# function to derive sunrise/set times for Pat and Melanie
# load R source and then call as:
# times<-getSunTimes()
# need to make sure that the excel dates are in the right format (see below) before csv export

getSunTimes<-function(){

# load dependencies
library(rgdal)
library(maptools)

# set up variables for coordinate conversion

ukgrid ="+init=epsg:27700"
latlong ="+init=epsg:4326"

# read in csv data (Easting, Northing, Date (in format YYYY-MM-DD))
df<-read.csv(file.choose(), header=TRUE)

# convert dates to POSIXct
df$Date<-as.POSIXct(as.character(df$Date))

# convert coordinates to lat long
# extract coords
coords <-cbind(Easting =as.numeric(as.character(df$Easting)), Northing =as.numeric(as.character(df$Northing)))
# convert to SP
coords_BNG<-SpatialPoints(coords, proj4string=CRS(ukgrid))
# convert to Lat Long
coords_LL<-spTransform(coords_BNG, CRS(latlong))
# update column names
colnames(coords_LL@coords)[colnames(coords_LL@coords)=="Easting"]<-"Longitude"
colnames(coords_LL@coords)[colnames(coords_LL@coords)=="Northing"]<-"Latitude"

# derive times of sunrise and sunset
# sunrise times
sr<-sunriset(coords_LL, df$Date, direction="sunrise", POSIXct.out=TRUE)
# sunset times
ss<-sunriset(coords_LL, df$Date, direction="sunset", POSIXct.out=TRUE)

# return new dataframe with added sunrise and set times
output<-cbind(df, sr$time, ss$time)
names(output)[4] <- "Sunrise"
names(output)[5] <- "Sunset"

return(output)

}