def pitch(ax, ay, az):
    """Angle of x-axis relative to ground (theta)

    Args
    ----
    ax: ndarray
        x-axis acceleration values
    ay: ndarray
        y-axis acceleration values
    az: ndarray
        z-axis acceleration values

    Returns
    -------
    pitch: ndarray
        Pitch angle in radians
    """
    import numpy
    return numpy.arctan(ax, numpy.sqrt(ay ** 2 + az ** 2))
