Final answer:
Ocean tides on Earth are caused by the gravitational forces exerted by the Moon and the Sun, resulting in two tides per day.
Step-by-step explanation:
Ocean tides on Earth are caused by the gravitational forces exerted by the Moon and, to a lesser extent, the Sun. The Moon's gravitational pull is strongest on the side of Earth closest to it, creating a high tide, while the water on the opposite side is also pulled away from Earth, resulting in another high tide. As Earth rotates, the tidal bulge keeps its orientation with the Moon, leading to two tides per day. The actual tidal period is about 12 hours and 25.2 minutes.