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Lunar magnitude fix. #270

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Lunar magnitude fix. #270

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211eb52
Improve lunar magnitude.
mkbrewer May 16, 2024
dccac68
Replace spaces with tabs. Fix comment style.
mkbrewer May 16, 2024
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31 changes: 25 additions & 6 deletions libastro/circum.c
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Original file line number Diff line number Diff line change
Expand Up @@ -570,7 +570,8 @@ moon_cir (Now *np, Obj *op)
double el; /* elongation, rads east */
double ms; /* sun's mean anomaly */
double md; /* moon's mean anomaly */
double i;
double pang; /* moon's geocentric phase angle */
double i, p;

moon (mjed, &lam, &bet, &edistau, &ms, &md); /* mean ecliptic & EOD*/
sunpos (mjed, &lsn, &rsn, NULL); /* mean ecliptic & EOD*/
Expand All @@ -586,15 +587,33 @@ moon_cir (Now *np, Obj *op)
op->s_sdist = (float) sqrt (edistau*edistau + rsn*rsn
- 2.0*edistau*rsn*cos(el));

/* TODO: improve mag; this is based on a flat moon model. */
i = -12.7 + 2.5*(log10(PI) - log10(PI/2*(1+1.e-6-cos(el))))
+ 5*log10(edistau/.0025) /* dist */;
set_smag (op, i);

/* find phase -- allow for projection effects */
i = 0.1468*sin(el)*(1 - 0.0549*sin(md))/(1 - 0.0167*sin(ms));
op->s_phase = (float)((1+cos(PI-el-degrad(i)))/2*100);

/* moon's geocentric magnitude based on methodology described in Lane,
* Adair P.; Irvine, William M. 1973. Monochromatic phase curves and
* albedos for the lunar disk. Astronomical Journal, Vol. 78, pp. 267-277.
* Using V band data from their Table V.
*/
pang = raddeg(acos(op->s_phase / 50.0 - 1.0)); /* need range 0-180 degrees */
if (pang <= 40.0) {
p = pang - 20.0;
i = -12.72 + 0.0267 * p + 0.534;
} else {
p = pang - 80.0;
i = -12.72 + p * (0.03188 + p * (1.9621e-4 + p * 1.7256e-6)) + 2.14;
}

#define mean_em_dist (60.2665 * ERAD / MAU) /* mean earth-moon distance in AU
(factor from above reference). */
/* correct for earth-moon distance. */
i += 5.0 * log10(edistau / mean_em_dist);
/* correct for moon-sun distance */
i += 5.0 * log10(op->s_sdist);

set_smag (op, i);

/* fill moon's ra/dec, alt/az in op and update for topo dist */
cir_pos (np, bet, lam, &edistau, op);

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