Moon phase calculation
Dec 21, 2013
Moon phase is calculated using percent illumination.
The illuminated fraction of the moon can be calculated with moonAngle() in the oce package, as illustrated graphically and in R code below.
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library(oce)
par(mar=c(3, 3, 1, 1), mgp=c(2, 0.7, 0)) # tighten margins
t <- as.POSIXct("2013-12-1", tz="UTC") + seq(0, 28*24*3600, 3600)
f <- moonAngle(t=t, longitude=-63.6,
latitude=44.65)$illuminatedFraction
plot(t, f, type="l", xlab="Day of 2013", ylab="Moon fraction")
grid()
For interest, add full-moon time as indicated in an almanac.
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full <- as.POSIXct("2013-12-17 05:29:00", tz="AST")
abline(v=full)
mtext("full", at=full, side=3)
Exercises (a) Try this for other locations. (b) Detect full moon by using uniroot() on the rate of change of illumination.