How Tide Clocks Work Tides are the rise and fall of ocean waters caused by the gravitational forces mainly between the Earth and the Moon (the Sun also generates tidal forces, but they are much smaller). These forces cause the oceans to stretch towards and away from the moon, forming two tidal bulges. The Earth rotates through these two tidal bulges causing two HIGH tides and two LOW tides every Lunar day. A Lunar day is 24 hours and 50 minutes long. This is longer than a solar day (24 hours) because the Moon revolves around the Earth in the same direction as the Earth rotates. It takes a little longer until the Moon is over the same position. Since the Moon has the greatest effect on the tides, ALL tide clocks are synchronised to the Lunar day and will indicate a HIGH tide every 12 hours and 25 minutes (two times per Lunar day). Think of it this way: A standard clock, set for your location, displays 12:00 when the Sun is directly overhead (noon), OR on the direct opposite side of the Earth (midnight). A Tide clock, set for your location, displays HIGH TIDE when the Moon is directly overhead OR on the direct opposite side of the Earth. Many factors can influence if you will experience the brightest time of day at 12:00 noon on your standard clock. Similarly, many factors can influence if you will experience the highest water of the day at HIGH TIDE on your Tide clock. One of these factors is that the large continental land masses interfere with the Westward travel of the tidal bulge and this causes many complex tidal cycles,especially on the West coast of North America. Ocean floor topography, bays and narrow inlets can cause resonance effects and even weather conditions can alter the timing of the tidal cycle. For these reasons, Tide clocks will be more accurate in some areas than others. The red outlined areas on the map are where you can expect to receive tide information within 15 - 30 minute accuracy..