def solar_constant():
    """
    Returns the solar constant. The solar constant is defined as the
    flux density of solar radiation at the mean distance from Sun
    to Earth. The solar constant is estimated to be 1367 W m\\ :sup:`-2`\\

    Parameters
    ----------
       
    Returns
    -------
    I0 : float
        solar constant
        :math:`I_{0}`
        [W m\\ :sup:`-2`\\]
    """
    return 1367.0
