High and low tides are caused by the gravitational pull of the moon on the earth.

The first high tide is caused by the earth rotating into a bulge of water caused by the gravity of the moon.

The second high tide occurs on the opposite side of the earth and is caused by inertia or by the earth being squeezed

Spring tides are higher tides that are caused when both the sun and moon align and the force of gravity from both pulls on the earth.

Neap tides are moderate tides that occur 7 days after a spring tide, when the sun and moon form a right angle to each other.

Both spring tides and neap tides occur twice every lunar month.