Calculates the distance from the earth to the sun using the
    NREL SPA algorithm described in [1].

    Parameters
    ----------
    unixtime : numpy array
        Array of unix/epoch timestamps to calculate solar position for.
        Unixtime is the number of seconds since Jan. 1, 1970 00:00:00 UTC.
        A pandas.DatetimeIndex is easily converted using .astype(np.int64)/10**9
    delta_t : float
        Difference between terrestrial time and UT. USNO has tables.
    numthreads : int
        Number to threads to use for calculation (if using numba)

    Returns
    -------
    R : array
        Earth-Sun distance in AU.

    References
    ----------
    [1] Reda, I., Andreas, A., 2003. Solar position algorithm for solar
    radiation applications. Technical report: NREL/TP-560- 34302. Golden,
    USA, http://www.nrel.gov.

def earthsun_distance(unixtime, delta_t, numthreads):
    """
    Calculates the distance from the earth to the sun using the
    NREL SPA algorithm described in [1].

    Parameters
    ----------
    unixtime : numpy array
        Array of unix/epoch timestamps to calculate solar position for.
        Unixtime is the number of seconds since Jan. 1, 1970 00:00:00 UTC.
        A pandas.DatetimeIndex is easily converted using .astype(np.int64)/10**9
    delta_t : float
        Difference between terrestrial time and UT. USNO has tables.
    numthreads : int
        Number to threads to use for calculation (if using numba)

    Returns
    -------
    R : array
        Earth-Sun distance in AU.

    References
    ----------
    [1] Reda, I., Andreas, A., 2003. Solar position algorithm for solar
    radiation applications. Technical report: NREL/TP-560- 34302. Golden,
    USA, http://www.nrel.gov.
    """

    R = solar_position(unixtime, 0, 0, 0, 0, 0, delta_t,
                       0, numthreads, esd=True)[0]

    return R