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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Index Error
From: Bill Noyce
Date: 2004 May 5, 10:35 -0400
From: Bill Noyce
Date: 2004 May 5, 10:35 -0400
I don't think you should be performing this step: 62.6 - 0.7 = 61.9 You observed two diameters. Let's correct each of them separately for the derived index error: 32.0 - 0.7 = 31.3 30.6 + 0.7 = 31.3 Therefore your measurements say twice the diameter is 62.6, which is the same as you get by simply adding the two raw observations together. Thus, your measured semidiameter is 15.65 -- a bit closer, but still not exact. If you measure the SD to be too small, it could be explained by flattening due to refraction. Basically, the altitude of the lower limb is less than the altitude of the upper limb, so their refraction corrections would be different. But the difference should be no more than 0.05' for reasonable altitudes. If you measure the SD to be too large, it could be explained by the illusion that brighter objects appear larger -- is this called illumination? -- Bill -----Original Message----- From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM] On Behalf Of Robert Eno Sent: Wednesday, May 05, 2004 4:57 AM To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM Subject: Re: Index Error Good point but this was taken into account. For example, yesterday, I came home for lunch and couldn't resist taking a quick noon sun shot. Index error calculations were as follows: 32.'0 on 30.'6 off Difference = 1.4 1.4/2 = 0.7 IC = -0.7 Now let's take the next confirmatory step: 32.'0 on + 30.'6 off = 62.6 62.6 - 0.7 = 61.9 61.9/4 = 15.5 (rounded up) Actual semi diameter = 15.9 add'l error = 0.4' In calculating my noon latitude, I used an IC of -0.7', ignoring the other mystery error. My noon latitude differed from actual by 0.1'. Pretty close and one could say: "fluke", but I have been getting this kind of consistent accuracy with my sextant. So the question is, where in the hell does this "other" error come from and if I wanted to factor it in to my IC, how do I go about doing this? Theoretically, the sum of my off and on readings should have been 63.6 but I got 62.6. I should know all of this by now but even someone with experience needs to go back to the fundamentals every once in a while. I am baffled. Additional comments welcome. >According to the 2004 nautical almanac the semi-diameter of the sun varies between 16.3' at perihelion to 15.8' at aphelion. Or the diameter varies between 31.6' and 32.6', for a difference of 1.0' or 2 X 0.5' > >Could this be the source of the error? ------------------- Email sent using AnyEmail (http://netbula.com/anyemail/) Netbula LLC is not responsible for the content of this email