At 07:22 am 21-03-05 -0900, you wrote: Rather than carry on this discursive thread in a jokey fashion and fill up the archives with lightweight stuff I thought I had better answer in a private post.
>>At 06:06 am 20-03-05 -0900, Horace wrote: >> >>>> >> >>>--------------------------------an accurate lunar >>> ephemieris... >> >> >>A case of two eyes being better than one, eh. <G> The point of my jokey remark arose as follows. I didn't know what the word "ephemieris" meant, so I had to look it up on Google. I cut and pasted but Google didn't give a definition so I realised it must have been misspelt. The correct spelling is ephemeris which only has one i. Hence my little joke which I have to admit is rather obscure if people haven't already spotted the misspelling. Still I do now know that ephemeris means ----------------------------------------------------- A table giving the coordinates of a celestial body at a number of specific times during a given period. ----------------------------------------------------- That's what I like about Vortex. One always learns something new. ;-) >>That would have really taxed Harrison's workmanship; > > >Yes indeed! I have a copy of *Longitude*, by Dava Sobel, the saga of John >Harrison and his clock, and it is a real comfort at times when the >inevitable depression comes from continually attempting and failing at >difficult or impossible things. There was a wonderful BBC program on Harrington and his clocks. If you ever get a chance to see it, do. I'm sure you would love it. > I am not aware of this pendulum phenomenon. The moon does have librations, > but these are not really pendulum effects, but rather periodic perspective > changes that occur due to ordinary orbital motions. Do you have specifics > or a reference? Your knowledge of the moon is obviously far superior to mine but google has come to my rescue and stopped me seeming a complete idiot since there are pendulum effects, albeit very small [like Gorings ***** in the wartime ribald song I remember from my boyhood. ;-) ] I have to admit that I was assuming that the geometrical librations were a pendulum effect which they obviously ain't as you correctly pointed out. >From http://www.phy6.org/stargaze/Smoon4.htm ================================================================ (An optional excursion to a somewhat specialized topic) 4b. Libration of the Moon Observers on Earth can see a little more than half the surface of the Moon, thanks to processes known as "librations." The term comes from "libra," Latin for scales. This too is the name of a constellation in the zodiac, supposedly resembling scales, and what we call "pound" also used to be called "libra, " and hence the abbreviation "lb". Two-pan weighing scales can oscillate like a pendulum, back and forth across their equilibrium position, and supposedly the libration of the Moon resembles such motion. In the equilibrium position, the long axis of the Moon points towards Earth, and libration temporarily shifts the earth-pointing tip of that axis to the north, south, east or west. Because the entire Moon follows that motion, librations reveal a little more of its surface. At any time, only half of the Moon's surface is visible from Earth, but librations allow us to "peek around the edges." Over time, up to 59% can be observed, although near the edge, where the line of view is very slanted, not much detail can be made out. Nowadays artificial lunar satellites have mapped the Moon in great detail, so such extra coverage is no big deal. But before the space age, when astronomers were denied any view of the back of the Moon, any trick for increasing their coverage was appreciated. "Libration" is used for any of several effects which allow us to "peek around the edge." Most of them, it turns out, are not associated with any pendulum-like motion of the axis, but rather represent a shift in viewing direction. Three such "geometrical librations" exist... ...Physical Librations In addition to the preceding modes, there also exist "physical" librations, actual pendulum-like nodding and wobbling of the Moon around its equilibrium position, like the spring-attached head of one of those "bobblehead dolls" popular as souvenirs. ================================================================ Keep up the good work Horace. If it wasn't for you and Jones, the Vortex group would be a dull place. :-) Cheers Frank
