solarpos {suntools}

Compute solar position

Description

Calculates the solar position, i.e., the sun's elevation and azimuth, at a specific geographical location and time.

Usage

solarpos(crds, dateTime, ...)

## S4 method for signature 'sf,POSIXct'
solarpos(crds, dateTime, ...)

## S4 method for signature 'matrix,POSIXct'
solarpos(crds, dateTime, crs = sf::st_crs(4326), ...)

## S4 method for signature 'SpatialPoints,POSIXct'
solarpos(crds, dateTime, ...)

Arguments

crds

Geographical coordinates. It can be an object of class sf, matrix, or SpatialPoints.

dateTime

A POSIXct object representing the date and time.

...

Additional arguments that are passed to methods.

crs

A CRS object representing the coordinate reference system. Default is sf::st_crs(4326).

Details

Methods are available for different classes of geographical coordinates, including:

• sf: an object of class sf.

• matrix: An unnamed matrix of coordinates, with each row containing a pair of geographical coordinates in c(lon, lat) order. See the example below.

• SpatialPoints: an object of class SpatialPoints. Input can consist of one location and at least one POSIXct time, or one POSIXct time and at least one location. solarDep is recycled as needed. Do not use the daylight savings time zone string for supplying dateTime, as many OS will not be able to properly set it to standard time when needed.

Compared to NOAA’s original Javascript code, the sunrise and sunset estimates from this translation may differ by +/- 1 minute, based on tests using selected locations spanning the globe. This translation does not include calculation of prior or next sunrises/sunsets for locations above the Arctic Circle or below the Antarctic Circle.

Solar position calculation

Details for the calculations are provided by NOAA here, which we repeat below as a reference.

The calculations in the NOAA Sunrise/Sunset and Solar Position Calculators are based on equations from Astronomical Algorithms, by Jean Meeus. The sunrise and sunset results are theoretically accurate to within a minute for locations between +/- 72° latitude, and within 10 minutes outside of those latitudes. However, due to variations in atmospheric composition, temperature, pressure and conditions, observed values may vary from calculations.

For the purposes of these calculators the current Gregorian calendar is extrapolated backward through time. When using a date before 15 October, 1582, you will need to correct for this. The year preceding year 1 in the calendar is year zero (0). The year before that is -1. The approximations used in these programs are very good for years between 1800 and 2100. Results should still be sufficiently accurate for the range from -1000 to 3000. Outside of this range, results may be given, but the potential for error is higher.

Atmospheric refraction correction

For sunrise and sunset calculations, we assume 0.833° of atmospheric refraction. In the solar position calculator, atmospheric refraction is modeled as:

Solar Elevation	Approximate Atmospheric Refraction Correction (°)
85° to 90°	0
5° to 85°	\frac{1}{3600}\left(\frac{58.1}{\tan(h)} - \frac{0.07}{\tan^3(h)} + \frac{0.000086}{\tan^5(h)}\right)
-0.575° to 5°	\frac{1}{3600}\left(1735 - 518.2 h + 103.4 h^2 - 12.79 h^3 + 0.711 h^4\right)
< -0.575°	\frac{1}{3600}\left(\frac{-20.774}{\tan(h)}\right)

The effects of the atmosphere vary with atmospheric pressure, humidity and other variables. Therefore the solar position calculations presented here are approximate. Errors in sunrise and sunset times can be expected to increase the further away you are from the equator, because the sun rises and sets at a very shallow angle. Small variations in the atmosphere can have a larger effect.

Value

matrix with the solar azimuth (in degrees from North), and elevation.

References

• Meeus, J. (1991) Astronomical Algorithms. Willmann-Bell, Inc.

• NOAA's solar position calculator. These algorithms include corrections for atmospheric refraction effects.

• NOAA's solar calculations details

Examples

# Solar position in Ithaca, NY, USA on June 1, 2023 at 08:00:00

solarpos(
 matrix(c(-76.4511, 42.4800), nrow = 1),
 as.POSIXct("2023-06-01 08:00:00", tz = "America/New_York")
)