def convert_wavelength_vacuum2air(wavelength_vacuum):
    """
    Convert vacuum wavelength to air wavelength
    Parameters
    -----------
    wavelength_vacuum: float
        Vacuum wavelength in Angstroms

    Returns
    --------
    float
        Air wavelength in Angstroms
    """
    sigma2 = (1e4/wavelength_vacuum)**2.
    fact = 1.0 + 5.792105e-2/(238.0185 - sigma2) + 1.67917e-3/(57.362 - sigma2)

    return wavelength_vacuum/fact