# Solar Long Term Almanac

This is an Awk script to calculate the position of the sun for a given date and time and location.

```# s2k.awk - Sun 2000 from Meeus Astronomical Algorithms
# s2k is Copyright Daniel K. Allen, 2000-2003.
#
#  6 Dec 2000 - Created by Dan Allen.
# 24 Dec 2000 - Updated to 2nd edition of Meeus, p. 163.
#  3 Jan 2001 - Fixed bug in azimuth calculation; equation of time added.
#  3 Jul 2001 - Time zone set for west coast by a month heuristic.
#  5 Jul 2001 - Ported to Mac OS X, added tz and current date/time.
#  2 Sep 2001 - More accurate Equation of Time.
# 21 Mar 2002 - eqt & sunLong shown mod 360.
# 14 Oct 2002 - Uses apparent longitude and obliquity of the ecliptic for greater accuracy.
#  5 Sep 2003 - Removed external tool dependency.
#  5 May 2004 - Prints sunLong, not appLong.
# 21 Jun 2016 - Handles US TZs approximately; loc now West Yarmouth.

BEGIN { # all arguments and results are in decimal degrees
CONVFMT = OFMT = "%10.6f"
PI = 4*atan2(1,1)
D2R = PI/180
R2D = 180/PI
if (lat == "") lat = 41.6441  # your default latitude here
if (lon == "") lon = 70.2247  # your default longitude here
if (ARGC == 2 && (index(ARGV[1],"/") || index(ARGV[1],".")))
t = J2000(ARGV[1],12)
else if (ARGC == 3 && (index(ARGV[1],"/") || index(ARGV[1],".")))
t = J2000(ARGV[1],ARGV[2])
else if (ARGC == 4) {
tz = ARGV[3]
t = J2000(ARGV[1],ARGV[2])
}
else {
print "Usage: awk -f s2k mm.ddyyyy | mm/dd/yyyy [hh[:mm[:ss]]] [tz]   Sun 2000"
exit 1
}
l = Mod(280.46646 + 36000.76983*t + 0.0003032*t*t,360) # mean longitude
m = Mod(357.52911 + 35999.05029*t + -0.0001537*t*t,360) # mean anomaly
e = (0.016708634 + -0.000042037*t + -0.0000001267*t*t) # eccentricity
c = (1.914602 + -0.004817*t + -0.000014*t*t) * Sin(m) # equation of center
c += (0.019993 + -0.000101*t) * Sin(m*2)
c += (0.000289) * Sin(m*3)
sunLong = Mod(l + c,360)
trueAnom = m + c
radius = (1.000001018 * (1 - e*e)) / (1 + e * Cos(trueAnom))
ecliptic = (84381.448 + -46.815*t - 0.00059*t*t + 0.001813*t*t*t) / 3600
omega = 125.04 - 1934.136*t
apparentLong = sunLong - 0.00569 - 0.00478*Sin(omega)
apparentEcliptic = ecliptic + 0.00256 * Cos(omega)
ra = Mod(ATan2(Cos(apparentEcliptic)*Sin(apparentLong),Cos(apparentLong)),360)
dec = ASin(Sin(apparentEcliptic)*Sin(apparentLong))
gst = (280.46061837 + 0.000387933*t*t + -2.58331180573E-8*t*t*t)
gst += t*36525*360.985647366
gst = Mod(gst,360)
lha = gst - lon - ra
eqt = (sunLong - ra) - (sunLong - l) # positive values occur before UT
if (eqt > 300) eqt = eqt - 360
alt = ASin(Sin(lat) * Sin(dec) + Cos(lat) * Cos(dec) * Cos(lha))
az = 180+ATan2(Sin(lha),(Cos(lha) * Sin(lat) - Cos(lat)*Tan(dec)))
print " Lon:",lon," Lat:",lat
print " GHA:",Mod(lha+lon,360)," EQT:",eqt
print "  RA:",ra, " Dec:",dec
print "  AZ:",az, " Alt:",alt
}

function Floor(x)  { return x < 0 ? int(x) - 1 : int(x) }
function Mod(x,y)  { return x - y * Floor(x/y) }
function Sin(x)    { return sin(x*D2R) }
function Cos(x)    { return cos(x*D2R) }
function Tan(x)    { return Sin(x)/Cos(x) }
function ASin(x)   { return atan2(x,sqrt(1 - x * x))*R2D }
function ATan2(y,x){ return atan2(y,x)*R2D }

function J2000(date,time,      m,d,y,a) {
if (index(date,"/") > 0) {      # mm/dd/yy or mm/dd/yyyy - Y2K compatible
split(date,a,"/")
m = a[1]
d = a[2]
y = a[3] < 50 ? 2000 + a[3] : a[3] < 100 ? 1900 + a[3] : a[3]
delete a
}
else if (index(date,".") > 0) { # mm.ddyyyy - HP calculator compatible
m = int(date)
d = int((date - m)*100)
y = int(1000000*(date - int(date) - d/100)+0.5)
}
if (tz == "") { # approximate tz code
tz = m > 3 && m < 11 ? 4 : 5 # EDT or EST
if (lon > 87) tz++ # CDT or CST
if (lon > 102) tz++ # MDT or MST
if (lon > 114.5) tz++ # PDT or PST
}
split(time,a,":")
print "Time:",y,m,d,a[1],a[2],a[3]," TZ:",tz
return (Julian(y,m,d,a[1]+tz,a[2],a[3]) - Julian(2000,1,1,12,0,0))/36525
}

function Julian(year,month,day,hr,min,sec,     m,y,a,b,jd)
{
if (month <= 2) { y = year - 1; m = month + 12 }
else { y = year; m = month }
a = int(y/100)
if (y < 1582 || y == 1582 && (m < 10 || m == 10 && day <=4)) b = 0
else b = 2 - a + int(a/4)
jd = int(365.25*(y+4716)) + int(30.6001*(m+1)) + day + b - 1524.5
jd += (hr-12)/24.0 + min/(24.0*60) + sec/(24.0*3600)
return jd
}
```