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39 lines
1.2 KiB
Fortran
39 lines
1.2 KiB
Fortran
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subroutine libration(jd,RA,Dec,xl,b)
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! Compute optical libration of moon at jd: that is, the sub-observer
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! point (xl,b) in selenographic coordinates. RA and Dec are
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! topocentric values.
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implicit real*8 (a-h,o-z)
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parameter (RADS=0.0174532925199433d0)
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parameter (TWOPI=6.28318530717959d0)
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real*8 jd,j2000,mjd,lambda
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j2000=2451545.0d0
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RA2000=RA
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Dec2000=Dec
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year=2000.0d0 + (jd-j2000)/365.25d0
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mjd=jd-2400000.d0
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call sla_PRECES('FK5',year,2000.d0,RA2000,Dec2000)
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call sla_EQECL(RA2000,Dec2000,mjd,lambda,beta)
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day=jd - j2000
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t = day / 36525.d0
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xi = 1.54242 * RADS
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ft = 93.2720993 + 483202.0175273 * t - .0034029 * t * t
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b= ft / 360
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a = 360 * (b - floor(b))
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if (a.lt.0.d0) a = 360 + a;
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f=a/57.2957795131d0
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omega=sla_dranrm(2.182439196d0 - t*33.7570446085d0 + t*t*3.6236526d-5)
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w = lambda - omega
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y = sin(w) * cos(beta) * cos(xi) - sin(beta) * sin(xi)
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x = cos(w) * cos(beta)
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a = sla_dranrm(atan2(y, x))
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xl = a - f
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if(xl.lt.-0.25*TWOPI) xl=xl+TWOPI !Fix 'round the back' angles
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if(xl.gt.0.25*TWOPI) xl=xl-TWOPI
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b = asin(-sin(w) * cos(beta) * sin(xi) - sin(beta) * cos(xi))
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return
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end subroutine libration
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