def trop_refr(theta):
    """
    Calculates the tropospheric refraction (delta theta).

    Parameters
    ----------
    theta : float | array_like
        Zenith angle in radians

    Returns
    -------
    dtheta : float | array_like
        Change of the angle theta due to tropospheric refraction (in radians).

    Notes
    -----
    Approximation is recommended by the ITU-R:
    https://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.834-7-201510-S!!PDF-E.pdf
    """
    a = 16709.51
    b = -19066.21
    c = 5396.33
    return 1 / (a + b * theta + c * theta * theta)