Functions for calculating the timing of solar positions, given geographical coordinates and dates. They can be also used to find the time for an arbitrary solar elevation between 90 and -90 degrees by supplying "twilight" angle(s) as argument.

day_night(date = lubridate::now(tzone = "UTC"),
  tz = lubridate::tz(date), geocode = tibble::tibble(lon = 0, lat =
  51.5, address = "Greenwich"), twilight = "none", unit.out = "hours")

day_night_fast(date, tz, geocode, twilight, unit.out)

noon_time(date = lubridate::today(), tz = lubridate::tz(date),
  geocode = tibble::tibble(lon = 0, lat = 51.5, address = "Greenwich"),
  twilight = "none", unit.out = "datetime")

sunrise_time(date = lubridate::today(), tz = lubridate::tz(date),
  geocode = tibble::tibble(lon = 0, lat = 51.5, address = "Greenwich"),
  twilight = "sunlight", unit.out = "datetime")

sunset_time(date = lubridate::today(), tz = lubridate::tz(date),
  geocode = tibble::tibble(lon = 0, lat = 51.5, address = "Greenwich"),
  twilight = "sunlight", unit.out = "datetime")

day_length(date = lubridate::now(), tz = "UTC",
  geocode = tibble::tibble(lon = 0, lat = 51.5, address = "Greenwich"),
  twilight = "sunlight", unit.out = "hours")

night_length(date = lubridate::now(), tz = "UTC",
  geocode = tibble::tibble(lon = 0, lat = 51.5, address = "Greenwich"),
  twilight = "sunlight", unit.out = "hours")

Arguments

date

"vector" of POSIXct times or Date objects, any valid TZ is allowed, default is current date at Greenwich.

tz

character vector indicating time zone to be used in output.

geocode

data frame with one or more rows and variables lon and lat as numeric values (degrees). If present, address will be copied to the output.

twilight

character string, one of "none", "rim", "refraction", "sunlight", "civil", "nautical", "astronomical", or a numeric vector of length one, or two, giving solar elevation angle(s) in degrees (negative if below the horizon).

unit.out

character string, One of "datetime", "day", "hour", "minute", or "second".

Value

A tibble with variables day, tz, twilight.rise, twilight.set, longitude, latitude, address, sunrise, noon, sunset, daylength, nightlength or the corresponding individual vectors.

noon_time, sunrise_time and sunset_time return a vector of POSIXct times

day_length and night_length return numeric a vector giving the length in hours

Details

Twilight names are interpreted as follows. "none": solar elevation = 0 degrees. "rim": upper rim of solar disk at the horizon or solar elevation = -0.53 / 2. "refraction": solar elevation = 0 degrees + refraction correction. "sunlight": upper rim of solar disk corrected for refraction, which is close to the value used by the online NOAA Solar Calculator. "civil": -6 degrees, "naval": -12 degrees, and "astronomical": -18 degrees. Unit names for output are as follows: "day", "hours", "minutes" and "seconds" times for sunrise and sunset are returned as times-of-day since midnight expressed in the chosen unit. "date" or "datetime" return the same times as datetime objects with TZ set (this is much slower than "hours"). Day length and night length are returned as numeric values expressed in hours when `"datetime"' is passed as argument to unit.out. If twilight is a numeric vector of length two, the element with index 1 is used for sunrise and that with index 2 for sunset.

Note

This function is an implementation of Meeus equations as used in NOAAs on-line web calculator, which are very precise and valid for a very broad range of dates. For sunrise and sunset the times are affected by refraction in the atmosphere, which does in turn depend on weather conditions. The effect of refraction on the apparent position of the sun is only an estimate based on "typical" conditions. The more tangential to the horizon is the path of the sun, the larger the effect of refraction is on the times of visual occlusion of the sun behind the horizon---i.e. the largest timing errors occur at high latitudes. The computation is not defined for latitudes 90 and -90 degrees, i.e. at the poles.

There exists a different R implementation of the same algorithms called "AstroCalcPureR" available as function astrocalc4r in package 'fishmethods'. Although the equations used are almost all the same, the function signatures and which values are returned differ. In particular, the present implementation splits the calculation into two separate functions, one returning angles at given instants in time, and a separate one returning the timing of events for given dates.

In the current implementation functions sunrise_time, noon_time, sunset_time and day_length are wrappers on day_night, so if more than one quantity is needed it is preferable to directly call day_night as it will be faster.

night_length returns the length of night-time conditions in one day (00:00:00 to 23:59:59), rather than the length of the night between two consecutive days.

Warning

Be aware that R's Date class does not save time zone metadata. This can lead to ambiguities in the current implementation based on time instants. The argument passed to date should be of class POSIXct, in other words an instant in time, from which the correct date will be computed based on the tz argument.

References

The primary source for the algorithm used is the book: Meeus, J. (1998) Astronomical Algorithms, 2 ed., Willmann-Bell, Richmond, VA, USA. ISBN 978-0943396613.

A different implementation is available at https://www.nefsc.noaa.gov/AstroCalc4R/ and in R paclage astrocalc4r. In 'fishmethods' (= 1.11-0) there is a bug in function astrocalc4r() that affects sunrise and sunset times.

An interactive web page using the same algorithms is available at https://www.esrl.noaa.gov/gmd/grad/solcalc/. There are small differences in the returned times compared to our function that seem to be related to the estimation of atmospheric refraction (about 0.1 degrees).

See also

Examples

library(lubridate)
#> #> Attaching package: 'lubridate'
#> The following object is masked from 'package:base': #> #> date
my.geocode <- data.frame(lat = 60, lon = 25) day_night(ymd("2015-05-30"), geocode = my.geocode)
#> # A tibble: 1 x 12 #> day tz twilight.rise twilight.set longitude latitude address sunrise #> <date> <chr> <dbl> <dbl> <dbl> <dbl> <chr> <dbl> #> 1 2015-05-30 UTC 0 0 25 60 NA 1.38 #> # ... with 4 more variables: noon <dbl>, sunset <dbl>, daylength <dbl>, #> # nightlength <dbl>
day_night(ymd("2015-05-30") + days(1:10), geocode = my.geocode, twilight = "civil")
#> # A tibble: 10 x 12 #> day tz twilight.rise twilight.set longitude latitude address #> <date> <chr> <dbl> <dbl> <dbl> <dbl> <chr> #> 1 2015-05-31 UTC -6 -6 25 60 NA #> 2 2015-06-01 UTC -6 -6 25 60 NA #> 3 2015-06-02 UTC -6 -6 25 60 NA #> 4 2015-06-03 UTC -6 -6 25 60 NA #> 5 2015-06-04 UTC -6 -6 25 60 NA #> 6 2015-06-05 UTC -6 -6 25 60 NA #> 7 2015-06-06 UTC -6 -6 25 60 NA #> 8 2015-06-07 UTC -6 -6 25 60 NA #> 9 2015-06-08 UTC -6 -6 25 60 NA #> 10 2015-06-09 UTC -6 -6 25 60 NA #> # ... with 5 more variables: sunrise <dbl>, noon <dbl>, sunset <dbl>, #> # daylength <dbl>, nightlength <dbl>
sunrise_time(ymd("2015-05-30"), geocode = my.geocode)
#> [1] "2015-05-30 01:12:23 UTC"
noon_time(ymd("2015-05-30"), geocode = my.geocode)
#> [1] "2015-05-30 10:17:30 UTC"
sunset_time(ymd("2015-05-30"), geocode = my.geocode)
#> [1] "2015-05-30 19:22:37 UTC"
day_length(ymd("2015-05-30"), geocode = my.geocode)
#> [1] 18.17045
day_length(ymd("2015-05-30"), geocode = my.geocode, unit.out = "day")
#> [1] 0.757102