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Use offset parameter in equation_of_time_pvcdrom and declination_cooper69 #2911

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Use offset parameter in equation_of_time_pvcdrom and declination_cooper69 #2911

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chenyi6758-source Oct 19, 2026
b4544f1
Add whatsnew entry for #2911
chenyi6758-source Oct 19, 2026
593d01b
Merge branch 'main' into fix/day-angle-offset
chenyi6758-source Oct 21, 2026
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7 changes: 6 additions & 1 deletion docs/sphinx/source/whatsnew/v0.16.2.rst
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Expand Up @@ -7,7 +7,6 @@ v0.16.2 (Anticipated December, 2026)
Breaking Changes
~~~~~~~~~~~~~~~~


Deprecations
~~~~~~~~~~~~

Expand All @@ -28,6 +27,12 @@ Enhancements
allowing for choosing between ERA5 and ERA5-Land datasets.
(:pull:`2780`)

* Pass ``offset`` directly to ``_calculate_simple_day_angle`` in
:py:func:`pvlib.solarposition.equation_of_time_pvcdrom` and
:py:func:`pvlib.solarposition.declination_cooper69` instead of
adding or subtracting equivalent multiples of 2*pi/365 afterwards.
(:issue:`2902`, :pull:`2911`)

Documentation
~~~~~~~~~~~~~
* Clarified in :py:func:`pvlib.tracking.singleaxis` that the tracker
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7 changes: 3 additions & 4 deletions pvlib/solarposition.py
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Expand Up @@ -1144,8 +1144,7 @@ def equation_of_time_pvcdrom(dayofyear):
equation_of_time_spencer71
"""
# day angle relative to Vernal Equinox, typically March 22 (day number 81)
bday = \
_calculate_simple_day_angle(dayofyear) - (2.0 * np.pi / 365.0) * 80.0
bday = _calculate_simple_day_angle(dayofyear, offset=81)

@AdamRJensen AdamRJensen Oct 19, 2026

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Why is offset=81 and not offset=80 as in the original code?

@echedey-ls echedey-ls Oct 19, 2026

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_calculate_simple_day_angle(dayofyear, offset=1) is the function signature, this is its implementation:
return (2. * np.pi / 365.) * (dayofyear - offset). Both offsets must be added for mathematical equivalence, I've checked that. I haven't checked against the original source.

# same value but about 2x faster than Spencer (1971)
return 9.87 * np.sin(2.0 * bday) - 7.53 * np.cos(bday) - 1.5 * np.sin(bday)

Expand Down Expand Up @@ -1236,8 +1235,8 @@ def declination_cooper69(dayofyear):
--------
declination_spencer71
"""
day_angle = _calculate_simple_day_angle(dayofyear)
dec = np.deg2rad(23.45 * np.sin(day_angle + (2.0 * np.pi / 365.0) * 285.0))
day_angle = _calculate_simple_day_angle(dayofyear, offset=-284)
dec = np.deg2rad(23.45 * np.sin(day_angle))
return dec


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