Function sun_angles() returns the solar angles and Sun to Earth relative distance for given times and locations using a very precise algorithm. Convenience functions sun_azimuth(), sun_elevation(), sun_zenith_angle() and distance_to_sun() are wrappers on sun_angles() that return individual vectors.

## Usage

sun_angles(
time = lubridate::now(tzone = "UTC"),
tz = lubridate::tz(time),
geocode = tibble::tibble(lon = 0, lat = 51.5, address = "Greenwich"),
use.refraction = FALSE
)

sun_angles_fast(time, tz, geocode, use.refraction)

sun_elevation(
time = lubridate::now(),
tz = lubridate::tz(time),
geocode = tibble::tibble(lon = 0, lat = 51.5, address = "Greenwich"),
use.refraction = FALSE
)

sun_zenith_angle(
time = lubridate::now(),
tz = lubridate::tz(time),
geocode = tibble::tibble(lon = 0, lat = 51.5, address = "Greenwich"),
use.refraction = FALSE
)

sun_azimuth(
time = lubridate::now(),
tz = lubridate::tz(time),
geocode = tibble::tibble(lon = 0, lat = 51.5, address = "Greenwich"),
use.refraction = FALSE
)

distance_to_sun(
time = lubridate::now(),
tz = lubridate::tz(time),
geocode = tibble::tibble(lon = 0, lat = 51.5, address = "Greenwich"),
use.refraction = FALSE
)

## Arguments

time

A "vector" of POSIXct Time, with any valid time zone (TZ) is allowed, default is current time.

tz

character string indicating time zone to be used in output.

geocode

data frame with variables lon and lat as numeric values (degrees), nrow > 1, allowed.

use.refraction

logical Flag indicating whether to correct for fraction in the atmosphere.

## Value

A data.frame with variables time (in same TZ as input),

TZ, solartime, longitude, latitude,

address, azimuth, elevation, declination,

eq.of.time, hour.angle, and distance. If a data frame with multiple rows is passed to geocode and a vector of times longer than one is passed to time, sun position for all combinations of locations and times are returned by sun_angles. Angles are expressed in degrees, solartime is a vector of class "solar.time",

distance is expressed in relative sun units.

## Details

This function is an implementation of Meeus equations as used in NOAAs on-line web calculator, which are precise and valid for a very broad range of dates (years -1000 to 3000 at least). The apparent solar elevations near sunrise and sunset 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 for the atmosphere. The computation is not defined for latitudes 90 and -90 degrees, i.e. exactly at the poles. The function is vectorized and in particular passing a vector of times for a single geocode enhances performance very much as the equation of time, the most time consuming step, is computed only once.

For improved performance, if more than one angle is needed it is preferable to directly call sun_angles instead of the wrapper functions as this avoids the unnecesary recalculation.

## Note

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.

## Important!

Given an instant in time and a time zone, the date is computed from these, and may differ by one day to that at the location pointed by geocode at the same instant in time, unless the argument passed to tz matches the time zone at this location.

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://github.com/NEFSC/READ-PDB-AstroCalc4R/.

An interactive web page using the same algorithms is available at https://gml.noaa.gov/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).