NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: It's Moon-landing Monday
From: Frank Reed
Date: 2009 Jul 24, 00:17 -0700
From: Frank Reed
Date: 2009 Jul 24, 00:17 -0700
I wrote previously: "What could you do while approaching the Moon if you could see tenth magnitude stars on it's limb?" And Greg you wrote: "I like observing orbiting satellites better." So do I. But let me elaborate a little on the dramatic effect of seeing so many faint stars. Take the 7x telescope, e.g., from the sextant case and aim it at the dark limb of the Moon. You will see hundreds of occultation events in just a few minutes. With fairly simple software, these would give you an accurate fix almost immediately. The small number of celestial objects visible when the horizon is also visible down here on Earth actually forces us to measure angles. If we would see tens of thousands of stars, then "coincidence events" like occultations and close appulses/conjunctions between objects would be much more useful. Similarly, if there are satellites in orbit around the Moon, typical ranges to them might be on the order of one or two thousand miles. If we have accurate orbital data on them, and if we can observe tow or more of their positions as they pass near known stars at a known instant of time to +/-1 or 2 minutes of arc, then we can get a position fix to within +/-0.3 miles at the low end up +/-1.2 miles at the high end. So toss the sextant, and keep the telescope. By the way, the satellite approach also works on Earth. I've described this before, but it doesn't hurt to repeat it --I hope. Suppose we have good orbital data on some satellites with typical ranges above us on the order of 500 miles. If we know their orbital data, and if we observe their positions with binoculars to +/-0.1 degrees (not difficult), then we can get a position fix to +/-1 mile. The usual uncertainty in orbital data is the mean longitude on the orbit which is nearly equivalent to an error in the time of the sight. That is, a satellite might be five seconds early but it's unlikely that the path will be much in error, just offset in time. Because of this possibility, it's better to plot LOPs from two satellites under an assumption of an error in the observation times. -FER --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---