Fix epoch naming (#38)

Author: tokoro10g

libsgp4/DateTime.h

@@ -626,14 +626,24 @@ class DateTime
         return Util::WrapTwoPI(Util::DegreesToRadians(gt / 240.0));
     }
 
+    /**
+     * Return the modified julian date since the j1900 epoch
+     * January 1, 1900, at 12:00 TT
+     * @returns the modified julian date
+     */
+    double ToJ1900() const
+    {
+        return ToJulian() - 2415020.0;
+    }
+
     /**
      * Return the modified julian date since the j2000 epoch
      * January 1, 2000, at 12:00 TT
      * @returns the modified julian date
      */
     double ToJ2000() const
     {
-        return ToJulian() - 2415020.0;
+        return ToJulian() - 2451545.0;
     }
 
     /**

libsgp4/SGP4.cc

@@ -721,7 +721,7 @@ void SGP4::DeepSpaceInitialise(
     /*
      * initialize lunar / solar terms
      */
-    const double jday = elements_.Epoch().ToJ2000();
+    const double jday = elements_.Epoch().ToJ1900();
 
     const double xnodce = Util::WrapTwoPI(4.5236020 - 9.2422029e-4 * jday);
     const double stem = sin(xnodce);

libsgp4/SolarPosition.cc

@@ -27,7 +27,7 @@ namespace libsgp4
 
 Eci SolarPosition::FindPosition(const DateTime& dt)
 {
-    const double mjd = dt.ToJ2000();
+    const double mjd = dt.ToJ1900();
     const double year = 1900 + mjd / 365.25;
     const double T = (mjd + Delta_ET(year) / kSECONDS_PER_DAY) / 36525.0;
     const double M = Util::DegreesToRadians(Util::Wrap360(358.47583