Smithsonian · attipaci · Mar 4, 2025 · Mar 4, 2025 · Mar 4, 2025
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -95,6 +95,10 @@ calculations.
  - #135: New `novas_str_hours()` and `novas_str_degrees()` for the simplest conversion of strings in decimal or 
    HMS/DMS formats to floating point values for SuperNOVAS (without parse position).
 
+ - #137: New `novas_epoch()` to convert string coordinate system specifications to the Julian date of the corresponding
+   epoch, and new `make_cat_object_sys()` and `make_redshifted_object_sys()` to make it simpler to define ICRS catalog 
+   sources in the coordinate system of choice.
+
  - Added `example-time.c` and `example-rise-set.c` under `examples/`, for demonstrating date/time handling functions
    and rise, set, and transit time calculations. 
 

diff --git a/README.md b/README.md
@@ -472,36 +472,21 @@ separators used between the components, e.g.:
 And, if you have LSR-based radial velocities instead of Solar-system Barycentric radial velocities, you may convert 
 these to SSB-based velocities for use in `make_cat_entry()` with `novas_lsr_to_ssb_vel()`.
 
-We must convert the older catalog coordinates to the now standard ICRS system for calculations in SuperNOVAS, first by 
-calculating equivalent J2000 coordinates, by applying the proper motion and the appropriate precession. Then, we apply 
-a small adjustment to convert from J2000 to ICRS coordinates.
-
-```c
- // First change the catalog coordinates (in place) to the J2000 (FK5) system... 
- transform_cat(CHANGE_EPOCH, NOVAS_JD_B1950, &star, NOVAS_JD_J2000, "FK5", &star);
-
- // Then convert J2000 coordinates to ICRS (also in place). Here the dates don't matter...
- transform_cat(CHANGE_J2000_TO_ICRS, 0.0, &star, 0.0, "ICRS", &star);
-```
-
-(Naturally, you can skip the transformation steps above if you have defined your source in ICRS coordinates from the 
-start.) Once the catalog entry is defined in ICRS, you can proceed wrapping it in a generic source structure (which
-handles both catalog and Solar-system sources).
-
 ```c
  object source;   // Common structure for a sidereal or an Solar-system source
 
- // Wrap it in a generic source data structure
- make_cat_object(&star, &source);
+ // Wrap it in a generic source data structure, with ICRS coordinates...
+ make_cat_object_sys(&star, "B1950", &source);
 ```
 
-Alternatively, for high-_z_ sources you might use `make_redshifted_cat_entry()` or `make_redshifted_object()` e.g.:
+Alternatively, for high-_z_ sources you might use `make_redshifted_cat_entry()` or `make_redshifted_object_sys()` 
+e.g.:
 
 ```c
  object quasar;
 
  // 12h29m6.6997s +2d3m8.598s (ICRS) z=0.158339
- make_redshifted_object("3c273", 12.4851944, 2.0523883, 0.158339, &quasar);
+ make_redshifted_object_sys("3c273", 12.4851944, 2.0523883, "ICRS", 0.158339, &quasar);
 ```
 
 <a name="specify-observer"></a>

diff --git a/examples/example-high-z.c b/examples/example-high-z.c
@@ -64,25 +64,11 @@ int main() {
   double ra0 = novas_str_hours("12h29m6.6997s");
   double dec0 = novas_str_degrees("+2d3m8.598s");
 
-  if(make_redshifted_object("3c273", ra0, dec0, 0.158339, &source) != 0) {
+  if(make_redshifted_object_sys("3c273", ra0, dec0, "ICRS", 0.158339, &source) != 0) {
     fprintf(stderr, "ERROR! defining cat_entry.\n");
     return 1;
   }
 
-  // If we did not use ICRS catalog coordinates, we would have to convert them to ICRS...
-
-  /* E.g. change B1950 to the J2000 (FK5) system...
-  if(transform_cat(CHANGE_EPOCH, NOVAS_JD_B1950, &source.star, NOVAS_JD_J2000, "FK5", &source.star) != 0) {
-    fprintf(stderr, "ERROR! converting B1950 catalog coordinates to J2000.\n");
-    return 1;
-  } */
-
-  /* Then convert J2000 coordinates to ICRS (also in place). Here the dates don't matter...
-  if(transform_cat(CHANGE_J2000_TO_ICRS, 0.0, &source.star, 0.0, "ICRS", &source.star) != 0) {
-    fprintf(stderr, "ERROR! converting J2000 catalog coordinates to ICRS.\n");
-    return 1;
-  } */
-
 
   // -------------------------------------------------------------------------
   // Define observer somewhere on Earth (we can also define observers in Earth

diff --git a/examples/example-rise-set.c b/examples/example-rise-set.c
@@ -83,22 +83,9 @@ int main(int argc, const char *argv[]) {
     return 1;
   }
 
-  // First change the catalog coordinates (in place) to the J2000 (FK5) system...
-  if(transform_cat(CHANGE_EPOCH, NOVAS_JD_B1950, &star, NOVAS_JD_J2000, "FK5", &star) != 0) {
-    fprintf(stderr, "ERROR! converting B1950 catalog coordinates to J2000.\n");
-    return 1;
-  }
-
-  // Then convert J2000 coordinates to ICRS (also in place). Here the dates don't matter...
-  if(transform_cat(CHANGE_J2000_TO_ICRS, 0.0, &star, 0.0, "ICRS", &star) != 0) {
-    fprintf(stderr, "ERROR! converting J2000 catalog coordinates to ICRS.\n");
-    return 1;
-  }
-
-
   // -------------------------------------------------------------------------
-  // Wrap the sidereal souce into an object structure...
-  if(make_cat_object(&star, &source) != 0) {
+  // Wrap the sidereal source into an object structure...
+  if(make_cat_object_sys(&star, "B1950", &source) != 0) {
     fprintf(stderr, "ERROR! configuring observed object\n");
     return 1;
   }

diff --git a/examples/example-star.c b/examples/example-star.c
@@ -66,28 +66,16 @@ int main() {
 
   // Convert string coordinates to hours/degrees...
   double ra0 = novas_str_hours("16h26m20.1918s");
-  double dec0 =  novas_str_degrees("-26d19m23.138s");
+  double dec0 = novas_str_degrees("-26d19m23.138s");
 
   if(make_cat_entry("Antares", "FK4", 1, ra0, dec0, -12.11, -23.30, 5.89, -3.4, &star) != 0) {
     fprintf(stderr, "ERROR! defining cat_entry.\n");
     return 1;
   }
-  // First change the catalog coordinates (in place) to the J2000 (FK5) system...
-  if(transform_cat(CHANGE_EPOCH, NOVAS_JD_B1950, &star, NOVAS_JD_J2000, "FK5", &star) != 0) {
-    fprintf(stderr, "ERROR! converting B1950 catalog coordinates to J2000.\n");
-    return 1;
-  }
-
-  // Then convert J2000 coordinates to ICRS (also in place). Here the dates don't matter...
-  if(transform_cat(CHANGE_J2000_TO_ICRS, 0.0, &star, 0.0, "ICRS", &star) != 0) {
-    fprintf(stderr, "ERROR! converting J2000 catalog coordinates to ICRS.\n");
-    return 1;
-  }
-
 
   // -------------------------------------------------------------------------
-  // Wrap the sidereal souce into an object structure...
-  if(make_cat_object(&star, &source) != 0) {
+  // Wrap the sidereal source into an object structure with ICRS coordinates.
+  if(make_cat_object_sys(&star, "B1950", &source) != 0) {
     fprintf(stderr, "ERROR! configuring observed object\n");
     return 1;
   }

diff --git a/include/novas.h b/include/novas.h
@@ -194,6 +194,30 @@
 /// The day prior to that was 4 October 1582 in the Julian Calendar.
 #define NOVAS_JD_START_GREGORIAN  2299160.5
 
+/// The ICRS system as a string
+/// @since 1.3
+#define NOVAS_SYSTEM_ICRS          "ICRS"
+
+/// The B1950 coordiante system as a string
+/// @since 1.3
+#define NOVAS_SYSTEM_B1950         "B1950"
+
+/// The J2000 coordinate syste, as a string
+/// @since 1.3
+#define NOVAS_SYSTEM_J2000         "J2000"
+
+/// The 4th catalog (FK4) coordinate system as a string
+/// @since 1.3
+#define NOVAS_SYSTEM_FK4           "FK4"
+
+/// The 5th catalog (FK5) coordinate system as a string
+/// @since 1.3
+#define NOVAS_SYSTEM_FK5           "FK5"
+
+/// The Hipparcos dataset coordinate system as a string
+/// @since 1.3
+#define NOVAS_SYSTEM_HIP           "HIP"
+
 #if !COMPAT
 // If we are not in the strict compatibility mode, where constants are defined
 // as variables in novascon.h (with implementation in novascon.c), then define
@@ -1775,6 +1799,13 @@ int novas_xyz_to_los(const double *xyz, double lon, double lat, double *los);
 
 int novas_xyz_to_uvw(const double *xyz, double ha, double dec, double *uvw);
 
+int make_cat_object_sys(const cat_entry *star, const char *restrict system, object *source);
+
+int make_redshifted_object_sys(const char *name, double ra, double dec, const char *restrict system, double z, object *source);
+
+double novas_epoch(const char *restrict system);
+
+
 // in parse.c
 double novas_hms_hours(const char *restrict hms);
 
@@ -1836,7 +1867,6 @@ enum novas_timescale novas_parse_timescale(const char *restrict str, char **rest
 int novas_print_timescale(enum novas_timescale scale, char *restrict buf);
 
 
-
 // <================= END of SuperNOVAS API =====================>
 
 
@@ -1878,6 +1908,7 @@ int novas_print_timescale(enum novas_timescale scale, char *restrict buf);
 #  define DEG360              360.0
 #  define JULIAN_YEAR_DAYS    365.25
 #  define JULIAN_CENTURY_DAYS 36525.0
+#  define BESSELIAN_YEAR_DAYS 365.2568983
 #  define ARCSEC              NOVAS_ARCSEC
 #  define DEGREE              NOVAS_DEGREE
 #  define HOURANGLE           NOVAS_HOURANGLE

diff --git a/src/novas.c b/src/novas.c
@@ -4544,6 +4544,7 @@ double rad_vel2(const object *restrict source, const double *pos_emit, const dou
  *
  * @sa nutation()
  * @sa frame_tie()
+ * @sa novas_epoch()
  * @sa tt2tdb()
  * @sa cio_basis()
  * @sa NOVAS_TOD
@@ -6318,6 +6319,7 @@ int novas_xyz_to_los(const double *xyz, double lon, double lat, double *los) {
  *
  * @sa transform_hip()
  * @sa make_cat_entry()
+ * @sa novas_epoch()
  * @sa NOVAS_JD_J2000
  * @sa NOVAS_JD_B1950
  * @sa NOVAS_JD_HIP
@@ -6967,7 +6969,7 @@ double norm_ang(double angle) {
  *
  * @sa novas_lsr_to_ssb_vel()
  * @sa make_redshifted_cat_entry()
- * @sa make_object()
+ * @sa make_cat_object_sys()
  * @sa transform_cat()
  */
 short make_cat_entry(const char *restrict star_name, const char *restrict catalog, long cat_num, double ra, double dec, double pm_ra, double pm_dec,