Newton's Third Law of Motion states that forces act in pairs. For every action force, there is a reaction force. Newton's third law of motion is that for every action there is an opposite reaction.
Explanation: when an object gets in front of a light, it temporarily breaks the beaks the beam from the light source to the user. the same occurs when a planet gets in front of a star. i don't feel like explaining the doppler effect but this is basically it.