ref: 61f4c085c402af665b436f6d010d61107551d8a3
dir: /sys/src/cmd/astro/nutate.c/
#include "astro.h"
void
nutate(void)
{
/*
* uses radian, radsec
* sets phi, eps, dphi, deps, obliq, gst, tobliq
*/
double msun, mnom, noded, dmoon;
/*
* nutation of the equinoxes is a wobble of the pole
* of the earths rotation whose magnitude is about
* 9 seconds of arc and whose period is about 18.6 years.
*
* it depends upon the pull of the sun and moon on the
* equatorial bulge of the earth.
*
* phi and eps are the two angles which specify the
* true pole with respect to the mean pole.
*
* all coeffieients are from Exp. Supp. pp.44-45
*/
mnom = 296.104608 + 13.0649924465*eday + 9.192e-3*capt2
+ 14.38e-6*capt3;
mnom *= radian;
msun = 358.475833 + .9856002669*eday - .150e-3*capt2
- 3.33e-6*capt3;
msun *= radian;
noded = 11.250889 + 13.2293504490*eday - 3.211e-3*capt2
- 0.33e-6*capt3;
noded *= radian;
dmoon = 350.737486 + 12.1907491914*eday - 1.436e-3*capt2
+ 1.89e-6*capt3;
dmoon *= radian;
node = 259.183275 - .0529539222*eday + 2.078e-3*capt2
+ 2.22e-6*capt3;
node *= radian;
/*
* long period terms
*/
phi = 0.;
eps = 0.;
dphi = 0.;
deps = 0.;
icosadd(nutfp, nutcp);
phi = -(17.2327+.01737*capt)*sin(node);
phi += sinadd(4, node, noded, dmoon, msun);
eps = cosadd(4, node, noded, dmoon, msun);
/*
* short period terms
*/
dphi = sinadd(4, node, noded, mnom, dmoon);
deps = cosadd(3, node, noded, mnom);
phi = (phi+dphi)*radsec;
eps = (eps+deps)*radsec;
dphi *= radsec;
deps *= radsec;
obliq = 23.452294 - .0130125*capt - 1.64e-6*capt2
+ 0.503e-6*capt3;
obliq *= radian;
tobliq = obliq + eps;
gst = 99.690983 + 360.9856473354*eday + .000387*capt2;
gst -= 180.;
gst = fmod(gst, 360.);
if(gst < 0.)
gst += 360.;
gst *= radian;
gst += phi*cos(obliq);
}