A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Star-sight discrepancy
From: Bill B
Date: 2005 Aug 25, 00:36 -0500
From: Bill B
Date: 2005 Aug 25, 00:36 -0500
T > I don't think it was due to Index Error or Dip. The I.E. (determined before > the sight, but not after) on my sextant was .8' on the arc, so I subtracted > that from Hs. > My height of eye was 40ft, taken from the outer deck of Bolsa > Chica State Beach Lifeguard Headquarters (N33 41 W118 03): 35ft to top of > balcony+ 5'10" for my own height= 40ft app.= DIP corrn -6.2' also subtracted > from Hs. The nice, neat, 35 ft figure sends up a red flag for me. Often structures on charts are measured from their base/ground plane up without regard for their base height above sea level (factor in tides--or lake level on the Great Lakes). What is important is your actual height-of-eye above water level. First you need to determine that as precisely as possible. Next wave height comes into play. Whether a 6 foot wave is measured from peak to trough, or mid point to peak, has been discussed on the list. Nonetheless, you might want to subtract some form of wave height from your height of eye (on a small boat you want to be on top of a crest and discount wave height--on a large ship split the difference). From the shore? Betting on 1/2 total broadcast wave height. I'll let the list gurus handle the short strokes on that issue. Working from memory, dip is .97 times the square root of height of eye above sea level, so in your case 6.2 is close enough for government work at 40'10" and would not account for your 5 mile? (from memory) discrepancy. As previously noted by another list member, 6 seconds time would account for an approx 1.5 nm error in latitude, possibly less in longitude. > The only thing > left is the horizon. Which definetly wasn't clear. There was a fogbank > hanging offshore. The eastern edge would have had to be somewhere between 3 > and 26 n.m.s offshore (I could see Oil Island Eva along with cargos lined up > offshore outside Long Beach, but strangely only Blackjack peak of Catalina > Is: low hanging haze/fog). I'm thinking maybe having the horizon 'cut > short' might have caused my error. I have tried to get a simple answer/rule of thumb from the list at least twice on how to adjust for a hazy horizon (sharp horizon obscured by a band of white) or anomalous dip. Others seeking an answer on "ideal" atmospheric conditions also seem to come up short. Good luck. > Approximately how > far away does fog need to be to get an accurate fix? Simplistically, that appears to depend a great deal on your height of eye. Working from memory again, the distance to your horizon is 1.169 times the square root of height of eye in feet, so nominally 7.4 nm away given your numbers. If your fog bank is 3 nm away, your horizon is obscured. If 26 nm away the horizon should be be clear against the fog bank. If not, the fog bank is probably within your range. I defer to the list's advanced pleasure boating and professional mariners on that issue. It seems there are differing views on how best to deal with fog--get as high as you can vs get as low as you can. Back to dip, which you had discounted. A fog bank 26 nm away is probably not the main problem, but you may have anomalous dip. One of the list gurus, Frank Reed, had written of spring and fall anomalous dip shifts off the CT coast similar to the magnitude of error you observed. (BTW, was the error towards or away from your fixed position?) My attempts to nail at least one of his brilliant feet to the floor long enough to obtain predictors and trends has so far failed. Perhaps a) The query was not intellectually stimulating enough to capture his attention (note the high level of intercourse in the recent thread on refraction), or b) I was not needy enough to appeal to his often-demonstrated and apparently well-developed sense of altruism. In a nutshell, and given Frank's posting on anomalous dip in CT, you may have done nothing wrong--Mother Nature may have been screwing with you. Why? Because she can!
As a novice myself, that's about the best I offer you. Hope it helps, and those more learned will fill in the gaps and correct any misinformation from my direction. Bill