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SuperNOVAS v1.5
The NOVAS C library, made better
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High-level and efficient astrometric calculations using observing frames. More...
Functions | |
int | novas_app_to_geom (const novas_frame *restrict frame, enum novas_reference_system sys, double ra, double dec, double dist, double *restrict geom_icrs) |
Converts an observed apparent sky position of a source to an ICRS geometric position, by undoing the gravitational deflection and aberration corrections. | |
int | novas_app_to_hor (const novas_frame *restrict frame, enum novas_reference_system sys, double ra, double dec, RefractionModel ref_model, double *restrict az, double *restrict el) |
Converts an observed apparent position vector in the specified coordinate system to local horizontal coordinates in the specified observer frame. | |
int | novas_change_observer (const novas_frame *orig, const observer *obs, novas_frame *out) |
Change the observer location for an observing frame. | |
int | novas_equ_track (const object *restrict source, const novas_frame *restrict frame, double dt, novas_track *restrict track) |
Calculates equatorial tracking position and motion (first and second time derivatives) for the specified source in the given observing frame. | |
double | novas_frame_lst (const novas_frame *restrict frame) |
Returns the Local (apparent) Sidereal Time for an observing frame of an Earth-bound observer. | |
int | novas_geom_posvel (const object *restrict source, const novas_frame *restrict frame, enum novas_reference_system sys, double *restrict pos, double *restrict vel) |
Calculates the geometric position and velocity vectors, relative to the observer, for a source in the given observing frame, in the specified coordinate system of choice. | |
int | novas_geom_to_app (const novas_frame *restrict frame, const double *restrict pos, enum novas_reference_system sys, sky_pos *restrict out) |
Converts an geometric position in ICRS to an apparent position on sky, by applying appropriate corrections for aberration and gravitational deflection for the observer's frame. | |
int | novas_hor_to_app (const novas_frame *restrict frame, double az, double el, RefractionModel ref_model, enum novas_reference_system sys, double *restrict ra, double *restrict dec) |
Converts an observed azimuth and elevation coordinate to right ascension (R.A.) and declination coordinates expressed in the coordinate system of choice. | |
int | novas_hor_track (const object *restrict source, const novas_frame *restrict frame, RefractionModel ref_model, novas_track *restrict track) |
Calculates horizontal tracking position and motion (first and second time derivatives) for the specified source in the given observing frame. | |
int | novas_invert_transform (const novas_transform *transform, novas_transform *inverse) |
Inverts a novas coordinate transformation matrix. | |
int | novas_make_frame (enum novas_accuracy accuracy, const observer *obs, const novas_timespec *time, double xp, double yp, novas_frame *frame) |
Sets up a observing frame for a specific observer location, time of observation, and accuracy requirement. | |
int | novas_make_transform (const novas_frame *frame, enum novas_reference_system from_system, enum novas_reference_system to_system, novas_transform *transform) |
Calculates a transformation matrix that can be used to convert positions and velocities from one coordinate reference system to another. | |
double | novas_moon_angle (const object *restrict source, const novas_frame *restrict frame) |
Returns the apparent angular distance of a source from the Moon from the observer's point of view. | |
double | novas_object_sep (const object *source1, const object *source2, const novas_frame *restrict frame) |
Returns the angular separation of two objects from the observer's point of view. | |
double | novas_rises_above (double el, const object *restrict source, const novas_frame *restrict frame, RefractionModel ref_model) |
Returns the UTC date at which a distant source appears to rise above the specified elevation angle. | |
double | novas_sets_below (double el, const object *restrict source, const novas_frame *restrict frame, RefractionModel ref_model) |
Returns the UTC date at which a distant source appears to set below the specified elevation angle. | |
int | novas_sky_pos (const object *restrict object, const novas_frame *restrict frame, enum novas_reference_system sys, sky_pos *restrict out) |
Calculates an apparent location on sky for the source. | |
double | novas_solar_illum (const object *restrict source, const novas_frame *restrict frame) |
Returns the Solar illumination fraction of a source, assuming a spherical geometry for the observed body. | |
double | novas_sun_angle (const object *restrict source, const novas_frame *restrict frame) |
Returns the apparent angular distance of a source from the Sun from the observer's point of view. | |
int | novas_track_pos (const novas_track *track, const novas_timespec *time, double *restrict lon, double *restrict lat, double *restrict dist, double *restrict z) |
Calculates a projected position, distance, and redshift for a source, given its near-term trajectory on sky, in the system for which the track was calculated. | |
int | novas_transform_sky_pos (const sky_pos *in, const novas_transform *restrict transform, sky_pos *out) |
Transforms a position or velocity 3-vector from one coordinate reference system to another. | |
int | novas_transform_vector (const double *in, const novas_transform *restrict transform, double *out) |
Transforms a position or velocity 3-vector from one coordinate reference system to another. | |
double | novas_transit_time (const object *restrict source, const novas_frame *restrict frame) |
Returns the UTC date at which a source transits the local meridian. | |
High-level and efficient astrometric calculations using observing frames.
An observing frame represents an observer location at a specific astronomical time (instant), which can be re-used again and again to calculate or transform positions of celestial sources in a a range of astronomical coordinate systems.
To use frames, you start with novas_make_frame()
with an astrometric time and an observer location, and optionally Earth-orientation parameters if precision below the arcsecond level is needed. E.g.:
Once a frame is defined, you can perform various astrometric calculation within it efficiently. For example, calculate apparent positions, in the coordinate system of choice, to predict where objects would be seen by the observer:
If your observer was defined on Earth (ground-based or airborne), you might continue to calculate Az/El positions using novas_app_to_hor()
, or you might just want to know the same position in another reference system also:
Or, you can calculate geometric positions, which are not corrected for aberration or gravitational deflection (but still corrected for light travel time in case of Solar-system sources):
You can also transform geometric positions / velocities to other reference frames using the same novas_transform
as above.
Or, perhaps you are interested when the source will rise (or transit, or set) next, for a ground-based or airborne observer:
Or, you might simply want to know how far your source is from the Sun or Moon (or another source) on the sky:
These are just some of the common use-case scenarios. There is even more possibilities with frames...
int novas_app_to_geom | ( | const novas_frame *restrict | frame, |
enum novas_reference_system | sys, | ||
double | ra, | ||
double | dec, | ||
double | dist, | ||
double *restrict | geom_icrs ) |
Converts an observed apparent sky position of a source to an ICRS geometric position, by undoing the gravitational deflection and aberration corrections.
frame | The observer frame, defining the location and time of observation | |
sys | The reference system in which the observed position is specified. | |
ra | [h] Observed ICRS right-ascension of the source | |
dec | [deg] Observed ICRS declination of the source | |
dist | [AU] Observed distance from observer. A value of <=0 will translate to 1015 AU (around 5 Gpc). | |
[out] | geom_icrs | [AU] The corresponding geometric position for the source, in ICRS. |
References grav_undo_planets(), NOVAS_CIRS, NOVAS_ITRS, NOVAS_J2000, NOVAS_MOD, NOVAS_REFERENCE_SYSTEMS, NOVAS_TIRS, NOVAS_TOD, radec2vector(), spin(), wobble(), and WOBBLE_ITRS_TO_TIRS.
int novas_app_to_hor | ( | const novas_frame *restrict | frame, |
enum novas_reference_system | sys, | ||
double | ra, | ||
double | dec, | ||
RefractionModel | ref_model, | ||
double *restrict | az, | ||
double *restrict | el ) |
Converts an observed apparent position vector in the specified coordinate system to local horizontal coordinates in the specified observer frame.
The observer must be located on the surface of Earth, or else the call will return with an error. The caller may optionally supply a refraction model of choice to calculate an appropriate elevation angle that includes a refraction correction for Earth's atmosphere. If no such model is provided the calculated elevation will be the astrometric elevation without a refraction correction.
frame | Observer frame, defining the time and place of observation (on Earth). | |
sys | Astronomical coordinate system in which the observed position is given. | |
ra | [h] Observed apparent right ascension (R.A.) coordinate | |
dec | [deg] Observed apparent declination coordinate | |
ref_model | An appropriate refraction model, or NULL to calculate unrefracted elevation. Depending on the refraction model, you might want to make sure that the weather parameters were set when the observing frame was defined. | |
[out] | az | [deg] Calculated azimuth angle. It may be NULL if not required. |
[out] | el | [deg] Calculated elevation angle. It may be NULL if not required. |
References novas_timespec::fjd_tt, novas_timespec::ijd_tt, itrs_to_hor(), NOVAS_AIRBORNE_OBSERVER, NOVAS_CIRS, NOVAS_GCRS, NOVAS_ICRS, NOVAS_ITRS, NOVAS_J2000, NOVAS_MOD, NOVAS_OBSERVER_ON_EARTH, NOVAS_REFRACT_ASTROMETRIC, NOVAS_TIRS, NOVAS_TOD, radec2vector(), spin(), wobble(), WOBBLE_PEF_TO_ITRS, and WOBBLE_TIRS_TO_ITRS.
int novas_change_observer | ( | const novas_frame * | orig, |
const observer * | obs, | ||
novas_frame * | out ) |
Change the observer location for an observing frame.
orig | Pointer to original observing frame | |
obs | New observer location | |
[out] | out | Observing frame to populate with a original frame data and new observer location. It can be the same as the input. |
References novas_frame::accuracy, grav_bodies_full_accuracy, grav_bodies_reduced_accuracy, NOVAS_FULL_ACCURACY, novas_get_time(), NOVAS_TDB, obs_planets(), novas_frame::obs_pos, novas_frame::observer, novas_frame::planets, novas_frame::state, and novas_frame::time.
int novas_equ_track | ( | const object *restrict | source, |
const novas_frame *restrict | frame, | ||
double | dt, | ||
novas_track *restrict | track ) |
Calculates equatorial tracking position and motion (first and second time derivatives) for the specified source in the given observing frame.
The position and its derivatives are calculated via the more precise IAU2006 method, and CIRS.
source | Observed source | |
frame | Observing frame, defining the observer location and astronomical time of observation. | |
dt | [s] Time step used for calculating derivatives. | |
[out] | track | Output tracking parameters to populate |
References sky_pos::dec, sky_pos::dis, novas_timespec::fjd_tt, ira_equinox(), NOVAS_CIRS, novas_make_frame(), novas_sky_pos(), NOVAS_TRUE_EQUINOX, novas_v2z(), sky_pos::ra, sky_pos::rv, and SKY_POS_INIT.
double novas_frame_lst | ( | const novas_frame *restrict | frame | ) |
Returns the Local (apparent) Sidereal Time for an observing frame of an Earth-bound observer.
frame | Observer frame, defining the location and time of observation |
References NOVAS_AIRBORNE_OBSERVER, and NOVAS_OBSERVER_ON_EARTH.
int novas_geom_posvel | ( | const object *restrict | source, |
const novas_frame *restrict | frame, | ||
enum novas_reference_system | sys, | ||
double *restrict | pos, | ||
double *restrict | vel ) |
Calculates the geometric position and velocity vectors, relative to the observer, for a source in the given observing frame, in the specified coordinate system of choice.
The geometric position includes proper motion, and for solar-system bodies it is antedated for light travel time, so it effectively represents the geometric position as seen by the observer. However, the geometric does not include aberration correction, nor gravitational deflection.
If you want apparent positions, which account for aberration and gravitational deflection, use novas_skypos() instead.
You can also use novas_transform_vector() to convert the output position and velocity vectors to a dfferent coordinate system of choice afterwards if you want the results expressed in more than one coordinate system.
It implements the same geometric transformations as place()
but at a reduced computational cost. See place()
for references.
NOTES:
source | Pointer to a celestial source data structure that is observed. Catalog sources should have coordinates and properties in ICRS. You can use transform_cat() to convert catalog entries to ICRS as necessary. | |
frame | Observer frame, defining the location and time of observation | |
sys | The coordinate system in which to return positions and velocities. | |
[out] | pos | [AU] Calculated geometric position vector of the source relative to the observer location, in the designated coordinate system. It may be NULL if not required. |
[out] | vel | [AU/day] The calculated velocity vector of the source relative to the observer in the designated coordinate system. It must be distinct from the pos output vector, and may be NULL if not required. |
References bary2obs(), d_light(), light_time2(), NOVAS_CATALOG_OBJECT, NOVAS_FULL_ACCURACY, novas_get_time(), NOVAS_JD_J2000, NOVAS_PLANET, NOVAS_REDUCED_ACCURACY, NOVAS_TDB, proper_motion(), and starvectors().
int novas_geom_to_app | ( | const novas_frame *restrict | frame, |
const double *restrict | pos, | ||
enum novas_reference_system | sys, | ||
sky_pos *restrict | out ) |
Converts an geometric position in ICRS to an apparent position on sky, by applying appropriate corrections for aberration and gravitational deflection for the observer's frame.
Unlike place()
the output reports the distance calculated from the parallax for sidereal sources. The radial velocity of the output is set to NAN (if needed use novas_sky_pos() instead).
frame | The observer frame, defining the location and time of observation | |
pos | [AU] Geometric position of source in ICRS coordinates | |
sys | The coordinate system in which to return the apparent sky location | |
[out] | out | Pointer to the data structure which is populated with the calculated apparent location in the designated coordinate system. It may be the same pounter as the input position. |
References grav_planets(), NOVAS_FULL_ACCURACY, NOVAS_REDUCED_ACCURACY, novas_vlen(), and vector2radec().
int novas_hor_to_app | ( | const novas_frame *restrict | frame, |
double | az, | ||
double | el, | ||
RefractionModel | ref_model, | ||
enum novas_reference_system | sys, | ||
double *restrict | ra, | ||
double *restrict | dec ) |
Converts an observed azimuth and elevation coordinate to right ascension (R.A.) and declination coordinates expressed in the coordinate system of choice.
The observer must be located on the surface of Earth, or else the call will return with an error. The caller may optionally supply a refraction model of choice to calculate an appropriate elevation angle that includes a refraction correction for Earth's atmosphere. If no such model is provided, the provided elevation value will be assumed to be an astrometric elevation without a refraction correction.
frame | Observer frame, defining the time and place of observation (on Earth). | |
az | [deg] Observed azimuth angle. It may be NULL if not required. | |
el | [deg] Observed elevation angle. It may be NULL if not required. | |
ref_model | An appropriate refraction model, or NULL to assume unrefracted elevation. Depending on the refraction model, you might want to make sure that the weather parameters were set when the observing frame was defined. | |
sys | Astronomical coordinate system in which the output is R.A. and declination values are to be calculated. | |
[out] | ra | [h] Calculated apparent right ascension (R.A.) coordinate |
[out] | dec | [deg] Calculated apparent declination coordinate |
References novas_timespec::fjd_tt, hor_to_itrs(), novas_timespec::ijd_tt, NOVAS_AIRBORNE_OBSERVER, NOVAS_CIRS, NOVAS_GCRS, NOVAS_ICRS, NOVAS_ITRS, NOVAS_J2000, NOVAS_MOD, NOVAS_OBSERVER_ON_EARTH, NOVAS_REFRACT_OBSERVED, NOVAS_TIRS, NOVAS_TOD, spin(), vector2radec(), wobble(), WOBBLE_ITRS_TO_PEF, and WOBBLE_ITRS_TO_TIRS.
int novas_hor_track | ( | const object *restrict | source, |
const novas_frame *restrict | frame, | ||
RefractionModel | ref_model, | ||
novas_track *restrict | track ) |
Calculates horizontal tracking position and motion (first and second time derivatives) for the specified source in the given observing frame.
The position and its derivatives are calculated via the more precise IAU2006 method, and CIRS, and then converted to local horizontal coordinates using the specified refraction model (if any).
source | Observed source | |
frame | Observing frame, defining the observer location and astronomical time of observation. | |
ref_model | Refraction model to use, or NULL for an unrefracted track. | |
[out] | track | Output tracking parameters to populate |
References sky_pos::dec, sky_pos::dis, novas_timespec::fjd_tt, ira_equinox(), NOVAS_AIRBORNE_OBSERVER, novas_app_to_hor(), NOVAS_CIRS, novas_make_frame(), NOVAS_OBSERVER_ON_EARTH, novas_sky_pos(), NOVAS_TOD, NOVAS_TRUE_EQUINOX, novas_v2z(), sky_pos::ra, sky_pos::rv, and SKY_POS_INIT.
int novas_invert_transform | ( | const novas_transform * | transform, |
novas_transform * | inverse ) |
Inverts a novas coordinate transformation matrix.
transform | Pointer to a coordinate transformation matrix. | |
[out] | inverse | Pointer to a coordinate transformation matrix to populate with the inverse transform. It may be the same as the input. |
References novas_transform::matrix.
int novas_make_frame | ( | enum novas_accuracy | accuracy, |
const observer * | obs, | ||
const novas_timespec * | time, | ||
double | xp, | ||
double | yp, | ||
novas_frame * | frame ) |
Sets up a observing frame for a specific observer location, time of observation, and accuracy requirement.
The frame is initialized using the currently configured planet ephemeris provider function (see set_planet_provider() and set_planet_provider_hp()), and in case of reduced accuracy mode, the currently configured IAU nutation model provider (see set_nutation_lp_provider()).
Note, that to construct full accuracy frames, you will need a high-precision ephemeris provider for the major planets (not just the default Earth/Sun), as without it, gravitational bending around massive plannets cannot be accounted for, and therefore μas accuracy cannot be ensured, in general. Attempting to construct a high-accuracy frame without a high-precision ephemeris provider for the major planets will result in an error in the 10–40 range from the required ephemeris() call.
NOTES:
novas_itrf_transform_eop()
to convert the EOP values as necessary. accuracy | Accuracy requirement, NOVAS_FULL_ACCURACY (0) for the utmost precision or NOVAS_REDUCED_ACCURACY (1) if ~1 mas accuracy is sufficient. | |
obs | Observer location | |
time | Time of observation | |
xp | [mas] Earth orientation parameter, polar offset in x, e.g. from the IERS Bulletins, and possibly corrected for diurnal and semi-diurnal variations, e.g. via novas_diurnal_eop() . (The global, undated value set by cel_pole() is not not used here.) You can use 0.0 if sub-arcsecond accuracy is not required. | |
yp | [mas] Earth orientation parameter, polar offset in y, e.g. from the IERS Bulletins, and possibly corrected for diurnal and semi-diurnal variations, e.g. via novas_diurnal_eop() . (The global, undated value set by cel_pole() is not not used here.) You can use 0.0 if sub-arcsecond accuracy is not required. | |
[out] | frame | Pointer to the observing frame to configure. |
References novas_frame::accuracy, novas_frame::deps0, novas_frame::dpsi0, novas_frame::dx, novas_frame::dy, novas_frame::earth_pos, novas_frame::earth_vel, novas_frame::ee, ephemeris(), era(), novas_frame::era, novas_timespec::fjd_tt, novas_frame::gst, novas_timespec::ijd_tt, mean_obliq(), novas_frame::mobl, NOVAS_BARYCENTER, novas_change_observer(), NOVAS_EARTH_INIT, novas_get_split_time(), NOVAS_JD_J2000, NOVAS_OBSERVER_PLACES, NOVAS_REDUCED_ACCURACY, NOVAS_SUN_INIT, NOVAS_UT1, nutation_angles(), novas_frame::state, novas_frame::sun_pos, novas_frame::sun_vel, novas_frame::time, novas_frame::tobl, tt2tdb(), and observer::where.
int novas_make_transform | ( | const novas_frame * | frame, |
enum novas_reference_system | from_system, | ||
enum novas_reference_system | to_system, | ||
novas_transform * | transform ) |
Calculates a transformation matrix that can be used to convert positions and velocities from one coordinate reference system to another.
frame | Observer frame, defining the location and time of observation | |
from_system | Original coordinate reference system | |
to_system | New coordinate reference system | |
[out] | transform | Pointer to the transform data structure to populate. |
References novas_frame::era, novas_transform::frame, novas_transform::from_system, novas_frame::gcrs_to_cirs, novas_frame::icrs_to_j2000, novas_matrix::M, novas_transform::matrix, NOVAS_CIRS, NOVAS_GCRS, NOVAS_ICRS, NOVAS_ITRS, NOVAS_J2000, NOVAS_MOD, NOVAS_REFERENCE_SYSTEMS, NOVAS_TIRS, NOVAS_TOD, novas_frame::nutation, novas_frame::precession, and novas_transform::to_system.
double novas_moon_angle | ( | const object *restrict | source, |
const novas_frame *restrict | frame ) |
Returns the apparent angular distance of a source from the Moon from the observer's point of view.
source | An observed source |
frame | Observing frame, defining the observer location and astronomical time of observation. |
References NOVAS_MOON_INIT, and novas_object_sep().
double novas_object_sep | ( | const object * | source1, |
const object * | source2, | ||
const novas_frame *restrict | frame ) |
Returns the angular separation of two objects from the observer's point of view.
The calculated separation includes light-time corrections, aberration and gravitational deflection for both sources, and thus represents a precise observed separation between the two sources.
source1 | An observed source |
source2 | Another observed source |
frame | Observing frame, defining the observer location and astronomical time of observation. |
References sky_pos::dec, sky_pos::dis, novas_equ_sep(), NOVAS_GCRS, novas_sky_pos(), sky_pos::ra, and SKY_POS_INIT.
double novas_rises_above | ( | double | el, |
const object *restrict | source, | ||
const novas_frame *restrict | frame, | ||
RefractionModel | ref_model ) |
Returns the UTC date at which a distant source appears to rise above the specified elevation angle.
The calculated time will account for the (slow) motion for Solar-system bodies, and optionally for atmospheric refraction also.
NOTES:
el | [deg] Elevation angle. |
source | Observed source |
frame | Observing frame, defining the observer location and astronomical time of observation. |
ref_model | Refraction model, or NULL to calculate unrefracted rise time. |
double novas_sets_below | ( | double | el, |
const object *restrict | source, | ||
const novas_frame *restrict | frame, | ||
RefractionModel | ref_model ) |
Returns the UTC date at which a distant source appears to set below the specified elevation angle.
The calculated time will account for the (slow) motion of Solar-system bodies, and optionally for atmopsheric refraction also.
NOTES:
el | [deg] Elevation angle. |
source | Observed source |
frame | Observing frame, defining the observer location and astronomical time of observation. |
ref_model | Refraction model, or NULL to calculate unrefracted setting time. |
int novas_sky_pos | ( | const object *restrict | object, |
const novas_frame *restrict | frame, | ||
enum novas_reference_system | sys, | ||
sky_pos *restrict | out ) |
Calculates an apparent location on sky for the source.
The position takes into account the proper motion (for sidereal source), or is antedated for light-travel time (for Solar-System bodies). It also applies an appropriate aberration correction and gravitational deflection of the light.
To calculate corresponding local horizontal coordinates, you can pass the output RA/Dec coordinates to novas_app_to_hor(). Or to calculate apparent coordinates in other systems, you may pass the result to novas_transform_sy_pos() after.
And if you want geometric positions instead (not corrected for aberration or gravitational deflection), you may want to use novas_geom_posvel() instead.
The approximate 'inverse' of this function is novas_app_to_geom().
This function implements the same aberration and gravitational deflection corrections as place()
, but at reduced computational cost. See place()
for references. Unlike place()
, however, the output always reports the distance calculated from the parallax for sidereal sources. Note also, that while place()
does not apply aberration and gravitational deflection corrections when sys
is NOVAS_ICRS (3), this routine will apply those corrections consistently for all coordinate systems (and you can use novas_geom_posvel() instead to get positions without aberration or deflection in any system).
NOTES:
object | Pointer to a celestial object data structure that is observed. Catalog sources should have coordinates and properties in ICRS. You can use transform_cat() to convert catalog entries to ICRS as necessary. | |
frame | The observer frame, defining the location and time of observation. | |
sys | The coordinate system in which to return the apparent sky location. | |
[out] | out | Pointer to the data structure which is populated with the calculated apparent location in the designated coordinate system. |
References grav_planets(), NOVAS_CATALOG_OBJECT, NOVAS_FULL_ACCURACY, novas_geom_posvel(), novas_geom_to_app(), NOVAS_ICRS, NOVAS_REDUCED_ACCURACY, novas_vlen(), rad_vel2(), and object::type.
double novas_solar_illum | ( | const object *restrict | source, |
const novas_frame *restrict | frame ) |
Returns the Solar illumination fraction of a source, assuming a spherical geometry for the observed body.
source | Observed source. Usually a Solar-system source. (For other source types, 1.0 is returned by default.) |
frame | Observing frame, defining the observer location and astronomical time of observation. |
References NOVAS_CATALOG_OBJECT, novas_geom_posvel(), NOVAS_ICRS, and novas_vlen().
double novas_sun_angle | ( | const object *restrict | source, |
const novas_frame *restrict | frame ) |
Returns the apparent angular distance of a source from the Sun from the observer's point of view.
source | An observed source |
frame | Observing frame, defining the observer location and astronomical time of observation. |
References novas_object_sep(), and NOVAS_SUN_INIT.
int novas_track_pos | ( | const novas_track * | track, |
const novas_timespec * | time, | ||
double *restrict | lon, | ||
double *restrict | lat, | ||
double *restrict | dist, | ||
double *restrict | z ) |
Calculates a projected position, distance, and redshift for a source, given its near-term trajectory on sky, in the system for which the track was calculated.
Thus if the input track was obtained with novas_hor_track()
it will calculate momentary azimuth and elevation (Az/El) for the specified proximate time. And, if you used novas_equ_track()
then it will give you theinstantaneous R.A. and declination for that time.
Using the quadratic trajectories via a novas_track
to project positions can be much faster than the repeated full recalculation of the source position, but still quite accurate over a suffciently brief period.
In SuperNOVAS terminology a 'track' is a 2nd order Taylor series expansion of the observed position and redshift in time. For most but the fastest moving sources, horizontal (Az/El) tracks are sufficiently precise for telescope positioning on minute timescales, whereas depending on the type of source, equatorial tracks can be precise for up to days, especially for sidereal (non-Solar-system) sources.
track | Tracking position and motion (first and second derivatives), obtained e.g. in horizontal (Az/El) coordinates with novas_hor_track() or in equatorial coordinates with novas_equ_track() . | |
time | Astrometric time of observation for which to calculate projected positions, around the time for which the track was obtained. | |
[out] | lon | [deg] projected observed Eastward longitude in the tracking coordinate system. Thus, if the track was calculated with novas_hor_track() it will return the projected azimuth coordinate for the given time. And, if you used novas_equ_track() , then this will be R.A. (in degrees, nevertheless). |
[out] | lat | [deg] projected observed latitude in the tracking coordinate system. Thus, if the track was calculated with novas_hor_track() it will return the projected elevation angle for the given time. And, if you used novas_equ_track() , then this will be declination. |
[out] | dist | [AU] projected apparent distance to source from observer. |
[out] | z | projected observed redshift (z = Δλ / λrest, e.g. for spectroscopy). |
References novas_track::accel, novas_observable::dist, novas_observable::lat, novas_observable::lon, novas_diff_time(), novas_track::pos, novas_track::rate, novas_track::time, and novas_observable::z.
int novas_transform_sky_pos | ( | const sky_pos * | in, |
const novas_transform *restrict | transform, | ||
sky_pos * | out ) |
Transforms a position or velocity 3-vector from one coordinate reference system to another.
in | Input apparent position on sky in the original coordinate reference system | |
transform | Pointer to a coordinate transformation matrix | |
[out] | out | Output apparent position on sky in the new coordinate reference system. It may be the same as the input. |
References sky_pos::dec, sky_pos::r_hat, sky_pos::ra, and vector2radec().
int novas_transform_vector | ( | const double * | in, |
const novas_transform *restrict | transform, | ||
double * | out ) |
Transforms a position or velocity 3-vector from one coordinate reference system to another.
in | Input 3-vector in the original coordinate reference system | |
transform | Pointer to a coordinate transformation matrix | |
[out] | out | Output 3-vector in the new coordinate reference system. It may be the same as the input. |
double novas_transit_time | ( | const object *restrict | source, |
const novas_frame *restrict | frame ) |
Returns the UTC date at which a source transits the local meridian.
The calculated time will account for the (slow) motion of Solar-system bodies.
NOTES:
source | Observed source |
frame | Observing frame, defining the observer location and astronomical time of observation. |