refactor: add _jd_j2000 variable instead of hard coded

pyTMD/astro.py

@@ -68,6 +68,8 @@
 _jd_mjd = 2400000.5
 # number of days between MJD and the J2000 epoch
 _mjd_j2000 = 51544.5
+# number of days between the Julian day epoch and J2000 epoch
+_jd_j2000 = _jd_mjd + _mjd_j2000
 # Julian century
 _century = 36525.0
 
@@ -589,8 +591,7 @@ def solar_ephemerides(MJD: np.ndarray, **kwargs):
     x, y, z = 1e3*(SSB_to_Sun.compute(ts.tt) - SSB_to_EMB.compute(ts.tt) -
         EMB_to_Earth.compute(ts.tt))
     # rotate to cartesian (ECEF) coordinates
-    # use UT1 time as input to itrs rotation function
-    rot_z = itrs((ts.ut1 - 2451545.0)/ts.century)
+    rot_z = itrs((ts.ut1 - _jd_j2000)/ts.century)
     X = rot_z[0,0,:]*x + rot_z[0,1,:]*y + rot_z[0,2,:]*z
     Y = rot_z[1,0,:]*x + rot_z[1,1,:]*y + rot_z[1,2,:]*z
     Z = rot_z[2,0,:]*x + rot_z[2,1,:]*y + rot_z[2,2,:]*z
@@ -768,7 +769,7 @@ def lunar_ephemerides(MJD: np.ndarray, **kwargs):
     x, y, z = 1e3*(EMB_to_Moon.compute(ts.tt) - EMB_to_Earth.compute(ts.tt))
     # rotate to cartesian (ECEF) coordinates
     # use UT1 time as input to itrs rotation function
-    rot_z = itrs((ts.ut1 - 2451545.0)/ts.century)
+    rot_z = itrs((ts.ut1 - _jd_j2000)/ts.century)
     X = rot_z[0,0,:]*x + rot_z[0,1,:]*y + rot_z[0,2,:]*z
     Y = rot_z[1,0,:]*x + rot_z[1,1,:]*y + rot_z[1,2,:]*z
     Z = rot_z[2,0,:]*x + rot_z[2,1,:]*y + rot_z[2,2,:]*z

test/test_solid_earth.py

@@ -365,3 +365,16 @@ def test_sidereal():
     # expected side real time in hours
     expected = 20.96154017401333
     assert np.isclose(expected, 24.0*ts.st).all()
+
+def test_epochs():
+    """Test that the epoch conversions match expected outputs
+    """
+    # Modified Julian Day (MJD)
+    assert np.isclose(pyTMD.astro._jd_mjd, 2400000.5) 
+    # J2000 time
+    mjd_j2000 = timescale.time.convert_calendar_dates(
+        *timescale.time._j2000_epoch,
+        epoch=timescale.time._mjd_epoch)
+    assert np.isclose(mjd_j2000, pyTMD.astro._mjd_j2000)
+    assert np.isclose(pyTMD.astro._mjd_j2000, 51544.5) 
+    assert np.isclose(pyTMD.astro._jd_j2000, 2451545.0)