This “forcing” moves around the Earth following the moon’s motion, and the end result is a tidal wave (rising and falling of the water level) that works its way around each ocean basin. This leads to the ocean surface dropping below a datum level away from this area (about 90 degrees of longitude to its east and west) and this in turn leads to the ocean level rising above the datum level on the opposite side of the Earth. The basic mechanism involved is that when the moon’s gravity is stronger in a given location, it pulls the ocean surface upward (toward the moon). Because the moon takes 27 days to revolve around the Earth, the tides do not occur at the same time every day but rather advance a little less than an hour (on average) each day. Because the moon is much closer to the Earth, its gravity is the dominant forcing factor and the sun’s gravity provides only a rather minor contribution. Tides are defined as the periodic rising and falling of the ocean surface, and they are produced by gravitational forces from the sun and the moon. As technology has advanced, there are many apps that provide tide information for multiple locations, so it is easier than it ever has been to keep track of the tides for critical areas. Mariners are quite familiar with tides in most parts of the world, and most probably have a tide table posted on the refrigerator door, in the truck or on board at the nav station.