Download or view DurinsDay.frink in plain text format
// Calculate Durin's Day, from The Hobbit.
//
// "The first day of the dwarves' New Year, said Thorin, is as all should
// know the first day of the last moon of Autumn on the threshold of Winter.
// We still call it Durin's Day when the last moon of Autumn and the sun are
// in the sky together. But this will not help us much, I fear, for it passes
// our skill in these days to guess when such a time will come again."
//
// Many of these definitions are vague, and we use some conventions to put
// the dates earlier than a strict astronomical reading might yield. The
// astronomical figures are rigorous, but the clarity of Thorin's definition
// is in some question.
use sun.frink
// How many years should be checked?
years = 25
// How chatty should we be?
verbose[] :=
{
// true: report everything
// false: report New Year's Days and Durin's Days
return true
}
y = parseInt[input["Enter year: ",2000]]
limit = y + years
// GPS-surveyed position for east side of MIT's Infinite corridor, Cambridge,
// Massachusetts.
lat = 42.36002 degrees North
long = 71.09332 degrees West
// NWS for Denver, CO
//lat = 39.78 degrees North
//long = 104.88 degrees West
// Greenwich, England
//lat = DMS[51,28,38] North
//long = 0 degrees West
// Loop until we find a New Year's Day with:
// a sunrise or sunset when the moon is entirely above the horizon
// AND the separation between sun and moon is at least 7.5 degrees,
// which has been proven to be minimal for seeing a crescent.
// See http://www.skyandtelescope.com/observing/objects/projects/3308686.html
// and http://www.earthsky.org/faq/young-moon-visibility
do
{
found = false
dawn = false
nyd = NewYearsDay[y]
print[" New Year's Day occurs on "]
if (verbose[])
println["$nyd."]
else
println[(nyd -> ###yyyy-MM-dd###) + "."]
if (verbose[])
printSunMoonPos[nyd, lat, long]
sunrise = sunrise[nyd, lat, long]
sunset = sunset[nyd, lat, long]
if (verbose[])
println[" Sunrise is at $sunrise"]
[moonalt, sep] = printSunMoonPos[sunrise, lat, long]
if (moonalt > moonRadiusAngle[sunrise]) and (sep > 7.5 degrees)
{
found = true
dawn = true
}
if (verbose[])
println[" Sunset is at $sunset"]
[moonalt, sep] = printSunMoonPos[sunset, lat, long]
if (moonalt > moonRadiusAngle[sunset]) and (sep > 7.5 degrees)
{
found = true
dawn = false
}
if (! found) and (verbose[])
{
println[" This is not Durin's Day."]
println[]
}
if (found)
{
print["Durin's day occurs at "]
if (dawn)
print["dawn"]
else
print["dusk"]
println[" on " + (nyd -> ### MMMM d, yyyy ###) + "."]
if (verbose[])
println[]
}
y = y + 1
} while (y < limit)
// Print the position of sun and moon at a specified time.
printSunMoonPos[nyd, lat, long] :=
{
[sunaz, sunalt] = refractedSunAzimuthAltitude[nyd, lat, long]
[moonaz, moonalt] = refractedMoonAzimuthAltitude[nyd, lat, long]
sep = sunMoonAngularSeparation[nyd, lat, long]
if (verbose[])
{
illum = moonIlluminatedFraction[nyd]
illum1 = moonIlluminatedFraction[nyd + 1 minute]
if (illum1 > illum)
indicator = "waxing"
else
indicator = "waning"
println[" At this time, the altitude of the moon is: " +
format[moonalt, "degrees" , 5]]
println[" Angular separation: " + format[sep,"degrees",3]]
println[" Moon illuminated fraction: " +
format[illum, percent, 3] + "% ($indicator)"]
println[]
}
return [moonalt, sep]
}
// Find New Year's Day, the first day of the last month before the
// cross-quarter day between the autumn equinox and winter solstice.
NewYearsDay[y] :=
{
autumn = autumnEquinox[y]
winter = winterSolstice[y]
endautumn = autumn + (winter-autumn)/2
// Find the last new moon of "autumn."
nextmoon = endautumn
do
{
newmoon = newMoon[nextmoon]
nextmoon = newmoon - lunarmonth
} while newmoon > endautumn
// Return the new moon
return newmoon
}
Download or view DurinsDay.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 20145 days, 6 hours, 12 minutes ago.