def _calc_phi(time, params):
    """Calculates orbital phase assuming zero eccentricity

    Args:
        time: observational time (same units at orbital period)
        params: dict of floats/numpy arrays, including
            params.per - orbital period (any units)
            params.T0 - mid-transit time (same units as period)

    Returns:
        orbital phase
    """

    T0 = params.T0
    per = params.per

    return ((time - T0) % per)/per