astro

astro

Minimal astronomy: angle conversions, mean sidereal time, and the equatorial-to-horizon transformation. Pure C99, double precision, link with -lm.

This is the math you need to point a small telescope or plot the sky for a given time and place. It is not a full reduction pipeline — there is no precession, no nutation, no aberration, and no atmospheric refraction. The RA / Dec you pass in is treated as the truth; the alt / az you get back is in the same epoch. Accuracy is about 0.01 degree on recent dates, which is enough to slew a mount but not enough for arcsecond astrometry. If you need that, reach for SOFA or NOVAS.

#include "astro.h"

/* Where will Vega (RA = 18h 36m 56s, Dec = +38d 47m 1s) appear
 * over Bangkok at 2026-05-04 13:30 UT? */
double ra_deg  = astro_hms_to_deg(18, 36, 56);
double dec_deg = astro_dms_to_deg(38, 47,  1);
double az, alt;
astro_eq_to_horizon(2026, 5, 4, 13, 30, 0,
                    /*lat =*/ 13.7563,
                    /*long=*/100.5018,
                    ra_deg, dec_deg,
                    &az, &alt);

Functions

Function	Purpose
astro_hms_to_deg(h, m, s)	RA in hours / minutes / seconds to decimal degrees. One hour of RA is 15 degrees.
astro_dms_to_deg(d, m, s)	Signed degrees / arc-minutes / arc-seconds to decimal degrees.
astro_gmst_deg(y, mo, d, h, mi, s)	Greenwich Mean Sidereal Time, IAU 1982 polynomial, degrees in [0, 360).
astro_lmst_deg(..., long_deg)	Local Mean Sidereal Time = astro_gmst_deg(...) + long_deg, wrapped.
astro_eq_to_horizon(...)	RA / Dec to azimuth (CW from north) and altitude. Uses atan2 so there is no special case at the zenith or the celestial poles.

All angle inputs and outputs are decimal degrees. Longitudes are east positive (the IAU convention). If your value comes in the navigational "west positive" form, negate it.

Either output pointer of astro_eq_to_horizon may be NULL.

Build the self-test

make test       # from this directory

Builds example/check.c, runs it, prints astro: All OK. on success and exits zero. The check covers:

• Trivial conversions (12h 30m 0s → 187.5 deg, signed DMS).
• GMST at the J2000.0 epoch (2000-01-01 12:00 UT) evaluates to 280.46061837 deg.
• A synthetic case where the object sits at the zenith (HA = 0, dec = lat) returns altitude = 90 deg.
• Meeus, Astronomical Algorithms 2nd ed., Example 13.b: Venus on 1987-04-10 at the U.S. Naval Observatory. Reference values are altitude 15.1249 deg, azimuth 248.0337 deg (CW from north).

Use in another project

Copy astro.h and astro.c into your sources, link with -lm. No other module dependencies.

License

MIT.