// This program calculates and plots the date of the autumn equinox // over the course of a few centuries. 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" startyear = 1800 endyear = 2150 g = new graphics g.font["Monospaced", .1 day] p = new polyline for y = startyear to endyear { autumnEquinox = secantInvert[longfunc, 0 degrees, autumnEquinox[y] - 1 hour, autumnEquinox[y] + 1 hour, angleres] d = autumnEquinox - parseDate["\$y \$tz"] println["\$y\t" + (autumnEquinox -> 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 = startyear to endyear step 50 { g.color[0,0,0,.3] g.line[y, -263 days, y, -267 days] g.color[0,0,0] g.text["\$y", y, -263 days, "center", "top"] } df = ### MM-dd ### for d = 263 days to 267 days step day { g.color[0,0,0,.3] g.line[startyear, -d, endyear, -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) ", startyear, -d, "right", "center"] } g.color[0,0,0,0] g.line[startyear-100, -263 days, startyear-100, -267 days] g.color[0,0,0] g.font["SansSerif", .15 day] g.text["Autumn Equinox (days after beginning of year, \$tz)", 1975, -267.1 days, "center", "bottom"] g.show[] stz = tz =~ %s/\//./g g.write["autumnequinox\$stz.png",2000,1000] g.write["autumnequinox\$stz.html",2000,1000] g.write["autumnequinox\$stz.svg",2000,1000] //browse["autumnequinox\$stz.html"] //browse["autumnequinox\$stz.svg"]