sunAngle: Solar Angle as Function of Space and Time

Description

This calculates solar angle, based on a NASA-provided Fortran program, which (according to comments in the code) is in turn based on "The Astronomical Almanac".

Usage

sunAngle(t, longitude = 0, latitude = 0, useRefraction = FALSE)

Arguments

time, a POSIXt object (converted to timezone "UTC", if it is not already in that timezone), a character or numeric value that corresponds to such a time.

longitude

observer longitude in degrees east.

latitude

observer latitude in degrees north.

useRefraction

boolean, set to TRUE to apply a correction for atmospheric refraction.

Value

A list containing the following:

time the time
azimuth, in degrees eastward of north, from 0 to 360.
altitude, in degrees above the horizon, ranging from -90 to 90.
diameter, solar diameter, in degrees.
distance to sun, in astronomical units.
declination angle in degrees, computed with sunDeclinationRightAscension().
rightAscension angle in degrees, computed with sunDeclinationRightAscension().

References

Regarding declination and rightAscension, see references in the documentation for sunDeclinationRightAscension(). The other items are based on Fortran code retrieved from ftp://climate1.gsfc.nasa.gov/wiscombe/Solar_Rad/SunAngles/sunae.f on 2009-11-1. Comments in that code list as references:

Michalsky, J., 1988: The Astronomical Almanac's algorithm for approximate solar position (1950-2050), Solar Energy 40, 227-235

The Astronomical Almanac, U.S. Gov't Printing Office, Washington, D.C. (published every year).

The code comments suggest that the appendix in Michalsky (1988) contains errors, and declares the use of the following formulae in the 1995 version the Almanac:

p. A12: approximation to sunrise/set times
p. B61: solar altitude (AKA elevation) and azimuth
p. B62: refraction correction
p. C24: mean longitude, mean anomaly, ecliptic longitude, obliquity of ecliptic, right ascension, declination, Earth-Sun distance, angular diameter of Sun
p. L2: Greenwich mean sidereal time (ignoring T^2, T^3 terms)

The code lists authors as Dr. Joe Michalsky and Dr. Lee Harrison (State University of New York), with modifications by Dr. Warren Wiscombe (NASA Goddard Space Flight Center).

Examples

Run this code

# NOT RUN {
rise <- as.POSIXct("2011-03-03 06:49:00", tz="UTC") + 4*3600
set <- as.POSIXct("2011-03-03 18:04:00", tz="UTC") + 4*3600
mismatch <- function(lonlat)
{
    sunAngle(rise, lonlat[1], lonlat[2])$altitude^2 + sunAngle(set, lonlat[1], lonlat[2])$altitude^2
}
result <- optim(c(1,1), mismatch)
lon.hfx <- (-63.55274)
lat.hfx <- 44.65
dist <- geodDist(result$par[1], result$par[2], lon.hfx, lat.hfx)
cat(sprintf("Infer Halifax latitude %.2f and longitude %.2f; distance mismatch %.0f km",
            result$par[2], result$par[1], dist))

# }

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