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Re: [Phys-l] One half of a tide?



chuck britton wrote:

Fascinating - looks like it happened back in April too.

Not quite. On April 15th the inverted low-high tide thing happened, which eliminated one tide cycle, but that time it simply left the day with two extrema (one cycle) as one might expect. On Friday it will leave us with only one extremum because of the fact that the adjacent tides straddle the two midnights. The latter effect is what leads, roughly once a week, to a three extrema day.

I notice, however, that next month there will be two days in a row with inverted tides and that they will again coincide with a midnight straddlng event. The result is that there will be one tide cycle (two extrema) on 9/11 followed by half a tide cycle (one extremum) on 9/12.

Thanks for the pointer to the interesting Babbage-Lovelace story, which I did not know.

John Mallinckrodt
Cal Poly Pomona

'Most of' the gulf of Mexico (usually) has only one tide per day.

But it's fun to scan around the coast of Florida to see how it varies.

Charles Babbage chose in interesting and commercially important
project for his calculating machine to address.

And Lady Ada did a fine job of implementing his ideas.

On Aug 12, 2009, at Aug 12(Wed) 10:43 , John Mallinckrodt wrote:

Yesterday I just happened to pick up a tide table for the Ventura
area (where our vacation single-wide is located) and noticed what I
was initially quite certain was a mistake. The tide table indicates
that the usual two tides (four tidal extrema) a day routine would be
disrupted this Friday (August 14, 2009.) On that day there the table
indicates no low tides at all and only one high tide in the late
afternoon. That is, the prediction is for just one half of "a tide"
on Friday. It turns out that the tide table is correct. The
following link to a graphical tide table shows how it works:

http://www.freetidetables.com/state/California/sid/14557594

Notice that Friday happens to be one of the very rare days when the
lower high tide (in the wee hours) happens to be lower than the
higher low tide (at dawn). But compounding the rarity even further,
the previous low tide occurs at 11:23 PM on the 13th (37 minutes
before the 14th begins) and the next low tide occurs at 12:39 AM on
the 15th (39 minutes after the 14th ends.) The result is a single
tidal extremum at 4:19 PM on the 14th.

John Mallinckrodt
Cal Poly Pomona