R: Times for sun positions

day_night {photobiology} | R Documentation

## Times for sun positions

### Description

Functions for calculating the timing of solar positions by means of function `sun_angles`, 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.

### Usage

    day_night(date = lubridate::now(tzone = "UTC"), tz = lubridate::tz(date),
      geocode = data.frame(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 = Sys.timezone(),
      geocode = data.frame(lon = 0, lat = 51.5, address = "Greenwich"),
      twilight = "none", unit.out = "datetime")

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

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

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

    night_length(date = lubridate::now(), tz = "UTC", geocode = data.frame(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
`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", "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".

### Details

Twilight names are interpreted as follows. "none": solar elevation = 0 degrees. "refraction": solar elevation = 0 degrees + refraction correction. "sunlight": upper rim of solar disk corrected for refraction. "civil": -6 degrees, "naval": -12 degrees, and "astronomical": -18 degrees. Unit names for output are as follows: "hours" times for sunrise and sunset are returned as times-of-day in hours since midnight. "date" or "datetime" return the same times as datetime objects with TZ set (this is much slower than the "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.

### Value

A `data.frame` with variables day, tz, twilight.rise, twilight.set, longitude, latitude, address, sunrise, noon, sunset, daylength, nightlength.

`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

### Warning

Be aware that R's `Date` class does not save time zone metadata. This can lead to ambiguities in the current implementation as 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.

### 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.

`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.

### See Also

`sun_angles`.

Other astronomy related functions: `format.solar_time`, `is.solar_time`, `print.solar_time`, `solar_time`, `sun_angles`

### Examples

    library(lubridate)
    my.geocode <- data.frame(lat = 60, lon = 25)
    day_night(ymd("2015-05-30"), geocode = my.geocode)
    day_night(ymd("2015-05-30") + days(1:10), geocode = my.geocode, twilight = "civil")
    sunrise_time(ymd("2015-05-30"), geocode = my.geocode)
    noon_time(ymd("2015-05-30"), geocode = my.geocode)
    sunset_time(ymd("2015-05-30"), geocode = my.geocode)
    day_length(ymd("2015-05-30"), geocode = my.geocode)
    day_length(ymd("2015-05-30"), geocode = my.geocode, unit.out = "day")

[Package photobiology version 0.9.21 Index]