When the moon is above our head, the downward gravitational ball of the earth acts on us as well as the upward gravitational force of the moon. As a result, the earth's attraction to us diminishes somewhat. And since weight depends on the mass and gravity of the object. (Gravity is the attraction of the earth with any object of the earth , here we are the object). That is, F = mg , so as the value of g decreases, we normally lose weight.