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Day Length from Latitude and Date

Source: R/agronomic_analysis.R
day_length.Rd

Maximum possible daylight hours (astronomical day length) from latitude and day of year, following FAO Irrigation and Drainage Paper 56 (equations 24, 25 and 34). Useful for photo-thermal units when measured day length is not available.

Usage

day_length(latitude, date)

Arguments

latitude

Latitude in decimal degrees (north positive).

date

A Date vector, or day of year (1-366) as numbers.

Value

Numeric vector of day length in hours.

References

Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. (1998). Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56. FAO, Rome.

Examples

# Bikaner (28.0 N) on 15 March and 15 June
day_length(28.0, as.Date(c("2026-03-15", "2026-06-15")))
#> [1] 11.80731 13.76581