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\name{sun-methods}
\docType{methods}
\alias{crepuscule}
\alias{sunriset}
\alias{solarnoon}
\alias{solarpos}
\alias{crepuscule-methods}
\alias{crepuscule,SpatialPoints,POSIXct-method}
\alias{crepuscule,matrix,POSIXct-method}
\alias{solarnoon-methods}
\alias{solarnoon,SpatialPoints,POSIXct-method}
\alias{solarnoon,matrix,POSIXct-method}
\alias{solarpos-methods}
\alias{solarpos,SpatialPoints,POSIXct-method}
\alias{solarpos,matrix,POSIXct-method}
\alias{sunriset-methods}
\alias{sunriset,SpatialPoints,POSIXct-method}
\alias{sunriset,matrix,POSIXct-method}
\title{Methods for sun ephemerides calculations}
\description{Functions for calculating sunrise, sunset, and times of
dawn and dusk, with flexibility for the various formal definitions.
They use algorithms provided by the National Oceanic & Atmospheric
Administration (NOAA).}
\usage{
\S4method{crepuscule}{SpatialPoints,POSIXct}(crds, dateTime, solarDep, direction=c("dawn", "dusk"),
POSIXct.out=FALSE)
\S4method{crepuscule}{matrix,POSIXct}(crds, dateTime,
proj4string=CRS("+proj=longlat +datum=WGS84"), solarDep,
direction=c("dawn", "dusk"), POSIXct.out=FALSE)
\S4method{sunriset}{SpatialPoints,POSIXct}(crds, dateTime, direction=c("sunrise", "sunset"),
POSIXct.out=FALSE)
\S4method{sunriset}{matrix,POSIXct}(crds, dateTime,
proj4string=CRS("+proj=longlat +datum=WGS84"),
direction=c("sunrise", "sunset"), POSIXct.out=FALSE)
\S4method{solarnoon}{SpatialPoints,POSIXct}(crds, dateTime, POSIXct.out=FALSE)
\S4method{solarnoon}{matrix,POSIXct}(crds, dateTime, proj4string=CRS("+proj=longlat +datum=WGS84"),
POSIXct.out=FALSE)
\S4method{solarpos}{SpatialPoints,POSIXct}(crds, dateTime, ...)
\S4method{solarpos}{matrix,POSIXct}(crds, dateTime, proj4string=CRS("+proj=longlat +datum=WGS84"), ...)
}
\arguments{
\item{crds}{a \code{SpatialPoints} or \code{matrix} object, containing
x and y coordinates (in that order).}
\item{dateTime}{a POSIXct object with the date and time associated to
calculate ephemerides for points given in crds.}
\item{solarDep}{numeric vector with the angle of the sun below the
horizon in degrees.}
\item{direction}{one of "dawn", "dusk", "sunrise", or "sunset",
indicating which ephemerides should be calculated.}
\item{POSIXct.out}{logical indicating whether POSIXct output should be
included.}
\item{proj4string}{string with valid projection string describing the
projection of data in \code{crds}.}
\item{\dots}{other arguments passed through.}
}
\details{NOAA used the reference below to develop their Sunrise/Sunset
\url{http://www.srrb.noaa.gov/highlights/sunrise/sunrise.html} and Solar
Position
\url{http://www.srrb.noaa.gov/highlights/sunrise/azel.html}
Calculators. The algorithms include corrections for atmospheric
refraction effects.
Input can consist of one location and at least one POSIXct times, or one
POSIXct time and at least one location. \var{solarDep} is recycled as
needed.
Do not use the daylight savings time zone string for supplying
\var{dateTime}, as many OS will not be able to properly set it to
standard time when needed.}
\section{Warning}{Compared to NOAA's original Javascript code, the
sunrise and sunset estimates from this translation may differ by +/- 1
minute, based on tests using selected locations spanning the globe.
This translation does not include calculation of prior or next
sunrises/sunsets for locations above the Arctic Circle or below the
Antarctic Circle.}
\value{\code{crepuscule}, \code{sunriset}, and \code{solarnoon} return a
numeric vector with the time of day at which the event occurs,
expressed as a fraction, if POSIXct.out is FALSE; otherwise they
return a data frame with both the fraction and the corresponding
POSIXct date and time. \code{solarpos} returns a matrix with the solar
azimuth (in degrees from North), and elevation.}
\note{NOAA notes that \dQuote{for latitudes greater than 72 degrees N
and S, calculations are accurate to within 10 minutes. For latitudes
less than +/- 72 degrees accuracy is approximately one minute.}}
\references{
Meeus, J. (1991) Astronomical Algorithms. Willmann-Bell, Inc.
}
\author{Sebastian P. Luque \email{spluque@gmail.com}, translated from
Greg Pelletier's \email{gpel461@ecy.wa.gov} VBA code (available from
\url{http://www.ecy.wa.gov/programs/eap/models.html}), who in turn
translated it from original Javascript code by NOAA (see Details).
Roger Bivand \email{roger.bivand@nhh.no} adapted the code to work with
\pkg{sp} classes.}
\examples{
## Location of Helsinki, Finland, in decimal degrees,
## as listed in NOAA's website
hels <- matrix(c(24.97, 60.17), nrow=1)
Hels <- SpatialPoints(hels, proj4string=CRS("+proj=longlat +datum=WGS84"))
d041224 <- as.POSIXct("2004-12-24", tz="EET")
## Astronomical dawn
crepuscule(hels, d041224, solarDep=18, direction="dawn", POSIXct.out=TRUE)
crepuscule(Hels, d041224, solarDep=18, direction="dawn", POSIXct.out=TRUE)
## Nautical dawn
crepuscule(hels, d041224, solarDep=12, direction="dawn", POSIXct.out=TRUE)
crepuscule(Hels, d041224, solarDep=12, direction="dawn", POSIXct.out=TRUE)
## Civil dawn
crepuscule(hels, d041224, solarDep=6, direction="dawn", POSIXct.out=TRUE)
crepuscule(Hels, d041224, solarDep=6, direction="dawn", POSIXct.out=TRUE)
solarnoon(hels, d041224, POSIXct.out=TRUE)
solarnoon(Hels, d041224, POSIXct.out=TRUE)
solarpos(hels, as.POSIXct(Sys.time(), tz="EET"))
solarpos(Hels, as.POSIXct(Sys.time(), tz="EET"))
sunriset(hels, d041224, direction="sunrise", POSIXct.out=TRUE)
sunriset(Hels, d041224, direction="sunrise", POSIXct.out=TRUE)
## Using a sequence of dates
Hels_seq <- seq(from=d041224, length.out=365, by="days")
up <- sunriset(Hels, Hels_seq, direction="sunrise", POSIXct.out=TRUE)
down <- sunriset(Hels, Hels_seq, direction="sunset", POSIXct.out=TRUE)
day_length <- down$time - up$time
plot(Hels_seq, day_length, type="l")
## Using a grid of spatial points for the same point in time
grd <- GridTopology(c(-179,-89), c(1,1), c(359,179))
SP <- SpatialPoints(coordinates(grd),
proj4string=CRS("+proj=longlat +datum=WGS84"))
wint <- as.POSIXct("2004-12-21", tz="GMT")
win <- crepuscule(SP, wint, solarDep=6, direction="dawn")
SPDF <- SpatialGridDataFrame(grd,
proj4string=CRS("+proj=longlat +datum=WGS84"),
data=data.frame(winter=win))
image(SPDF, axes=TRUE, col=cm.colors(40))
}
\keyword{methods}
\keyword{manip}
\keyword{utilities}
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