What causes the tides in Earth's oceans?

TIDES are caused primarily by the GRAVITATIONAL PULL of the MOON (and to a lesser extent the Sun) on Earth's oceans. ASVAB General Science includes earth science/astronomy. The Moon's gravity pulls ocean water toward it, creating high tides; there are generally two high and two low tides per day. SPRING TIDES (higher) occur when the Sun and Moon align; NEAP TIDES (lower) when they're at right angles. The Moon's gravitational influence on tides is a commonly tested concept.