Volcanoes 101 . Site map . The Moon pulls tirelessly at Earth's oceans, a key ingredient of the changing tides. Earth's 24-hour rotation causes the daily change in tides. The Moon's gravity pulls on Earth's oceans.
High tide aligns with the Moon as Earth spins underneath.
Another high tide occurs on the opposite side of the planet because gravity pulls Earth toward the Moon more than it pulls the water. The Sun's gravity has a similar but smaller effect. When the Moon is at first or third quarter, smaller neap tides form. The Moon's 29.5-day orbit around the Earth is not quite circular. E-mail us with general site comments or suggestions.