Tide Clocks
Hours and minutes aren't the only way to measure the passage of time. For those who live or work near the sea, the change of the tide can matter far more than the hour of the day.
Time and Tide
Anyone who has spent any time near the coast will know that in most places the high and low tides don't occur "in synch" with the normal day. High tide will shift gradually from one day to the next, as will low tide. Why is this?Tides are a gravity based phenomenon. They are created by the gravitational forces acting on the Earth's oceans. There are two major gravitational forces at work: the Sun and the Moon. The Sun is obviously far larger than the Moon, but also further away. This means that the relative position of both bodies must be considered to determine the current tide.
If the moon orbitted the earth exactly once a day life would be easy - there would be no daily variation in tide times. However the moon's orbit actually takes around 24 hours and 50 minutes. Successive high tides in many places thus occur at around 12 hours and 24 minutes apart. Unfortunately there is a tidal "lag" and the relative effect of solar and lunar tides depends on your exact location. So in many places the daily variation won't be exactly 24 minutes and a tide clock will thus only give an approximation unless specifically calibrated for your location. If accuracy matters then you should buy a clock intended for your specific location.
What Is A Tide Clock?
The most common form of tide clock you can buy measures tides only and is marked to show the number of hours until the next high or low tide. Another form you can buy incorporates the tide marker as an extra hand on a normal clock.Times of high tides vary from place to place so before use a tide clock must be calibrated - set at either high or low tide. As mentioned above, the interaction between sun and moon varies from place to place so for most people the tide clock will become gradually less accurate as the moon approaches its first and third quarters, being most accurate at full and new moon when solar and lunar tides are synchronised. It's recommended that the clock be set at such times.
Note: Remember, tides vary considerably from location to location! If in doubt, always ask a local expert before buying or depending on any clock. Safety first.