Satellite Geolocation for Animal Tracking

Description Usage Arguments Details Value Examples

Estimate time of sunrise or sunset for a given day and location

twilight(tm, lon, lat, rise, zenith = 96, iters = 3, closest = FALSE)

sunrise(tm, lon, lat, zenith = 96, iters = 3, closest = FALSE)

sunset(tm, lon, lat, zenith = 96, iters = 3, closest = FALSE)

`tm`	vector of approximate times of twilight.
`lon`	vector of longitudes.
`lat`	vector of latitudes.
`rise`	logical vector indicating whether to compute rise or set.
`zenith`	the solar zenith angle that defines twilight.
`iters`	number of iteratve refinements made to the initial approximation.
`closest`	if `TRUE`, attempt to find the twilight closest to `tm`.

twilight uses an iterative algorithm to estimate times of sunrise and sunset.

Note that these functions return the twilight that occurs on the same date GMT as tm, and so sunset may occur before sunrise, depending upon latitude.

Solar declination and equation of time vary slowly over the day, and so the values of the Solar declination and equation of time at sunrise/sunset are well approximated by their values at 6AM/6PM local time. The sun's hour angle and hence sunrise/sunset for the required zenith can then be caclulates from these approximations. The calculation is then repeated using the approximate sunrise/sunset times to derive more accurate values of the Solar declination and equation of time and hence better approximations of sunrise/sunset. The process is repreated and is accurate to less than 2 seconds within 2 or 3 iterations.

It is possible that sunrise or sunset does occur for a given date and location. When closest is FALSE, the twilight returned on or before the (UTC) date of tm. When closest is TRUE, twilight attempts to return the twilight closest to the input time tm.

sunrise and sunset are simple wrappers for twilight.

a vector of twilight times.

## Approx location of Santa Barbara
lon <- -119.7022
lat <- 34.4191
## Sunrise and sunset for 8th April 2013 at Santa Barbara
day <- as.POSIXct("2013-04-08","GMT")
sunrise(day,lon,lat)
sunset(day,lon,lat)