A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Bubble Horizon Follow Up
From: Bill B
Date: 2004 Jul 5, 17:16 -0500
From: Bill B
Date: 2004 Jul 5, 17:16 -0500
Thank you for sharing your knowledge. Some of it was a bit advanced for a would-be practitioner that is still working his way towards beginner.
Thanks to your collective help, I think I have the Sun bubble-horizon observation under control and the suggestions given pass my common sense test. The moon is still a bit of a puzzle to me, despite your collective efforts. Following up on the Nautical Almanac's instructions (below) and George Huxtable's advise, "The Moon has an immense parallax of up to a degree, which can't possibly be neglected!" my question is: If using the almanac's method, should the Moon Altitude Correction be factored in for a Moon-center bubble-horizon observation as well as the average of the HP value? The second paragraph below is not clear (in my mind) about that. NAUTICAL ALMANAC INSTRUCTIONS The correction is in two parts; the first correction is taken from the upper part of the table with argument apparent altitude, and the second from the lower part, with argument HP, in the same column as that from which the first correction was taken. Separate corrections are given in the lower part for lower (L) and upper(U) limbs. All 30' degrees is to be subtracted from the altitude of the upper limb. For bubble sextant observations ignore dip, take the mean of upper and lower limb corrections and subtract 15'from the altitude. RELATED QUESTIONS It appears to me that the tables favor LL Sun/Moon observations and then adjust to give the center of the body. I note the -30' (approx. 2X SD) correction for UL Moon observations. So the almanac's -15' for a center observation makes sense in that light. Howell's suggestion that you have already observed the center so no corrections are need except for parallax (HP average) and refraction also make sense. Guess it all depends on the table's reference point. While updated many times the Howell original was published in 1979 and uses 1976 tables. Another puzzle to me are the Moon's Altitude and HP (parallax?)correction tables. For the planets, Sun and stars refraction correction gets lower as the body approaches the observer's zenith, ranging from 38' to 0'. Howell states parallax also declines as the Moon moves towards the observers zenith. She states the change in semidiameter of the Moon is only 0.3' from horizon to zenith, and the range of possible semidiameters is 14.7' to 16.8'. That above makes sense to me. But in the moon altitude correction tables we start with a 34.5' correction on the horizon, climb to maximum of 62.8' at approx. 15 degrees, and then move down to 10.9' at 90 degrees. What is lumped into this correction that causes it to be so large and behave as it does? I apologize if the level of my questions on celestial navigation are an order or two of magnitude below the rather amazing level of knowledge the members of this forum demonstrate. However, I assume you all had to start somewhere and a kind soul or two helped you to make sense out of what appeared to be magic at the time. Thanks Bill FOR WHAT IT'S WORTH Believe I am developing a feel for the lack of precision with the bubble horizon, both academically and experientially. I am able to cheat a bit here with photo gear. Level up a tripod head, mount the sextant to the head with an articulated arm and clamp, and mount a small LED to light the bubble on another articulated arm. I can also ensure the sextant is perpendicular to the horizon. (Swinging the sextant to arc the body on the horizon line while handholding AND keeping the bubble level is an art form I have yet to master.) A Rube Goldberg transit if you will. By selecting a star or planet in the 30 to 60 degree range and noting its Hc for my time and position from Omar's site, and then working backwards from Ho to Hs, I can preset the sextant and make adjustments to the bubble level. By adjusting the level so the bubble end aligns with one of the limiting lines I can get very good repeatability. Testing it without the bubble by measuring angles of distant objects with known heights at known distances and doing the trig, I find its accuracy to be *much* better than the designer's claim of a possible 8'. Closer to 3' with IC accounted for. Best to date: within 5 ft of the stated height of a 10-story building. (Better lucky than good!) So I have a pretty good feel for the slop this bubble introduces. Once I have the instrument calibrated it gives me feedback on various stances and positions that I am experimenting with for handholding. Even though the cardboard model is no match for the real deal, it is teaching me a great deal about stable shooting positions, and turning a mental exercise (Let me think. The mirror image is too high so I need to move the index arm forward to increase the angle the sextant sees therefore lowering the image...) into a reflex action. If I had to stop and think about which way to turn the focusing ring on a camera lens to follow focus as subjects move toward or away from me, I doubt I would have many shots in focus. I understand that some of the minor corrections are superfluous under my conditions, given the accuracy of the sextant and the lack of precision of my bubble horizon, but will be good to know in the long run. With a large band of sailing buddies, it is not out of the realm of the possible that one day a sextant with a $900 adjustable/lighted bubble horizon will find its way into my sweaty little palms.