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Solar zenith angle using NOAA / Meeus equations

Source: R/geolight_solar.R
geolight_solar.Rd

Compute solar zenith angles from time and location using the formulation implemented in the NOAA Solar Calculator, itself based on the astronomical algorithms of Meeus.

Usage

geolight_solar(date, lat, lon)

Arguments

date

POSIXct vector of times.

lat

Numeric vector of latitudes (degrees north).

lon

Numeric vector of longitudes (degrees east).

Value

Solar zenith angle(s) in degrees, corrected for atmospheric refraction.

Details

The computation is split internally into three steps:

  1. geolight_solar_constants(): Time-only solar quantities (solar time and declination)

  2. geolight_solar_zenith(): Geometric solar zenith angle

  3. geolight_solar_refracted(): Approximate atmospheric refraction correction

Conventions

  • lon is expressed in degrees east (positive eastward).

  • lat is expressed in degrees north.

  • All angles are in degrees unless stated otherwise.

  • date is provided as POSIXct and internally converted to Julian day.

When lat and lon are vectors, the result is a 3-D array with dimensions:

c(length(lat), length(lon), length(date))

When lat and lon are scalars, the result reduces to a vector of length length(date).

Solar time and hour angle

Internally, the solar time is computed as NOAA's true solar time expressed as an angle (degrees) at Greenwich, without longitude adjustment. Longitude enters later through the hour angle:

hour_angle = solar_time + lon - 180

Values are intentionally not wrapped to [0, 360) because all downstream trigonometric functions are periodic.

Atmospheric refraction

Atmospheric refraction is applied using NOAA's standard piecewise approximation as a function of solar elevation. This correction is approximate and does not account for local pressure or temperature.

References

NOAA Global Monitoring Laboratory. Solar Calculation Details. https://gml.noaa.gov/grad/solcalc/calcdetails.html

Meeus, J. (1998). Astronomical Algorithms. Willmann-Bell.

See also

Other geolight: geolight_fit_location(), geolight_map(), plot_twl_calib(), twilight_create(), twilight_label_read(), twilight_label_write()

Examples

date <- as.POSIXct(
  c("2020-06-21 12:00:00", "2020-12-21 12:00:00"),
  tz = "UTC"
)
z <- geolight_solar(date, lat = 46, lon = 6)
z
#> , , 1
#> 
#>          [,1]
#> [1,] 22.99545
#> 
#> , , 2
#> 
#>          [,1]
#> [1,] 69.63826
#>