def _julian_day(year, month, day):
    """
    Given a Anno Dominei date, computes the Julian Date in days.

    Parameters
    ----------
    year : int
    month : int
    day : int

    Returns
    -------
    float
        Julian Date
    """

    # Formula below from
    # http://scienceworld.wolfram.com/astronomy/JulianDate.html
    # Also agrees with https://gist.github.com/jiffyclub/1294443
    return (367*year - int(7*(year + int((month+9)/12))/4)
            - int((3*(int(year + (month - 9)/7)/100)+1)/4)
            + int(275*month/9) + day + 1721028.5)