Moving SKSpriteNode in a semi circle with a virtual joystick

Question

I am making a game that contains a virtual joystick. My joystick is composed by two SKSpriteNode, one for the stick the other for the base panel. ( see the picture below )

Obviously my joystick can only be moved following the green semi-circle on the panel.

My problem here is that i'm trying to make it happen, here is my code for now:

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
    for touch: AnyObject in touches {
        let location = touch.location(in: self)

        guard isTracking else {
            return
        }

        let maxDistantion = panel.radius
        let realDistantion = sqrt(pow(location.x, 2) + pow(location.y, 2))
        let needPosition = realDistantion <= maxDistantion ? CGPoint(x: location.x, y: location.y) : CGPoint(x: location.x / realDistantion * maxDistantion, y: location.y / realDistantion * maxDistantion)

        if (needPosition.y > 0) {
            stick.position = needPosition
        } else {
            stick.position = CGPoint(x: needPosition.x, y: 0)
        }

        data = AnalogJoystickData(velocity: needPosition, angular: -atan2(needPosition.x, needPosition.y))
    }
}

With this code, I can move into the semi-circle, but I would to only move my stick onto the green line, if someone can help me with my poor mathematics

Thank you