Tides occur during the gravitational pull between the __________?

• A. Earth, Moon, and Sun
• B. Moon and Sun
• C. Earth and Sun
• D. Moon

Answer: (A)

Tides are caused by the gravity of celestial bodies acting on Earth's oceans, and the relevant interaction involves three bodies: Earth, Moon, and Sun. The Moon is close enough to exert a strong tidal pull, creating bulges on the side of Earth nearest the Moon and on the opposite side as well. The Sun, though far away, is so massive that its gravity also contributes to those bulges. The observed tides come from the combined, competing influences of both Moon and Sun on Earth’s oceans, so describing the system as involving Earth, Moon, and Sun best captures the full picture. When the Sun and Moon align, their pulls reinforce each other and tides are larger; when they are at right angles, their effects partially cancel, and tides are smaller.