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Re: Timing Lunars with a Rock
From: George Huxtable
Date: 2005 Jul 18, 07:58 +0100
From: George Huxtable
Date: 2005 Jul 18, 07:58 +0100
At 02:37 18/07/2005, Jared wrote: >Frank, iircif the length of >the pendulum is some precise length (vaguely16 inches?) the period *will* be >one second. So if you can find out the correct pendulum length and carry >that bit of strong with your sextant... ... > >Obtaining a willing "native" may be a bit harder these days. Much confusion here. The author quoted by Frank got it right when he said that a pendulum that "beats" seconds was 39 inches long. (it varies slightly with position on the Earth, because gravity varies, but not enough to make a significant difference for that purpose). Beating seconds means that it takes a second to swing left-to-right, then another second to swing right-to-left; a total period, for a complete oscillation, of 2 seconds. That provides a useful rate that's relatively easy to count. Instead, Jared has referred to a PERIOD of one second, which is quite a different matter. Half a second each way: much more frantic, and oscillations will die out much quicker. And then he has quoted a "precise length" as "vaguely 16 inches" for that pendulum. Far from it. It's very simple. The period is proportional to the square root of the length. So halve the period, quarter the length. So the length of the faster pendulum that Jared refers to is NOT "vaguely 16 inches", it's a quarter of the 39-inch length feferred to in Frank's quotation, or just less than 10 inches. If it had been Jared who found himself without a watch by an African lake, he would have made rather a mess of the observation. George. =============================================================== Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222, or from within UK 01865 820222. Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.