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Sun and moon paths in daily sky

Dan Kelley Blog/R
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Introduction

This blog started because I was interested in the sunrise position on the winter solstice of 2013. With the Spring equinox, I’m trying something different: plotting the paths of the sun and moon through the day.

The code shown here produces a daily graph, and I have a cron job running on a machine so that this graph is visible to anyone at my website.

Procedure

The Oce package has functions called moonAngle() and sunAngle() that do the calculations. The rest of this code sets up and graphs the results. The horizon is on the outer edge of the circle. Hours (local time) are marked along the sun and moon paths. East-west and North-south lines are shown, and they intersect at the zenith.

1
library(oce)

## Loading required package: methods

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angles <- function(day = Sys.Date(), lon = -63.61, lat = 44.67, tz = "America/Halifax", 
    sun = TRUE) {
    tUTC <- t <- seq(as.POSIXct(paste(day, "00:00:00"), tz = tz), length.out = 240, 
        by = "6 min")
    attributes(tUTC)$tzone <- "UTC"
    a <- if (sun) 
        sunAngle(tUTC, lon = lon, lat = lat) else moonAngle(tUTC, lon = lon, lat = lat)
    invisible <- a$altitude < 0
    a$altitude[invisible] <- NA
    a$azimuth[invisible] <- NA
    list(t = t, altitude = a$altitude, azimuth = a$azimuth)
}

day <- Sys.Date()
tz <- "America/Halifax"
s <- angles()
m <- angles(sun = FALSE)
max <- 1.04 * max(c(s$altitude, m$altitude), na.rm = TRUE)

par(mar = rep(0.5, 4))
theta <- seq(0, 2 * pi, length.out = 24 * 10)
radiusx <- cos(theta)
radiusy <- sin(theta)

# Horizon and labels+lines for EW and NS
plot(radiusx, radiusy, type = "l", col = "gray", asp = 1, axes = FALSE, xlab = "", 
    ylab = "")
lines(c(-1, 1), c(0, 0), col = "gray")
lines(c(0, 0), c(-1, 1), col = "gray")
D <- 1.06
text(0, -D, "S", xpd = TRUE)  # xpd so can go in margin
text(-D, 0, "W", xpd = TRUE)
text(0, D, "N", xpd = TRUE)
text(D, 0, "E", xpd = TRUE)

## Moon
mx <- (90 - m$altitude)/90 * cos(pi/180 * (90 - m$azimuth))
my <- (90 - m$altitude)/90 * sin(pi/180 * (90 - m$azimuth))
lines(mx, my, col = "blue", lwd = 3)
t <- s$t
mlt <- as.POSIXct(sprintf("%s %02d:00:00", day, 1:24), tz = tz)
ti <- unlist(lapply(mlt, function(X) which.min(abs(X - t))))
points(mx[ti], my[ti], pch = 20, cex = 3, col = "white")
text(mx[ti], my[ti], 1:24, cex = 3/4)

## Sun
sx <- (90 - s$altitude)/90 * cos(pi/180 * (90 - s$azimuth))
sy <- (90 - s$altitude)/90 * sin(pi/180 * (90 - s$azimuth))
lines(sx, sy, col = "red", lwd = 3)
slt <- as.POSIXct(sprintf("%s %02d:00:00", day, 1:24), tz = tz)
si <- unlist(lapply(slt, function(X) which.min(abs(X - t))))
points(sx[ti], sy[ti], pch = 20, cex = 3, col = "white")
text(sx[ti], sy[ti], 1:24, cex = 3/4)

mtext(paste("Halifax NS", day, sep = "\n"), side = 3, adj = 0, line = -2)
mtext("Red sun\nBlue moon", side = 3, adj = 1, line = -2)

Resources

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