I've no clue why the mailing list software didn't like this. Bill Ackerman's note is at the bottom. Good luck untangling the quoted stuff in the middle. --Rich ---- Date: Thu, 05 Mar 2015 13:24:37 -0500 Subject: Re: [math-fun] newton rotating liquid mirror From: Bill Ackerman <wbackerman@gmail.com> On 05-Mar-15 12:50, Eugene Salamin via math-fun wrote: [Hide Quoted Text] A lake at the South Pole would have the shape of the geoid, the equipotential surface due to gravity and centrifugal force. Would you expect the lake to somehow have a concave surface? Think about cost and maintainability. Which is easier, a liquid mirror on Earth with an extremely elaborate servo controller, or one on an asteroid? -- Gene From: Henry Baker <hbaker1@pipeline.com> To: math-fun <math-fun@mailman.xmission.com> Sent: Thursday, March 5, 2015 9:37 AM Subject: Re: [math-fun] newton rotating liquid mirror Couldn't we build such a mirror at the South Pole using just the Earth's rotation? It would want to be pretty big, but there's lots of open space down there. Even if you didn't care about visible light wavelengths, you could still provide a bunch of liquid-filled tubes to distribute the correct height to each location where you could put a small radio antenna. Alternatively, you go through the asteroid belt to find a decent size spinning asteroid and install your mirror there. Yes, you don't have much control over where it points, but there are some types of investigations where that might not matter. At 08:56 AM 3/5/2015, Warren D Smith wrote: Adam P.G's blog seems amazinger and amazinger. I don't understand his life census, but it too seems amazing. I noticed some post by him about the spinning liquid parabolic mirror being invented by Isaac Newton. I actually made one of those mirrors once using a phonograph turntable and plastic resin. The mirror was quite decent, although I daresay it was not good enough for a quality telescope. A lot of people may be willing to donate an old phonograph turntable no longer useful to them for music. _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com https://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com https://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun Ignoring Gene Salamin's objection that the shape of the mirror has to follow a geoid (or, equivalently, that, if the mirror is large, one has to compensate for the fact that gravity doesn't pull "straight up" when far from the center), the focal length of the mirror would be about 1/4 of an astronomical unit (distance from Earth to Sun). There are better ways of utilizing the required technology to make good telescopes. The idea of faithfully recreating very old historical techniques and artifacts with modern equipment comes up every once in a while. Presumably for nostalgic or entertainment reasons. Perhaps the most famous of these was the reconstruction of Charles Babbage's mechanical "difference engine" at the British museum in 1991. (The bicentennial of Babbage's birth.) The machine works, following Babbage's plans almost exactly. Imagine--getting the design for a computer exactly right, the first time. Take that, Intel!