Merge pull request #34 from observerly/feature/refraction/GetAirmass

feat: add GetAirmass() to refraction module in @observerly/sidera

pkg/refraction/refraction.go

@@ -42,3 +42,27 @@ func GetRefraction(
 }
 
 /*****************************************************************************************************************/
+
+func GetAirmass(target common.HorizontalCoordinate) float64 {
+	alt := target.Altitude
+
+	if alt < 0 {
+		return math.Inf(1)
+	}
+
+	// Convert the altitude to radians:
+	z := common.Radians(target.Altitude)
+
+	// Get the tangent of the altitude:
+	tanZ := math.Tan(z)
+
+	// Airmass approaches infinity as the altitude approaches 0 degrees (horizon):
+	if tanZ == 0 {
+		return math.Inf(1)
+	}
+
+	// Get the airmass using the Ciddor (1996) equation of state for air:
+	return 1 / (math.Sin(z) + 0.0001184*(1/tanZ) + 0.003188*(1/tanZ*tanZ))
+}
+
+/*****************************************************************************************************************/

pkg/refraction/refraction_test.go

@@ -131,3 +131,50 @@ func TestRefractionAtTargetWithPressureAndTemperature(t *testing.T) {
 }
 
 /*****************************************************************************************************************/
+
+func TestAirmassAtTargetBelowHorizon(t *testing.T) {
+	// Test the airmass at a target below the horizon:
+	Z := GetAirmass(common.HorizontalCoordinate{
+		Altitude: -10.0,
+		Azimuth:  180.0,
+	})
+
+	if Z != math.Inf(1) {
+		t.Errorf("Expected airmass to be infinite, got %f", Z)
+	}
+}
+
+/*****************************************************************************************************************/
+
+func TestAirmassAtTargetAboveHorizon(t *testing.T) {
+	// Test the airmass at a target above the horizon:
+	Z := GetAirmass(target)
+
+	if Z == math.Inf(1) {
+		t.Errorf("Expected airmass to be finite, got %f", Z)
+	}
+
+	if Z <= 0 {
+		t.Errorf("Expected airmass to be positive, got %f", Z)
+	}
+
+	if Z > 1.5 {
+		t.Errorf("Expected airmass to be less than 1.5, got %f", Z)
+	}
+}
+
+/*****************************************************************************************************************/
+
+func TestAirmassAtTargetAtHorizon(t *testing.T) {
+	// Test the airmass at a target below the horizon:
+	Z := GetAirmass(common.HorizontalCoordinate{
+		Altitude: 0.0,
+		Azimuth:  180.0,
+	})
+
+	if Z != math.Inf(1) {
+		t.Errorf("Expected airmass to be infinite, got %f", Z)
+	}
+}
+
+/*****************************************************************************************************************/