For the most part, the rise and fall of the tides is caused by the gravitational influence of the Moon. The Moon’s gravity pulls the ocean surface upward, creating a bulge in the water — high tide. Locations on Earth perpendicular to the Earth-Moon line experience low tide.
That’s not the end of the story, however. The Sun also tugs on Earth’s oceans, though to a lesser extent. When both Sun and Moon are lined up with Earth, as occurs during the full and new moons, their combined effects yield higher tides than normal, called spring tides. Conversely, when the Sun is at right angles to Earth and Moon, as at the first- and last-quarter phase, the Sun’s gravitational influence works against the Moon, and we experience an unusually low, or neap tide.