The earth-sun distance for a particular day of the year is computed based on one of several empirical formulas.
calcEarthSunDist(date, formula = c("Spencer", "Mather", "ESA", "Duffie"))Numeric earth-sun distance (in AU) or, if formula = "Duffie",
the relative squared earth--sun distance on the given day.
Date of the sensor overpass; either a character string in a
native date format (e.g. "YYYY-MM-DD", see as.Date) or a POSIX*
object (see as.POSIXct).
Formula to be applied, specified through the name of the author, i.e. one of "Spencer", "Mather", "ESA" or "Duffie" (see 'Details').
Computation of earth-sun distance using formulas provided by
Spencer (1971), Mather (2005) or ESA. If formula = "Duffie", the
inverse squared relative earth--sun distance is returned as proposed by
Duffie and Beckman (1980).
The formulas are taken from the following sources:
Spencer: Spencer JW (1971) Fourier series representation of the position of the sun. Search 2/5. Taken from https://goo.gl/lhi9UI.
Mather: Mather PM (2005) Computer Processing of Remotely-Sensed Images: An Introduction. Wiley: Chichester, ISBN: 978-0-470-02101-9, https://eu.wiley.com/WileyCDA/WileyTitle/productCd-0470021012.html.
ESA: ESA Earth Observation Quality Control: Landsat frequently asked questions.
Duffie: Duffie JA, Beckman WA (2013) Solar Engineering of Thermal Processes. Wiley: Hoboken, New Jersey, ISBN: 978-0-470-87366-3, https://eu.wiley.com/WileyCDA/WileyTitle/productCd-0470873663.html.
See also: Bird R, Riordan C (1984) Simple solar spectral model for direct and diffuse irradiance on horizontal and tilted planes at the Earth's surface for cloudless atmospheres. Task No. 3434.10, Solar Energy Research Institute: Golden, Colorado, http://www.nrel.gov/docs/legosti/old/2436.pdf.
calcEarthSunDist(date = "2015-01-01", formula = "Spencer") # absolute
calcEarthSunDist(date = "2015-01-01", formula = "Duffie") # relative
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