def rjd_to_mjd(rjd):
    """
    RJD (Reduced Julian Date)
        days elapsed since 1858-11-16T12Z (JD 2400000.0)
    MJD (Modified Julian Date)
        days elapsed since 1858-11-17T00Z (JD 2400000.5)
    This function transforms RJD in MJD
    """
    return rjd - 0.5