For the record, I pored through several historical geology books and some astronomy texts and came up with the following about how many days there were in a year at different points of geologic time. Most of this is from counting daily growth rings on corals, but some similar (and supportive) information comes from looking at things like brachiopods, bryozoans and pelecypods (clams).
As the Moon moves farther away from the Earth, the rate of rotational slowing decreases because the tides grow weaker. But a question arises: how close was the Moon to the Earth when it was at its closest; and when was it closest? On this I found little info (if anybody has better, I'd be interested). Sources give the time when the Moon was closest in the whole range from 4 billion years ago to as little as 400 million years ago. A bit of a range.
More interesting, I think, is that business about what kind of tides there would have been when the Moon was at its closest. Here, different sources range from 1/2 the present distance to as little as 1/10 the present distance. Wow. As previously noted, tides are a function of distance cubed. So if the Moon once WAS only 25,000 miles away, imagine daily tides measuring thousands of feet between high and low. Seems hard to believe, but..... phooh to the Bay of Fundy!
Anyway, once upon a time the "daily" tides were certainly much stronger than they are now, AND they went through their cycle much faster. Interesting world.
Scott Bryan
Victor Valley College
sbryan@victor.cc.ca.us
TSBryan@aol.com
Recently here there were discussions about how the length of the day/number of days per year has changed through geological-astronomical time. With that were some (mostly unstated) implications regarding the corresponding change in the Earth-Moon distance and (finally) with that how the tides must have changed as a result.
Just received is the September 2000 (vol. 28, no. 9) issue of Geology magazine, a monthly journal published by the Geological Society of America. In it, on pages 831-834 is an article titled _Quantifying the oldest tidal record: The 3.2 Ga Moodies Group, Barberton Greenstone Belt, South Africa_. As with all GSA articles, you should be able to access this entire article as a PDF file at: http://www.geosociety.org/pubs/journals.htm
Very briefly here: analysis of these inferred tidal sedimentary deposits implies that 3.2 billion years ago, the anomalistic month was between 18 and 20 days long (as compared to the 27.55 days of now). If I have done my math correctly [ and it is entirely possible that I have not ;-) ], then at that time the Moon was on the order of 189,000 miles from Earth and the tides were roughly 2.7 times as extreme as at present. Close enough to that, anyway, and good enough to maybe give you some of you some presentation ideas.
T. Scott Bryan
Victor Valley College
Victorville, California