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.

## 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://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).

## See also

Other astronomy related functions:
`day_night()`

,
`format.solar_time()`

## Examples

```
library(lubridate)
sun_angles()
#> # A tibble: 1 × 12
#> time tz solartime longitude latitude address azimuth
#> <dttm> <chr> <solar_tm> <dbl> <dbl> <chr> <dbl>
#> 1 2023-10-21 10:15:26 UTC 10:30:47 0 51.5 Greenwich 156.
#> # ℹ 5 more variables: elevation <dbl>, declination <dbl>, eq.of.time <dbl>,
#> # hour.angle <dbl>, distance <dbl>
sun_azimuth()
#> [1] 155.7248
sun_elevation()
#> [1] 24.89774
sun_zenith_angle()
#> [1] 65.10225
sun_angles(ymd_hms("2014-09-23 12:00:00"))
#> # A tibble: 1 × 12
#> time tz solartime longitude latitude address azimuth
#> <dttm> <chr> <solar_tm> <dbl> <dbl> <chr> <dbl>
#> 1 2014-09-23 12:00:00 UTC 12:07:35 0 51.5 Greenwich 182.
#> # ℹ 5 more variables: elevation <dbl>, declination <dbl>, eq.of.time <dbl>,
#> # hour.angle <dbl>, distance <dbl>
sun_angles(ymd_hms("2014-09-23 12:00:00"),
geocode = data.frame(lat=60, lon=0))
#> # A tibble: 1 × 12
#> time tz solartime longitude latitude address azimuth
#> <dttm> <chr> <solar_tm> <dbl> <dbl> <chr> <dbl>
#> 1 2014-09-23 12:00:00 UTC 12:07:35 0 60 NA 182.
#> # ℹ 5 more variables: elevation <dbl>, declination <dbl>, eq.of.time <dbl>,
#> # hour.angle <dbl>, distance <dbl>
sun_angles(ymd_hms("2014-09-23 12:00:00") + minutes((0:6) * 10))
#> # A tibble: 7 × 12
#> time tz solartime longitude latitude address azimuth
#> <dttm> <chr> <solar_tm> <dbl> <dbl> <chr> <dbl>
#> 1 2014-09-23 12:00:00 UTC 12:07:35 0 51.5 Greenwich 182.
#> 2 2014-09-23 12:10:00 UTC 12:17:35 0 51.5 Greenwich 186.
#> 3 2014-09-23 12:20:00 UTC 12:27:35 0 51.5 Greenwich 189.
#> 4 2014-09-23 12:30:00 UTC 12:37:35 0 51.5 Greenwich 192.
#> 5 2014-09-23 12:40:00 UTC 12:47:35 0 51.5 Greenwich 195.
#> 6 2014-09-23 12:50:00 UTC 12:57:35 0 51.5 Greenwich 198.
#> 7 2014-09-23 13:00:00 UTC 13:07:36 0 51.5 Greenwich 201.
#> # ℹ 5 more variables: elevation <dbl>, declination <dbl>, eq.of.time <dbl>,
#> # hour.angle <dbl>, distance <dbl>
```