Answer:
When the Earth and sun are perfectly lined up, then it will happen. They can tell when it's going to happen.
Explanation:
This is why it only happens in some places. Some days it's not sunny out, so it's not going to happen.
Here’s my work to your question. I used Newton’s Second Law and a kinematics equation to arrive at the answer.
Momentum = mass x velocity
So both mass and velocity affect an object's momentum.