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Re: Example from About Lunars, Part 2
From: George Huxtable
Date: 2002 Mar 5, 01:17 +0000
From: George Huxtable
Date: 2002 Mar 5, 01:17 +0000
OK, Chuck, just some hints is what you asked for, so I won't work it out for you. But remember how fast the Moon moves with respect to everything else, whereas Mars moves slowly. Moon motion should be something of the order of a half-degree in each hour, so over the three hour period the Moon-Mars angle should change by about 1.5 deg. Your numbers quoted below, for D1 and D2, don't change by anything like that much over the 3-hour period. So something is up with your values for D1 and D2. Check them again. If you can't find what's gone wrong, post your working for D1 and D2 and I will try to discover what it is. Two other things to note. You shouldn't be subtracting Mars parallax from the lunar distance (they are in different directions in the sky). One of the weaknesses of Letcher's method is that it considers the parallax of the "other body" to be negligible, and so takes no account of it. Whan the other body is the Sun, then the error is never much more than 0.1 min, but for Mars it can be greater, especially near perigee. However, there's no way I know of for accounting for Mars' parallax using Letcher's method, so a possible error of up to 0.2 min (and somewhat more nearer perigee) just has to be accepted. So forget that Mars parallax of 0.2 min and see what answer comes up at the end. You say "I'm subtracting semidiameter (of the Moon presumably) and Mars' parallax from the lunar distance". Subtracting the Moon's semidiameter is correct, because it was stated that the lunar distance was measured ACROSS the Moon. But somehow, after that subtraction, you have got a result of 65.406, which is greater than the raw lunar distance specified in the problem, of 65 deg 11.9 min. So have you actually added where you intended to subtract? Chuck shouldn't feel despondent. He is going through the same process we have all had to suffer, when starting with lunars. We all need to rediscover that facility with numbers that we (well, some of us) had in our youth, but then lost as calculators and computers took over our arithmetic. >I'm working my way through the Moon-Mars example now from Part 2 and I'm not >feeling too sure of myself. I feel pretty good about my calculated D1 and D2, >65.459 and 65.920 respectively. But, I'm wrestling with using Letcher's method >to find D and I'm outside of that range. I'm subtracting semidiameter and >Mars' >paralax from the lunar distance and I end up with a D of 65.406. Can >anyone give >me some hints? > >Chuck Griffiths > ------------------------------ george@huxtable.u-net.com George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. Tel. 01865 820222 or (int.) +44 1865 820222. ------------------------------