#19 ra/dec to alt/az conversion

[Andre Moutinho]

Dear support,

I am trying to run the transform example code that is included in libnova distribution.
I also have made the same ra/dec to alt/az conversion using Stellarium and an online conversion tool but unfortunatelly I have got different results.

Here is libnova example result:
JD 2453121.013067
(Alnilam) Equ to Horiz ALT 24.410553
(Alnilam) Equ to Horiz AZ 247.293709
ALT 24:24:37.991195 AZ 247:17:37.352136
(Alnilam) Horiz to Equ RA 84.109089
(Alnilam) Horiz to Equ DEC -1.200000
RA 5:36:26.181311 DEC 1:12:0.000000

Please se attached picture showing different result: ALT 26:34:34.9 AZ 119:26:56.9

I have also used this online calculator
http://www.convertalot.com/celestial_horizon_co-ordinates_calculator.html.
Its result was consistent with Stellarium but also different from your
LIBNOVA example:
Online tool result: ALT 26.65 AZ 119.55

Notice I had to put my local computer time (GMT -3) to get the correct JD time.

Hope you can help me.

Best
Andre

[Andre Moutinho]

Here is the example code:
struct lnh_equ_posn hobject, hequ;
struct lnh_lnlat_posn hobserver;
struct ln_equ_posn object, equ;
struct ln_hrz_posn hrz;
struct lnh_hrz_posn hhrz;
struct ln_lnlat_posn observer;
struct ln_date date;
double JD;

/*
        * observers position
        * longitude is measured positively eastwards
        * i.e. Long 5d36m30W (Leon, Spain) = 354d24m30
        * Lat for Leon = Lat 42d35m40 N
        */
hobserver.lng.degrees = -5;
hobserver.lng.minutes = 36;
hobserver.lng.seconds = 30.0;
hobserver.lat.degrees = 42;
hobserver.lat.minutes = 35;
hobserver.lat.seconds = 40.0;
/* Alnilam */
hobject.ra.hours = 5;
hobject.ra.minutes = 36;
hobject.ra.seconds = 27.0;
hobject.dec.neg = 1;
hobject.dec.degrees = 1;
hobject.dec.minutes = 12;
hobject.dec.seconds = 0.0;
/* UT date and time */
date.years = 2004;
date.months = 4;
date.days = 25;
date.hours = 12;
date.minutes = 18;
date.seconds = 49.0;
JD = ln_get_julian_day(&date);
fprintf(stdout, "JD %f\n", JD);
ln_hequ_to_equ(&hobject, &object);
ln_hlnlat_to_lnlat(&hobserver, &observer);
ln_get_hrz_from_equ(&object, &observer, JD, &hrz);
fprintf(stdout, "(Alnilam) Equ to Horiz ALT %f\n", hrz.alt);
fprintf(stdout, "(Alnilam) Equ to Horiz AZ %f\n", hrz.az);
ln_hrz_to_hhrz(&hrz, &hhrz);
fprintf(stdout, "ALT %d:%d:%f  AZ %d:%d:%f\n",
        hhrz.alt.degrees, hhrz.alt.minutes, hhrz.alt.seconds,
        hhrz.az.degrees, hhrz.az.minutes, hhrz.az.seconds);
ln_get_equ_from_hrz (&hrz, &observer, JD, &equ);
fprintf(stdout, "(Alnilam) Horiz to Equ RA %f\n", equ.ra);
fprintf(stdout, "(Alnilam) Horiz to Equ DEC %f\n", equ.dec);
ln_equ_to_hequ(&equ, &hequ);
fprintf(stdout, "RA %d:%d:%f  DEC %d:%d:%f\n",
        hequ.ra.hours, hequ.ra.minutes, hequ.ra.seconds,
        hequ.dec.degrees, hequ.dec.minutes, hequ.dec.seconds);