#-----------
# Variables to enter: Sunset calculator for local sunset (both LST and UTC relative)
# ----------
let day=15
let month=7
let year=2005
let lat2=20.50
let lon2=-65.50

pi=$(echo "scale=10; 4*$((atan(1)))" | bc)

let n1=$((floor(275*$month/9)))
let n2=$((floor(($month+9)/12)))
let n3=$(((1+floor(($year-4*$((floor($year/4)))+2)/3))))
let n=$n1-($n2*$n3)+$day-30

lngHour=$(echo "scale=5; ($lon2/15)" | bc)

let t=$n+((18-($lngHour))/24)
let M=0.9856*$t-3.289
let L=$M+(1.916*$((sin($pi/180*$M)))+(0.02*$((sin($pi/180*2*$M)))))+282.634-360
let RA=180/$pi*$((atan(0.91764*$((tan($pi/180*$L))))))
let Lquadrant=$((floor($L/90)))*90
let RAquadrant=$((floor($RA/90)))*90
let RA2=$RA+($Lquadrant-($RAquadrant))
let RAhr=$RA2/15
let sinDec=0.39782*$((sin($pi/180*$L)))
let cosDec=$((cos($pi/180*(180/$pi)*$((asin($sinDec))))))

let cosHa=$((cos($pi/180*90.833)))-($sinDec*$((sin($pi/180*$lat2))))
let cosHb=cosDec*$((cos($pi/180*$lat2)))
let cosH=$cosHa/$cosHb

let H=180/$pi*$((acos($cosH)))
let H2=$H/15
let T=$H2+$RAhr-(0.06571*$t)-6.622

let utc=24+$T-($lngHour)
let utchh=$((floor($utc)))
let utcmm=60*($utc-($utchh))

printf %.0f $lngHour | read timezoneadj
let localthh=$utchh+$timezoneadj
printf %.0f $utcmm | read localtmm

echo
echo Local sunset at the current storm position is: $localthh":"$localtmm LST
echo
echo Local sunset at the current storm position is: $localthh":"$localtmm UTC