Expert: Philip Stahl - 9/28/2005

Question
how do tides change at a point on the seashore over the course of a day?

Answer
Hello.


The question is actually somewhat too vague to answer in any specific way. For example, a key point omitted from the quesiton is what day are you choosing? E.g. is it a day on which the Moon is full (so 'spring tide' applies), or some other phase? This makes a huge difference. (Since at the 'New' phase, for example, tides would be reinforced by the Sun).

Also, what is the ocean floor like in the vicinity of the local? Are there deep shaft nearby where the sea drops off in level, or is there a lot of coral? A place like the Bay of Fundy would expect to always have huge tides, but not necessarily in many other other places.

Also what do you mean by "in the course of a day"? 12 hours - over daylight? The whole 24-hr. period?

In general, since the Earth is rotating once every 24 hrs in the direction of the Moon's movement (which displaces roughly 12 degrees/day) , the combination of these two periods produces two high tides every 24h 50 mins.

The 50 mins. differential is the 'delay' time of the Moon in rising on subsequent nights. This, of course, leaves out a lot of details - such as the fact that Earth's axis is not at right angles to the Moon's orbital plane. However, it does provide the best *simple* explanation for why the Moon has two high tides, and also why they occur at the time intervals they do.