# fromSolarZenith: Compute hour angle and time from the solar zenith angle In environmentalinformatics-marburg/satelliteTools: What the package does (short line)

## Description

Compute the solar hour angle and the time difference from noon associated therewith from the sun zenith angle.

## Usage

 ```1 2 3 4 5 6 7 8``` ```## S4 method for signature 'numeric' fromSolarZenith(theta, phi, n, formula = c("Cooper", "Spencer")) ## S4 method for signature 'Raster' fromSolarZenith(theta, phi, n, formula = c("Cooper", "Spencer"), daytime = c("AM", "PM"), origin = strptime("1993-01-01", "%Y-%m-%d"), ...) ```

## Arguments

 `theta` `numeric` or `Raster*` object, sun zenith angle in degrees (see Note). `phi` `numeric` or `Raster*` object, latitude in EPSG:4326 (see http://spatialreference.org/ref/epsg/wgs-84/). `n` `Date` object from which to extract the day of the year (DOY), passed to `declination`. `formula` `character`, formula from which to calculate the angular declination of the sun, see `declination`. `daytime` `character`, possible options are "AM" (i.e., morning scene) and "PM". `origin` A `POSIX*` object that defines the origin of `x`, see `solarHourAngle`. `...` If `theta` is a `Raster*` object, further arguments passed on to `writeRaster`.

## Note

Negative and positive solar hour angles indicate solar zenith angles during the morning and afternoon, respectively. Note that the sun zenith angle can easily be derived from the elevation angle through `sun zenith = 1 - elevation`.

## References

PVEducation (2016) Elevation angle. Available online at http://www.pveducation.org/pvcdrom/properties-of-sunlight/elevation-angle.

`declination`, `solarHourAngle`.
 ```1 2 3 4 5``` ```theta <- 30 # 30 deg zenith angle delta <- declination(Sys.Date(), "Spencer", "degrees") # angular declination phi <- 0 # geographic latitude, i.e. equator fromSolarZenith(theta, delta, phi) ```