A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Bubble Horizon Altitude Corrections
From: Bill B
Date: 2004 Jul 4, 18:09 -0500
From: Bill B
Date: 2004 Jul 4, 18:09 -0500
BACKGROUND After attending an excellent seminar by Ken of Celestaire at Chicago's Strictly Sail show this winter, I decided it was time to quit toying with the idea and actually add celestial navigation to my bag of tricks. My first step was to peruse astronomy books and sights and get a feeling for how SHA/RA, declination, GHA etc. fit together while learning the celestial bodies. My assumption--if I can't identify it, I can't shoot it. A 2102-D star finder is on my wish list when I take the show on the water, but for backyard practice David Chandler's planisphere and the Navigator Star Finder on Omar Reis's superb web site are more than adequate. The next step in getting my feet wet was to acquire HO229 sight reduction tables, a Nautical Almanac, Susan Howell's Practical Celestial Navigation, and the AstroMedia/Wurzburg cardboard sextant with a bubble horizon from Celestaire. With a little tweaking (added a horizontal slit over the eyepiece to stabilize the eye position and bring the horizon line into focus) and practice I am getting within 15-20' of the reference Hc taken from Omar's sight. My location is determined by my Garmin GPS 76 and plugged in along with GMT. For sight-reduction-form practice I find I can take Omar's HC for the body and use it as the Ho, and then proceed as usual. If done correctly, the plotted LOP is dead-nuts on in reference to my plotted "DR" position, so errors are immediately evident. So far I am encouraged enough with my progress to consider a real sextant and 2102-D before my summer cruise(s) on the Great Lakes. PROBLEM AREAS Now the other shoe drops. I am observing the center of the sun and moon as it is easier with the cardboard sextant and bubble horizon and marker line, especially before I added the slit to bring the marker horizon line into focus. Given the slop of a bubble horizon and ? 5 to 10 minutes accuracy I am not able to resolve the issues attendant to using a bubble horizon empirically; and don't know enough about how parallax, semidiameter, and refraction are employed to arrive at the Sun and Moon altitude corrections and HP tabular values, so can't work it out logically. To add to the confusion, Howell and the instructions in the Nautical Almanac Moon Corrections page seem to be--for the neophyte--at odds. (Excerpts from both follow.). Given, Howell is mainly addressing an artificial horizon and the almanac a bubble horizon, but it strikes me there is something missing. I am guessing the almanac gives no instructions for bubble horizon observations of the Sun as they are assuming you are using a real horizon with the Sun, while you may be using a lighted bubble for the Moon. I understand why dip would be ignored in either case. Howell suggests ignoring temp/pressure corrections while the almanac says to include them. Perhaps the artificial horizons double angle cancels that out while a direct shot with a bubble would not? The almanac does not state which limb of the moon is observed with the bubble horizon. Are bubble observations traditionally made on the center of the body? I am guessing the center of the moon, as they have the reader average the UL and LL corrections and subtract 15', while and UL sight would subtract 30'. Howell says that since you are observing the center of the sun, no correction is needed except for refraction taken from the star table and moon parallax. No mention of 15'. It strikes me in the case of the Sun the total of UL and LL corrections (UL treated as a positive) is greater that 2x the semidiameter, and that the UL correction is usually greater than SD, and the LL correction is less than the SD, so the actual center of the Sun is lower than its observed center by the difference of LL corrections - LL corrections. That value differs from the refraction correction for stars. Suffice it to say I am deeply confused about what corrections to make to the Sun and Moon observations made from the center of the body with a bubble horizon. I need to rely on the kindness of strangers to solve this conundrum. 1. Can anyone provide a cookbook version on what corrections I need to make or ignore? Temp and pressure, altitude and/or HP correction for the Moon, and star refraction or UL/LL difference for the Sun? And nothing or -15' for the moon? 2. If time and temperament allow, a bit of theory as to why in terms a beginner might grasp? Thank you Bill EXCERPTS From Susan Howell's Practical Celestial Navigation In correcting the altitude of an artificial horizon sights first apply the index correction. There is no dip correction or temperature correction necessary. The sextant altitude with I.C. applied is then divided by two after which the main correction is applied. The main correction is figured as for a normal sight except in the case when the Sun or Moon are superimposed upon themselves. Here the center of the body has already been observed so there is no correction needed for semidiameter, phase or augmentation. For the Moon, there is still a parallax correction needed. The values for upper and lower limb corrections are averaged. Refraction correction is still necessary for the superimposed Sun and Moon sights, this refraction value taken from the star altitude correction table. Therefore, for most artificial horizon sights Ho = (Hs ? I.C.)/2 ? main. Ho is the corrected sextant altitude. From the Nautical Almanac Moon Correction Table 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 corrections for pressure and temperature see page A4. For bubble sextant observations ignore dip, take the mean of upper and lower limb corrections and subtract 15'from the altitude. App. Alt. = Apparent altitude = Sextant altitude corrected for index error and dip.