Update get_extra_radiation references

docs/sphinx/source/whatsnew/v0.12.1.rst

@@ -23,6 +23,8 @@ Documentation
 * Add a supporting reference to :py:func:`pvlib.atmosphere.get_relative_airmass` (:issue:`2390`, :pull:`2424`)
 * Documented how `np.nan` values are handled by :py:func:`~pvlib.spectrum.average_photon_energy`
   (:issue:`2423`, :pull:`2426`)
+* Update references in :py:func`~pvlib.irradiance.get_extra_radiation`
+  (:issue:`2333`, :pull:`2347`)
 
 Testing
 ~~~~~~~

pvlib/irradiance.py

@@ -34,26 +34,31 @@
                         method='spencer', epoch_year=2014, **kwargs):
     """
     Determine extraterrestrial radiation from day of year.
+
+    Specific references for each method of caclulating the earth-sun distance
+    are cited in the parameter descriptions below, while a more general
+    discussion of the different models may also be found in [1]_ and [2]_.
 
     Parameters
     ----------
     datetime_or_doy : numeric, array, date, datetime, Timestamp, DatetimeIndex
         Day of year, array of days of year, or datetime-like object
 
     solar_constant : float, default 1366.1
-        The solar constant.
+        The solar constant. [Wm⁻²]
 
-    method : string, default 'spencer'
-        The method by which the ET radiation should be calculated.
-        Options include ``'pyephem', 'spencer', 'asce', 'nrel'``.
+    method : string, default `spencer`
+        The method by which the extraterrestrial radiation should be
+        calculated. Options include: `pyephem`, `spencer` [3]_, `asce` [4]_,
+        'nrel' [6]_.
 
     epoch_year : int, default 2014
         The year in which a day of year input will be calculated. Only
-        applies to day of year input used with the pyephem or nrel
+        applies to day of year input used with the `pyephem` or `nrel`
         methods.
 
     kwargs :
-        Passed to solarposition.nrel_earthsun_distance
+        Passed to :py:func:`~pvlib.solarposition.nrel_earthsun_distance`.
 
     Returns
     -------
@@ -68,19 +73,24 @@
     .. [1] M. Reno, C. Hansen, and J. Stein, "Global Horizontal Irradiance
        Clear Sky Models: Implementation and Analysis", Sandia National
        Laboratories, SAND2012-2389, 2012.
+       :doi:`10.2172/1039404`
 
-    .. [2] http://solardata.uoregon.edu/SolarRadiationBasics.html, Eqs.
-       SR1 and SR2
+    .. [2] J. A. Duffie, W. A. Beckman, N. Blair, "Solar Radiation", in Solar
+       Engineering of Thermal Processes, Photovoltaics and Wind, 5th ed,
+       New York, USA: J. Wiley and Sons, 2020, pp. 3-44.
+       :doi:`10.1002/9781119540328`
 
-    .. [3] Partridge, G. W. and Platt, C. M. R. 1976. Radiative Processes
-       in Meteorology and Climatology.
+    .. [3] J. W. Spencer, "Fourier series representation of the sun," Search,
+       vol. 2, p. 172, 1971.
 
-    .. [4] Duffie, J. A. and Beckman, W. A. 1991. Solar Engineering of
-       Thermal Processes, 2nd edn. J. Wiley and Sons, New York.
+    .. [4] R. G. Allen, Environmental, and E. Water Resources institute. Task
+       Committee on Standardization of Reference, The ASCE standardized
+       reference evapotranspiration equation. Reston, Va.: American Society of
+       Civil Engineers, 2005. :doi:`10.1061/9780784408056`
 
-    .. [5] ASCE, 2005. The ASCE Standardized Reference Evapotranspiration
-       Equation, Environmental and Water Resources Institute of the American
-       Civil Engineers, Ed. R. G. Allen et al.
+    .. [6] I. Reda, A. Andreas, "Solar position algorithm for solar
+       radiation applications" NREL Golden, USA. NREL/TP-560-34302,
+       Revised 2008. :doi:`10.2172/15003974`
     """
 
     to_doy, to_datetimeindex, to_output = \