diff --git a/tractor/tractortime.py b/tractor/tractortime.py
index 6bc423a4..d0b98c10 100644
--- a/tractor/tractortime.py
+++ b/tractor/tractortime.py
@@ -17,8 +17,8 @@ class TAITime(ScalarParam, ArithmeticParams):
 
     MJD = TAI/(24*3600)
     '''
-    equinox = 53084.28  # mjd
-    daysperyear = 365.25
+    equinox = 53084.28  # mjd of the spring equinox in 2004
+    daysperyear = 365.25  # Julian years, by definition
 
     mjd2k = datetomjd(J2000)
 
diff --git a/tractor/wcs.py b/tractor/wcs.py
index 266bb4b7..f0679aef 100644
--- a/tractor/wcs.py
+++ b/tractor/wcs.py
@@ -154,6 +154,8 @@ def __init__(self, wcs):
 
         cd = self.wcs.get_cd()
         self.cd = np.array([[cd[0], cd[1]], [cd[2], cd[3]]])
+
+        self.cd_inverse = np.linalg.inv(cd)
         self.pixscale = self.wcs.pixel_scale()
 
     def hashkey(self):
@@ -219,6 +221,9 @@ def cdAtPixel(self, x, y):
         '''
         return self.cd
 
+    def cdInverseAtPixel(self, x, y):
+        return self.cd_inverse
+
     def pixel_scale(self):
         return self.pixscale