Draw a simple pendulum with a line in the middle. When the simple pendulum swings at a small angle, the angle with this line is set to a.

Let the length of the pendulum be l.

Distance from the sphere of a simple pendulum to the center line =l*a(a is arc system)

When a is small, it also satisfies that the distance between the ball and the center line of a simple pendulum =l*sin(a).

Because A is very small, a = Sina (A)

Therefore, it is easy to regard it as a simple pendulum ball doing horizontal movement.

It may not be clear just by listening, but you can understand by drawing a picture yourself.