NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar Distances with Alex's SNO-T
From: Bill B
Date: 2006 Nov 09, 02:08 -0500
From: Bill B
Date: 2006 Nov 09, 02:08 -0500
Left town unexpectedly for family matters. I apologize for the delay in response. Frank wrote: > So there's no > problem calculating its exact angular size in the sky. BUT you can't use the > value in the Nautical Almanac. Is that sacrilege? Isn't the Nautical Almanac > nearly Holy Scripture for celestial navigation? Some people treat it that way > sometimes, but in fact, the modern Nautical Almanac is no more than a > well-honed tool for a particular class of celestial navigation observations, > namely, ordinary altitude observations. And here and there in the tables of > the Nautical Almanac, you will find that some quantities are inaccurate by as > much as three-tenths of a minute of arc, because that level of error is not > critical for ordinary altitude observations. > > To get the correct semi-diameter of the Moon, just take out the Moon's HP from > the almanac for the correct hour of observation. Multiply that by 27.27%. Then > apply the augmentation. You can calculate this or just use a short lookup > table as follows: if the Moon's altitude is between 10 and 30 degrees, add 0.1 > minutes of arc, between 30 and 60 add 0.2', above 60 add 0.3' (see "Easy > Lunars" on my web site). That's close enough in most cases. Note that if the > Moon is lower than 15 degrees or so, you have to take refractional flattening > into account, but you could just as well wait until the Moon's higher than > that. I'll take the word of a guy that knows the exact diameter of the moon and its distance from the observer at any given time. And I would bet the farm you know how to determine that. For those of us OCD in other areas, if I recall you stated your formula was approximate. You said 27.27%, Alex posted 27.277. Is his added digit meaningful? How "approximate" is "approximate?" You wrote, "Actually, the correct horizon SD at the time of the observation was 15.9 (calculated from 27.27% of the HP --you should always use a calculated SD when doing lunars). Then you need to augment it for altitude which gives 16.0 minutes of arc. The result is then quite a bit closer, but yes, a little off --an error of 0.14 minutes of arc in the Moon's diameter. "0.14 minutes of arc." Off what standard, please? Thanks again Bill --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---