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# EPHEM -- Calculate ephemeris data for the sun, return latitude and
# longitude of sub-earth point.
procedure ephem (month, day, year, hour, minute, second, image_r,
bn_degrees, cldc_degrees, verbose)
int month # time of observation
int day #
int year #
int hour #
int minute #
int second #
real image_r # image radius
real bn_degrees # solar latitude of sub-earth point (degrees)
real cldc_degrees # Carrington longitude of disk center
bool verbose # verbose flag
double radians_per_degree, pi, two_pi, st, d, dd
double ma, sin_ma, sin_two_ma, ml, e, e_squared, e_cubed
double ep, ea, r, image_r_squared, tl
double lan, bn, p, p_degrees
double sl1, sl2, cldc, cos_bn, x, cl1
double sin_three_ma, sec_bn, y
double dd_squared, dd_cubed, c, s, cl2, sln
int mac[12]
data mac/0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334/
begin
# This version ignores lunar and planetary perturbations.
radians_per_degree = .017453292519943d+0
pi = 3.1415926536d+0
two_pi = pi + pi
d = double(365 * year + (year - 1)/4 + mac[month] + day)
if (month >= 3 && mod(year, 4) == 0)
d = d + 1.d+0
st = double(second / 3600. + minute / 60. + hour)
d = d + st/24.d+0 -.5d+0
dd = d / 10000.d+0
dd_squared = dd * dd
dd_cubed = dd * dd * dd
# Mean anomaly.
ma = radians_per_degree * (358.475845d+0 + .985600267d+0 *
d - 1.12d-5 * dd_squared - 7.d-8 * dd_cubed)
ma = mod(ma, two_pi)
sin_ma = sin(ma)
sin_two_ma = sin(2.d+0 * ma)
sin_three_ma = sin(3.d+0 * ma)
# Mean longitude.
ml = radians_per_degree *
(279.696678d+0 + .9856473354d+0 * d + 2.267d-5 * dd_squared)
ml = mod(ml, two_pi)
# Ecentricity.
e = 0.01675104d+0 - 1.1444d-5 * dd - 9.4d-9 * dd_squared
e_squared = e * e
e_cubed = e_squared * e
# Obliquity.
ep = radians_per_degree * (23.452294d+0 -
3.5626d-3 * dd - 1.23d-7 * dd_squared + 1.03d-8 * dd_cubed)
# Eccentric anomaly.
ea = ma + (e - e_cubed/8.d+0) * sin_ma + e_squared * sin_two_ma/2.d+0 +
3.d+0 * e_cubed * sin_three_ma/8.d+0
# Radius vector.
r = 1.00000003d+0 * (1.d+0 - e * cos(ea))
# Image radius.
image_r = real(961.18d+0 / r)
image_r_squared = double(image_r * image_r)
# True longitude.
tl = ml + (2.d+0 * e - e_cubed/4.d+0) * sin_ma + 5.d+0 * e_squared *
sin_two_ma/4.d+0 + 13.d+0 * e_cubed * sin_three_ma/12.d+0
tl = mod(tl, two_pi)
# Longitude of ascending node of solar equator.
lan = radians_per_degree * (73.666667d+0 + 0.0139583d+0 *
(year + 50.d+0))
# Solar latitude of sub-earth point.
bn = asin(sin(tl - lan) * .12620d+0)
bn_degrees = real(bn / radians_per_degree)
if (verbose) {
call printf("B0 (degrees) = %10.5f\n")
call pargr(bn_degrees)
}
# Position angle of rotation axis.
p = atan(-cos(tl) * tan(ep)) + atan(-cos(tl - lan) * .12722d+0)
p_degrees = p/radians_per_degree
if (verbose) {
call printf("P-angle (degrees) = %10.5f\n")
call pargr(real(p_degrees))
}
# Carrington longitude of disk center.
sl1 = (d + 16800.d+0) * 360.d+0/25.38d+0
sl2 = mod(sl1, 360.d+0)
sln = 360.d+0 - sl2
sln = radians_per_degree * sln
cos_bn = cos(bn)
sec_bn = 1.d+0/cos_bn
c = +1.d+0
s = +1.d+0
x = -sec_bn * cos(tl - lan)
if (x < 0.)
c = -1.d+0
y = -sec_bn * sin(tl - lan) * .99200495d+0
if (y < 0.)
s = -1.d+0
cl1 = tan(tl - lan) * 0.99200495d+0
cl2 = atan(cl1)
if (s == 1.d+0 && c == 1.d+0)
cldc = sln + cl2
if (s == -1.d+0 && c == -1.d+0)
cldc = sln + cl2 + pi
if (s == 1.d+0 && c == -1.d+0)
cldc = sln + cl2 + pi
if (s == -1.d+0 && c == 1.d+0)
cldc = sln + cl2
if (cldc < 0.d+0)
cldc = cldc + two_pi
if (cldc > two_pi)
cldc = mod(cldc, two_pi)
cldc_degrees = real(cldc / radians_per_degree)
if (verbose) {
call printf ("L0 (degrees) = %10.5f\n")
call pargr (cldc_degrees)
}
end
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