The diameter therefore of the moon _q o r_ will be perpendicular to the strait line _t a_. Wherefore the moon is carried, by reason of the motion of the earth, from _o_ towards _p_. But by reason of the motion of the sun, if it were in _p_ it would at the same time be carried from _p_ towards _o_; and by these two contrary movents the strait line _q r_ will be turned about; and, in a quadrant of the circle _m n o p_, it will be turned so much as makes the fourth part of its whole conversion. Wherefore when the moon is in _p_, _q r_ will be parallel to the strait line _m o_. Secondly, when the moon is in _m_, the strait line _q r_ will, by reason of the motion of the earth, be in _m o_. But by the working of the sun's motion upon it in the quadrant _p m_, the same _q r_ will be turned so much as makes another quarter of its whole conversion. When, therefore, the moon is in _m_, _q r_ will be perpendicular to the strait line _o m_. By the same reason, when the moon is in _n_, _q r_ will be parallel to the strait line _m o_; and, the moon returning to _o_, the same _q r_ will return to its first place; and the body of the moon will in one entire period make also one entire conversion upon her own axis. In the making of which, it is manifest, that one and the same face of the moon is always turned towards the earth. And if any diameter were taken in that little circle, in which the moon were supposed to be carried about with simple motion, the same effect would follow; for if there were no action from the sun, every diameter of the moon would be carried about always parallel to itself. Wherefore I have given a possible cause why one and the same face of the moon is always turned towards the earth. But it is to be noted, that when the moon is without the ecliptic, we do not always see the same face precisely. For we see only that part which is illuminated. But when the moon is without the ecliptic, that part which is towards us is not exactly the same with that which is illuminated. [Sidenote: The cause of the tides of the ocean.]