springEquinoxPlot.frink

View or download springEquinoxPlot.frink in plain text format


// This program calculates and plots the date of the spring equinox
// over the course of a few centuries.  I made this because I'm old
// and I remember the equinoxes usually being on the 21st (probably
// in UTC.)  This shows how the date of the equinox drifts over
// time (and is corrected back by leap year rules.)

use sun.frink
use planets.frink
use secant.frink

longfunc = {|date| highAccuracySunApparentLongitude[date]}
// Let's calculate the longitude angle that corresponds to 1 millisecond of
// time
angleres = ms circle/solaryear

tz = "US/Mountain"

g = new graphics
g.font["Monospaced", .1 day]
p = new polyline
for y = 1800 to 2150
{
   springEquinox = secantInvert[longfunc,
                                180 degrees,
                                springEquinox[y] - 1 hour,
                                springEquinox[y] + 1 hour,
                                angleres]
   d = springEquinox - parseDate["$y $tz"]
   println["$y\t" + (springEquinox -> tz) + "\t" + format[d, "days", 2]]
   p.addPoint[y,-d]
   g.fillEllipseCenter[y, -d, 1, .01 day] 
}

g.color[0,0,0,.3]
g.add[p]

for y = 1800 to 2150 step 50
{
   g.color[0,0,0,.3]
   g.line[y, -77 days, y, -80 days]
   g.color[0,0,0]
   g.text["$y", y, -77 days, "center", "top"]
}

df = ### MM-dd ###
for d = 77 days to 80 days step day
{
   g.color[0,0,0,.3]
   g.line[1800, -d, 2150, -d]
   usualDate = (parseDate["2015 $tz"] + d) -> [df, tz]
   leapDate = (parseDate["2016 $tz"] + d) -> [df, tz]
   g.color[0,0,0]
   g.text[format[d,"days",0] + "   \n(usually " + usualDate + "   \n on leap years, $leapDate)  ", 1800, -d, "right", "center"]
}

g.color[0,0,0,0]
g.line[1700, -77 days, 1700, -80 days]

g.color[0,0,0]
g.font["SansSerif", .15 day]
g.text["Spring Equinox (days after beginning of year, $tz)", 1975, -80.1 days, "center", "bottom"]
g.show[]

stz = tz =~ %s/\//./g

g.write["springequinox$stz.png",2000,1000]
g.write["springequinox$stz.html",2000,1000]
g.write["springequinox$stz.svg",2000,1000]
browse["springequinox$stz.html"]
browse["springequinox$stz.svg"]
 


View or download springEquinoxPlot.frink in plain text format


This is a program written in the programming language Frink.
For more information, view the Frink Documentation or see More Sample Frink Programs.

Alan Eliasen was born 18661 days, 7 hours, 33 minutes ago.