fix wrong deltat coefficient (#600)

* fix wrong deltat coefficient

* include year 2150 in deltat calculation

* add 2050 & 2150 for test_calculate_deltat

* update whatsnew for calculate_deltat updates

Author: cedricleroy

docs/sphinx/source/whatsnew/v0.6.1.rst

@@ -43,6 +43,8 @@ Enhancements
 Bug fixes
 ~~~~~~~~~
 * Fix when building documentation using Matplotlib 3.0 or greater.
+* `~pvlib.spa.calculate_deltat`: Fix constant coefficient of the polynomial expression for years >= 1860
+  and < 1900, fix year 2050 which was returning 0.
 * Fix and improve :func:`~pvlib.solarposition.hour_angle` (:issue:`598`)
 * Fix error in :func:`pvlib.clearsky.detect_clearsky` (:issue:`506`)
 
@@ -56,6 +58,7 @@ Contributors
 ~~~~~~~~~~~~
 * Will Holmgren (:ghuser:`wholmgren`)
 * Leland Boeman (:ghuser:`lboeman`)
+* Cedric Leroy (:ghuser:`cedricleroy`)
 * Ben Ellis (:ghuser:`bhellis725`)
 * Cliff Hansen (:ghuser:`cwhanse`)
 * Mark Mikofski (:ghuser:`mikofski`)

pvlib/spa.py

@@ -1369,7 +1369,7 @@ def calculate_deltat(year, month):
 
     deltat = np.where((1860 <= year) & (year < 1900),
 
-                      7.6+0.5737*(y-1860)
+                      7.62+0.5737*(y-1860)
                       - 0.251754*(y-1860)**2
                       + 0.01680668*(y-1860)**3
                       - 0.0004473624*(y-1860)**4
@@ -1418,7 +1418,7 @@ def calculate_deltat(year, month):
                       -20+32*((y-1820)/100)**2
                       - 0.5628*(2150-y), deltat)
 
-    deltat = np.where(year > 2150,
+    deltat = np.where(year >= 2150,
 
                       -20+32*((y-1820)/100)**2, deltat)
 

pvlib/test/test_spa.py

@@ -79,13 +79,17 @@
 Phi = 194.340241
 year = 1985 
 month = 2
-year_array = np.array([-499, 500, 1000, 1500, 1800, 1900, 1950, 1970, 1985, 1990, 2000, 2005])
-month_array = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12])
+year_array = np.array([-499, 500, 1000, 1500, 1800, 1860, 1900, 1950,
+                       1970, 1985, 1990, 2000, 2005, 2050, 2150])
+# `month_array` is used with `year_array` in `test_calculate_deltat`.
+# Both arrays need to have the same length for the test, hence the duplicates.
+month_array = np.array([1, 2, 3, 4, 5, 6, 6, 7, 8, 9, 10, 11, 12, 12, 12])
 dt_actual = 54.413442486
 dt_actual_array = np.array([1.7184831e+04, 5.7088051e+03, 1.5730419e+03,
-                          1.9801820e+02, 1.3596506e+01, -2.1171894e+00,
-                          2.9289261e+01, 4.0824887e+01, 5.4724581e+01,
-                          5.7426651e+01, 6.4108015e+01, 6.5038015e+01])
+                            1.9801820e+02, 1.3596506e+01, 7.8316585e+00,
+                            -2.1171894e+00, 2.9289261e+01, 4.0824887e+01,
+                            5.4724581e+01, 5.7426651e+01, 6.4108015e+01,
+                            6.5038015e+01, 9.4952955e+01, 3.3050693e+02])
 mix_year_array = np.full((10), year)
 mix_month_array = np.full((10), month)
 mix_year_actual = np.full((10), dt_actual)
@@ -352,7 +356,7 @@ def test_calculate_deltat(self):
         result_array = self.spa.calculate_deltat(year_array, month_array)
         assert_almost_equal(dt_actual_array, result_array, 3)
 
-        result_scalar = self.spa.calculate_deltat(year,month)
+        result_scalar = self.spa.calculate_deltat(year, month)
         assert_almost_equal(dt_actual, result_scalar)
 
 class NumpySpaTest(unittest.TestCase, SpaBase):
@@ -417,4 +421,3 @@ def test_solar_position_multithreaded(self):
             , self.spa.solar_position(
                 times, lat, lon, elev, pressure, temp, delta_t,
                 atmos_refract, numthreads=8, sst=True)[:3], 5)
-