import math


def solve_tangent_angle(distance, radius):
    """
    
    Helper function to calculate the angle between the
    centre of a circle and the tangent point, as seen from
    a point a certain distance from the circle.
    
    :Parameters:
    
    distance: float
        Distance of point from centre of circle.
    radius: float
        Radius of circle

    :Returns:
    
    tangent_angle: float
        The tangent angle in radians, or None if there is
        no solution.
    
    """
    sinangle = float(radius) / float(distance)
    if abs(sinangle) <= 1.0:
        angle = math.asin(sinangle)
    else:
        angle = None
    return angle