libnova  v 0.16.0

Examples of how to use comet functions.

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU Library General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.
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along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
Copyright (C) 2003 Liam Girdwood <>
A simple example showing some comet calculations.
Comet Encke
#include <stdio.h>
#include <libnova/comet.h>
#include <libnova/julian_day.h>
#include <libnova/rise_set.h>
#include <libnova/transform.h>
#include <libnova/elliptic_motion.h>
/* use the orbital example from Meeus book */
#define MEEUS 0
static void print_date(const char *title, struct ln_zonedate *date)
fprintf(stdout, "\n%s\n",title);
fprintf(stdout, " Year : %d\n", date->years);
fprintf(stdout, " Month : %d\n", date->months);
fprintf(stdout, " Day : %d\n", date->days);
fprintf(stdout, " Hours : %d\n", date->hours);
fprintf(stdout, " Minutes : %d\n", date->minutes);
fprintf(stdout, " Seconds : %f\n", date->seconds);
fprintf(stdout, " GMT offset %ld\n", date->gmtoff);
int main (int argc, const char *argv[])
struct ln_equ_posn equ;
struct ln_rst_time rst;
struct ln_zonedate rise, set, transit;
struct ln_date epoch_date, date;
struct ln_lnlat_posn observer;
struct ln_ell_orbit orbit;
struct ln_rect_posn posn;
double JD, e_JD;
double E, v, V, r, l, dist, M;
/* observers location (Edinburgh), used to calc rst */ = 55.92; /* 55.92 N */
observer.lng = -3.18; /* 3.18 W */
date.years = 1990;
date.months = 10;
date.days = 6;
date.hours = 0;
date.minutes = 0;
date.seconds = 0;
JD = ln_get_julian_day(&date);
/* get Julian day from local time */
fprintf(stdout, "JD %f\n", JD);
/* calc epoch JD */
epoch_date.years = 1990;
epoch_date.months = 10;
epoch_date.days = 28;
epoch_date.hours = 12;
epoch_date.minutes = 30;
epoch_date.seconds = 0;
e_JD = ln_get_julian_day(&epoch_date);
e_JD = 2456617.5;
fprintf(stdout, "Epoch JD %f diff %f\n", e_JD, JD - e_JD);
/* Encke orbital elements */
orbit.JD = e_JD;
orbit.a = 2.2091404;
orbit.e = 0.8502196;
orbit.i = 11.94525; = 334.75006;
orbit.w = 186.23352;
orbit.n = 0;
orbit.JD = e_JD;
orbit.a = 2.214743;
orbit.e = 0.848232;
orbit.i = 11.7790; = 334.5731;
orbit.w = 186.5356;
orbit.n = 0;//0.2990330;
/* get mean anomaly */
if (orbit.n == 0.0)
orbit.n = ln_get_ell_mean_motion(orbit.a);
M = ln_get_ell_mean_anomaly(orbit.n, JD - orbit.JD);
"(Mean Anomaly) M when n is %f and JD diff is %f = %f\n",
orbit.n, JD - orbit.JD, M);
/* solve kepler for orbit */
E = ln_solve_kepler(orbit.e, M);
"(Equation of kepler) E when e is %f and M is %f = %f\n",
orbit.e, M, E);
/* true anomaly */
v = ln_get_ell_true_anomaly(orbit.e, E);
"(True Anomaly) v when e is %f and E is %f = %f\n", orbit.e, E, v);
/* radius vector */
r = ln_get_ell_radius_vector(M, orbit.e, E);
"(Radius Vector) r when e is %f and E is %f = %f\n", orbit.e, E, r);
/* geocentric rect coords */
ln_get_ell_geo_rect_posn(&orbit, JD, &posn);
"(Geocentric Rect Coords X) for comet Encke %f\n", posn.X);
"(Geocentric Rect Coords Y) for comet Encke %f\n", posn.Y);
"(Geocentric Rect Coords Z) for comet Encke %f\n", posn.Z);
/* rectangular coords */
ln_get_ell_helio_rect_posn(&orbit, JD, &posn);
"(Heliocentric Rect Coords X) for comet Encke %f\n", posn.X);
"(Heliocentric Rect Coords Y) for comet Encke %f\n", posn.Y);
"(Heliocentric Rect Coords Z) for comet Encke %f\n", posn.Z);
/* ra, dec */
ln_get_ell_body_equ_coords(JD, &orbit, &equ);
fprintf(stdout, "(RA) for comet Encke %f\n", equ.ra);
fprintf(stdout, "(Dec) for comet Encke %f\n", equ.dec);
/* orbit length */
l = ln_get_ell_orbit_len(&orbit);
fprintf(stdout, "(Orbit Length) for comet Encke in AU %f\n", l);
/* orbital velocity at perihelion */
V = ln_get_ell_orbit_pvel(&orbit);
"(Orbit Perihelion Vel) for comet Encke in kms %f\n", V);
/* orbital velocity at aphelion */
V = ln_get_ell_orbit_avel(&orbit);
fprintf(stdout, "(Orbit Aphelion Vel) for comet Encke in kms %f\n", V);
/* average orbital velocity */
V = ln_get_ell_orbit_vel(JD, &orbit);
fprintf(stdout, "(Orbit Vel JD) for comet Encke in kms %f\n", V);
/* comet sun distance */
dist = ln_get_ell_body_solar_dist(JD, &orbit);
fprintf(stdout, "(Body Solar Dist) for comet Encke in AU %f\n", dist);
/* comet earth distance */
dist = ln_get_ell_body_earth_dist(JD, &orbit);
fprintf(stdout, "(Body Earth Dist) for comet Encke in AU %f\n", dist);
/* rise, set and transit */
if (ln_get_ell_body_rst(JD, &observer, &orbit, &rst) != 0)
fprintf(stdout, "Comet is circumpolar\n");
else {
ln_get_local_date(rst.rise, &rise);
ln_get_local_date(rst.transit, &transit);
ln_get_local_date(rst.set, &set);
print_date("Rise", &rise);
print_date("Transit", &transit);
print_date("Set", &set);
return 0;