Observer.new(
+        latitude: Astronoby::Angle.zero,
+        longitude: Astronoby::Angle.zero,
+        elevation: Astronoby::Distance.zero
+      )
       time = Time.new
       true_coordinates = Astronoby::Coordinates::Equatorial.new(
         right_ascension: Astronoby::Angle.zero,
@@ -51,9 +53,7 @@
       distance = Astronoby::Distance.from_meters(100_000_000)
 
       apparent_coordinates = described_class.for_equatorial_coordinates(
-        latitude: latitude,
-        longitude: longitude,
-        elevation: elevation,
+        observer: observer,
         time: time,
         coordinates: true_coordinates,
         distance: distance
@@ -68,9 +68,11 @@
     #  Edition: Cambridge University Press
     #  Chapter: 39 - Calculating correction for parallax
     it "returns the corrected equatorial coordinates for the Moon" do
-      latitude = Astronoby::Angle.from_degrees(50)
-      longitude = Astronoby::Angle.from_degrees(-100)
-      elevation = Astronoby::Distance.from_meters(60)
+      observer = Astronoby::Observer.new(
+        latitude: Astronoby::Angle.from_degrees(50),
+        longitude: Astronoby::Angle.from_degrees(-100),
+        elevation: Astronoby::Distance.from_meters(60)
+      )
       time = Time.utc(1979, 2, 26, 16, 45)
       true_coordinates = Astronoby::Coordinates::Equatorial.new(
         right_ascension: Astronoby::Angle.from_hms(22, 35, 19),
@@ -81,9 +83,7 @@
       )
 
       apparent_coordinates = described_class.for_equatorial_coordinates(
-        latitude: latitude,
-        longitude: longitude,
-        elevation: elevation,
+        observer: observer,
         time: time,
         coordinates: true_coordinates,
         distance: distance
@@ -101,9 +101,11 @@
     #  Edition: Cambridge University Press
     #  Chapter: 39 - Calculating correction for parallax
     it "returns the corrected equatorial coordinates for the Sun" do
-      latitude = Astronoby::Angle.from_degrees(50)
-      longitude = Astronoby::Angle.from_degrees(-100)
-      elevation = Astronoby::Distance.from_meters(60)
+      observer = Astronoby::Observer.new(
+        latitude: Astronoby::Angle.from_degrees(50),
+        longitude: Astronoby::Angle.from_degrees(-100),
+        elevation: Astronoby::Distance.from_meters(60)
+      )
       time = Time.utc(1979, 2, 26, 16, 45)
       true_coordinates = Astronoby::Coordinates::Equatorial.new(
         right_ascension: Astronoby::Angle.from_hms(22, 36, 44),
@@ -112,9 +114,7 @@
       distance = Astronoby::Distance.from_au(0.9901)
 
       apparent_coordinates = described_class.for_equatorial_coordinates(
-        latitude: latitude,
-        longitude: longitude,
-        elevation: elevation,
+        observer: observer,
         time: time,
         coordinates: true_coordinates,
         distance: distance