NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Chile and deltaT
From: George Huxtable
Date: 2010 Mar 2, 21:20 -0000
From: George Huxtable
Date: 2010 Mar 2, 21:20 -0000
Peter Hakel commented on an item- "One Earth day is about 24 hours long. Over the course of a year, the length of a day normally changes gradually by one millisecond. It increases in the winter, when the Earth rotates more slowly, and decreases in the summer, Gross has said in the past." and comments- "I did not know about this annual variation until now. As a physicist who does NOT work in the area of celestial mechanics, I would really like to understand where this variation comes from. Rotation is about angular momentum, which is conserved in an isolated system. The perigees and apogees of the lunar orbit do not correlate with seasons - as far as I know (please correct me, wherever necessary). Leap seconds are added because the tidal friction gradually slows down Earth's rotation; the "lost" angular momentum is picked up by the Moon which as a result is receding from us at about 4cm per year. So I don't think this is it. In the (northern) winter the Earth is closer to the Sun and its orbital speed increases. Is there a tiny piece of angular momentum that gets deposited into and withdrawn from Earth's rotation then during the course of a year? On the other hand, Earth's rotation is prograde (or normal? don't have Meeus's book with me here to check terminology…), meaning that its orbital and rotational angular momenta are parallel (with the difference of 23.5 degrees) rather than antiparallel. Yet Gross says that the day gets a bit LONGER in the winter… Clearly I am missing something here. I would really appreciate if NavList members would be kind enough to elaborate on the subject." ==================== comment from George- If there really is such a seasonal effect, which sounds somewhat unlikely, its cause may well lie much closer to home than Peter has guessed. A seasonal variation in the overall angular momentum, clockwise or anticlockwise, of the atmospheric winds or of the ocean tides, by of the order of one part or so in 100 million of the overall angular momentum of the Earth, would cause such an effect. This is because the overall angular momentum of the whole isolated system must stay constant. It would require some asymmetry to make the effect of Northern Summer differ from Southern Summer, but then the layout of oceans and continents is asymmetrical between the hemispheres. But a back-of-envelope guess at the relative moments of inertia of the air-mass, and water-mass, with respect to that of the solid Earth-body, seems to require unfeasibly large differences in the mean velocities of the air and water circulations. So, in my view, we should retain a healthy scepticism about such a claim until we have seen an explanation of the mechanism behind it. But then, I'm sceptical about almost everything; a bit of a sad case, in that way... George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.