Tides are caused by the combined gravitational forces of the sun and moon on Earth's oceans. There are two types of tides: spring tides that occur during a new or full moon when the sun and moon are aligned, producing the highest high tides and lowest low tides; and neap tides that happen during a first quarter or third quarter moon when the sun and moon are at a right angle, resulting in smaller tidal ranges with higher low tides and lower high tides than normal.