The daily periodic rise and fall of water level caused by the gravitational pull of the Sun and the Moon is called what?

• A. Current
• B. Wave
• C. Flood
• D. Tide

Answer: (D)

Tides are the regular rise and fall of ocean water caused by the gravity of the Moon and the Sun. The Moon’s gravity pulls on Earth's oceans, creating bulges on opposite sides of the planet; as Earth rotates, different areas move into and out of those bulges, producing high and low tides about twice each day. The Sun also exerts gravitational force, which strengthens or weakens the effect depending on the Sun–Moon alignment. When they align, tides are higher (spring tides); when they’re at right angles, tides are lower (neap tides). So the described daily cycle is tides. A current is a steady flow of water in a direction, a wave is a disturbance traveling through the water, and a flood is an overflow of water onto land—none of these capture the regular gravitational rise and fall.