Equation of time from PVCDROM.

    `PVCDROM`_ is a website by Solar Power Lab at Arizona State
    University (ASU)

    .. _PVCDROM: http://www.pveducation.org/pvcdrom/2-properties-sunlight/solar-time

    Parameters
    ----------
    dayofyear : numeric

    Returns
    -------
    equation_of_time : numeric
        Difference in time between solar time and mean solar time in minutes.

    References
    ----------
    [1] Soteris A. Kalogirou, "Solar Energy Engineering Processes and
    Systems, 2nd Edition" Elselvier/Academic Press (2009).

    See Also
    --------
    equation_of_time_Spencer71

def equation_of_time_pvcdrom(dayofyear):
    """
    Equation of time from PVCDROM.

    `PVCDROM`_ is a website by Solar Power Lab at Arizona State
    University (ASU)

    .. _PVCDROM: http://www.pveducation.org/pvcdrom/2-properties-sunlight/solar-time

    Parameters
    ----------
    dayofyear : numeric

    Returns
    -------
    equation_of_time : numeric
        Difference in time between solar time and mean solar time in minutes.

    References
    ----------
    [1] Soteris A. Kalogirou, "Solar Energy Engineering Processes and
    Systems, 2nd Edition" Elselvier/Academic Press (2009).

    See Also
    --------
    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
    # 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)