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Re: Moon's angular speed
From: Frank Reed CT
Date: 2005 Nov 22, 01:22 EST
From: Frank Reed CT
Date: 2005 Nov 22, 01:22 EST
Ken Gebhart wrote: "Don't you mean 13.2? per HOUR instead of per day?" I probably should have included a little more preamble. I was refering to the rate of the Moon's angular motion among the stars. For example, if I have the Moon's (geocentric) RA and Dec at 10pm and it's RA and Dec at 11pm (or SHA and Dec), how far apart are those two locations? It's obviously not a big deal to calculate that directly if I have the almanac data, but the "little method" I described yields a very good result with almost no calculation. The average change in an hour is 33' of arc. If I know that the HP at the present moment is 56.0', which is 1 minute below the average value of 57.0, that's a decrease of 1.75%. So I double that and get 3.5%. The Moon will be moving 3.5% slower than the average speed of 33.0' per hour. In an hour, its position will have changed 31.8 minutes of arc. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars
