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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Photo sextant sights
From: Paul Hirose
Date: 2008 Jul 31, 14:42 -0700
From: Paul Hirose
Date: 2008 Jul 31, 14:42 -0700
George Huxtable wrote: > Wolfgang Köberer is right to question that example of an intended lunar > distance measurement by camera, in Navigator's Newsletter 99. Andres Ruiz > has provided the relevant link in- > http://www.starpath.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=31;t=000040 That's a conventional altitude shot for a LOP. Also, the location is at opposite corner of the U.S. from Florida. This page seems to be the right one: http://www.starpath.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=31;t=000017 > Now look at the spacing between the Moon and Jupiter, that's being measured > in the photo in an attempt to determine the Moon's position along its track. > You can see that it's nowhere near that optimum direction; in fact, it's not > far short of being at right-angles to it. The Moon isn't travelling towards > or away from Jupiter, it's simply going past it. My lunar program confirms the bad geometry. As the output (below) shows, the lunar distance rate of change at the observer is only about 1" per minute! However, the geocentric rate is 14" per minute due to the different point of view. I came up with the idea of "velocity efficiency" as a figure of merit for the geometry between the bodies. It's the rate of change of lunar distance, divided by the total angular velocity of the bodies. If they're moving directly toward or away from each other, velocity efficiency is 100%. But it's only 4% in the photo. Checking the computed position angles vs. the image shows that it seems to preserve the original orientation you would have seen that night. That is, it hasn't been rotated so it could be cropped for display. The observed lunar distance reported by the program is a little different from the value on the web page. That's because I repeated the pixel computations but rounded the result to a higher precision, and added Jupiter's refracted semidiameter to simulate an observation from the Moon's near limb to Jupiter's far limb. That latter correction is necessary the program has no provision for a center to (Moon) limb observation, unless the other body has zero semidiameter. From the relative orientations of the bodies and the direction of the Sun, the correct limbs to observe are determined. If the actual observation was different, the measured angle has to be adjusted. In this case, I believe the center of Jupiter was measured rather than its limb. The difference is only a couple pixels, though. It's interesting that if the web page computation is repeated, but with the refracted Moon diameter from my program instead of the USNO value, the measured lunar distance is only 22" different from the prediction! This shows the difficulty of determining image scale from such a small object. In fact, I think they have the cart before the horse. It would be better to first use lunar distance photos to determine the scale and distortion of the camera. Then see how well you can measure an unknown angle. Program Lunar1. 2006-03-19T05:38:23.00 UTC (Gregorian) + 0:00.28 delta UT (UT1-UTC) 2453813.735743 JD (TT) 2006-03-19T05:39:28.19 TT (Gregorian) +00:01:04.90 delta T (TT-UT1) 2006-03-19T05:38:23.28 UT1 (Gregorian) -05:20:53.60 LMT-UT1 2006-03-19T00:17:29.68 LMT (Gregorian) -07:52.71 equation of time 2006-03-19T05:30:30.57 GAT (Gregorian) 2006-03-19T00:09:36.97 LAT (Gregorian) Moon, apparent geocentric (true equinox) 14h48m52.97s -19°54'57.2" RA, dec 55'33.9" 15'08.1" HP, SD Jupiter, apparent geocentric (true equinox) 15h05m47.63s -16°05'53.1" RA, dec 0'01.9" 0'20.8" HP, SD geocentric separation angle: 5°32'37.3" (center to center) geocentric angular rate, velocity efficiency: -13.7" per minute 44% of total angular velocity is utilized observer position: +27°12'12.0" - 80°13'24.0" north lat, east lon - 80°29'40.2" ephemeris east lon 0 meters above ellipsoid atmosphere at observer: 20°C 68°F temperature 1010.0 mb 29.83" Hg altimeter setting 1010.0 mb 29.83" Hg station pressure Moon illumination: 48° phase angle (0 = full Moon) 152° position angle to Sun (0 = 12 o'clock) 5° -63° Sun az, alt Jupiter illumination: 8° phase angle (0 = full) 151° Sun position angle (0 = 12 o'clock from Jupiter) position angles: 80° Moon to Jupiter 263° Jupiter to Moon topocentric unrefracted angles: 5°37'05.9" center to center 5°22'11.7" Moon near limb to target far limb topocentric angular rate, velocity efficiency: -1.1" per minute 4% of total angular velocity is utilized topocentric refracted angles: 15'14.8" moon SD (refracted) 0'20.8" target SD (refracted) 5°37'00.0" center to center 5°22'06.0" Moon near limb to target far limb 5°23'55.7" observed lunar distance 0°01'49.7" error (observed - predicted) -- I block messages that contain attachments or HTML. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---