Q: If tides are caused by the...

Q: If tides are caused by the moon’s gravitational pull on the oceans, how can there be two high tides every day?

A: Tides are caused by the moon’s gravity and the highest tide is, in fact, at the point on the Earth’s surface directly below the moon. That occurs because the water is slightly closer to the moon than the solid Earth and is thus attracted more strongly, creating a bulge or tide. But there is also a simultaneous, although somewhat smaller, high tide on the opposite side the Earth from the moon because the solid Earth is slightly nearer the moon and is attracted more strongly than the ocean, thereby creating another bulge. Because the Earth rotates, each point on its surface has two high tides every day. If the moon were not moving in its own orbit, the high tides would occur exactly 12 hours apart. Because of the moon’s motion, they occur every 12 hours and 25 minutes.