feat: add J2000 timestamp constant to epoch module in @observerly/sidera.

pkg/epoch/epoch.go

@@ -27,3 +27,19 @@ and fractions of days from the start of their epoch.
 const J1970 float64 = 2440587.5
 
 /*****************************************************************************************************************/
+
+/*
+the epoch of J2000.0 i.e., 1 January 2000 12:00:00 UTC.
+
+The Julian Date for J2000.0 is 2,451,545.0. This date is often used as a reference epoch in
+astronomy and celestial mechanics. It is particularly useful for calculating the positions of
+celestial objects and for defining the equatorial coordinate system.
+
+This date is notably significant because J2000 is used as a reference epoch for the
+International Celestial Reference System (ICRS), which is the current standard for defining
+the positions of celestial objects. J2000 is also used as the reference epoch for the
+equatorial coordinate system, which is based on the celestial equator and the vernal equinox.
+*/
+const J2000 float64 = 2451545.0
+
+/*****************************************************************************************************************/

pkg/epoch/epoch_test.go

@@ -24,3 +24,10 @@ func TestGetJ1970(t *testing.T) {
 }
 
 /*****************************************************************************************************************/
+
+func TestGetJ2000(t *testing.T) {
+	// Test the Julian Date calculation for the J2000.0 epoch:
+	assert.Equal(t, J2000, 2451545.0)
+}
+
+/*****************************************************************************************************************/