Tides are caused by gravitational pull of the moon and the sun

ship stranded on a sandbar

The rise and fall of the tides play an important role in the natural world and can have a marked effect on maritime-related activities. Here, a ship's crew inspects the hull of their vessel which became stranded on a sandbar following a rapidly receding tide.

Tides are one of the most reliable phenomena in the world. As the sun rises in the east and the stars come out at night, we are confident that the ocean waters will regularly rise and fall along our shores.

Tides are very long-period waves that move through the oceans in response to the forces exerted by the moon and sun. Tides originate in the oceans and progress toward the coastlines where they appear as the regular rise and fall of the sea surface.

When the highest part, or crest, of the wave reaches a particular location, high tide occurs; low tide corresponds to the lowest part of the wave, or its trough. The difference in height between the high tide and the low tide is called the tidal range.

For more information:
Tides and Water Levels, NOS Education
Tides Online, Center for Operational Oceanographic Products and Services