Quote:
On 2002-03-03 13:32, GrapesOfWrath wrote:
Quote:
On 2002-03-03 07:30, Donnie B. wrote:
Page 70: "The time between high and low tides changes every day by about half an hour."
Shouldn't this be "fifteen minutes"?
The time of the "same" high tide advances by about an hour a day. With two tides per day, the time between subsequent high tides advances about a half hour. So the time between high and low tide advances by about 15 minutes, relative to when it would occur with a stationary moon.
Or did this old salt misunderstand?
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I don't have my copy yet, so I may have lost some of the context, but what does that mean anyway? "The time between high and low tides" changes? Is that the length of the interval between the two tides, or is it the actual time of the particular tide? It seems to refer to the former, but I would have thought it would be the latter.
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The chapter describes how tides work, as you have inferred. It begins with simple ideas (tides are caused by the moon's gravity), and proceeds to explain why there are two high tides per day, not just one.
This particular paragraph adds the next complication. If the moon were stationary, the tides would occur at the same time each day (say, highs at noon and midnight, lows at 0600 and 1800). It's the moon's orbital motion that causes subsequent tides to be stretched to 12.5 hour spacings rather than 12 hour. (Numbers approximate.)
So my point was, the time from high tide to the subsequent low tide is 6:15, rather than 6:00 as would be expected without taking the moon's orbital motion into account. The text seems to be trying to say that this interval is 6:30.
Of course, the wording of the text is somewhat ambiguous, and can be read to mean that each day the
interval from high to low tide grows by 30 minutes, e.g. if today's high-to-low interval is 6 hours, tomorrow's will be 6:30, and the next day's 7:00. I'm sure this is not what Phil intended, and not what I'm criticizing. Although if you fix the error, BA, you might want to reword it to avoid the ambiguity.
Perhaps something like, "So the time from one high tide to the next is about a half-hour longer than you'd expect if the moon weren't moving; this makes the time of high and low tide change from day to day."