Is it not common knowledge that the sun essentially remains fixed for all intents and purposes and the earth does the moving?
Do sextants work in the southern hemisphere? Dwight Veinot C&C 35 MKII, Alianna Head of St. Margaret's Bay, NS _____ From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Antoine Rose Sent: January 28, 2013 12:40 AM To: cnc-list@cnc-list.com Subject: Re: Stus-List Sextant Back to your original question. For every moment of every day, the sun is at the zenith somewhere on the earth. Since the earth rotation covers 360 degree in 24 hours, the sun nadir point is traveling 15 degrees over the earth surface every hour. The ephemeris gives you precisely that, the location of the sun on earth for every day and hours of the year. Interpolation tables will give you minutes and seconds. That's it and that is the first part. The reference for the ephemeris is the Greenwich meridian. The second part is a calculation based on an estimated position close to where you really are. Based on this estimated position, calculation will give you two things: the first is the azimuth, meaning that, if you were exactly at the estimated position at he time of the sextant reading, the nadir point of the sun will be on a precise bearing from that position. Typically, in north hemisphere, the azimuth of a sight taken at ten in the morning would be in the vicinity of a 140 degree bearing. A fix made at 2 in the afternoon will look like 220 degree. To visualize this, consider the course of the sun. Sunrise is on the east side somewhere like 100 degrees, depending on time of the year and latitude. The sun then travel south to be at 180 bearing at noon and sunsetting is around 260 degrees out at west. The second thing the calculation will give you is the height of the sun that you would measure with a sextant, again if you located precisely at the estimated position. I know, you're not at that estimated position and where does the sextant comes into play. Now that you know the bearing and the calculated hight of the sun for the nearby estimated position, you compared the height given by the sextant to the calculated one. If the hight given by the sextant is higher that the calculated one, you are closer to the sun nadir that the estimated position and conversely in the measured height is smaller. On the chart, you start with the estimated position used for the calculation. From that point, you trace the azimuth given by the calculation (remember, it should be southeast in the morning and south west in the afternoon). Second, the difference between the calculated height and the measured height of the sun is translated into a number of miles toward or away from the sun. From your estimated point, on the azimuth, you will measured to miles added or subtracted and trace a second line, perpendicular to the azimuth, the intercept line. Voilà, you are located somewhere on this line. To get a position fix, you need a second intercept line calculated later. The line drawn in the morning need to be transpose along your course by the estimated number of miles made between the two fixes. To summarize, to get a position, you need a sextant to measure the height of the sun. You need a watch to get the precise timing of your sight (remember a difference of four seconds between your watch and accurate time translated into a one mile difference. My thirty years old Seiko watch have a discrepancy of between 7 to 8 seconds per month). You need a compass to follow your course )please don't change course in between two sights) and a loch to know travelled distance between the two fixes. You need ephemeris and either sight reduction tables or a calculator. Principles are fairly simple in the end, but much less simple than opening the GPS. Antoine (C&C 30 Cousin) Le 2013-01-27 à 13:24, Chuck S a écrit : My understanding is that the sextant measures the angle between the Moon or Sun and the horizon. If you're careful reading the solar or lunor tables it gives you the latitude where you are. Longitude is something else. I know it requires a good timepiece, but how does one convert the time to longitude? Chuck Resolute 1990 C&C 34R Atlantic City, NJ _____ From: "Frederick G Street" <f...@postaudio.net> To: cnc-list@cnc-list.com Sent: Sunday, January 27, 2013 12:16:33 PM Subject: Re: Stus-List Mounting chartplotter now Compass Deviation Pricey little bugger I picked up a slightly used Astra 3B several years ago on eBay, from a guy in Malaysia. Nice wooden box (which I replaced with a Pelican case), and the thing was smack on for calibration. One thing I did do was to replace the old incandescent arc illumination lamp with a red LED; it required some redo of the battery compartment in the handle, but I shouldn't have to change batteries in a long time. Of course, it's not much use on Lake Superior -- as big as the lake is, you can often see the mountains (large hills, actually) on the North and South shores. I had a chance to run it through its paces on a trip to the Pacific Northwest, where we actually had a decent horizon to play with. But I'd need to get familiar with either the paper fix calculation charts or newer celestial nav software in order to get even remotely proficient again. Still, it's a cool gizmo to keep around, in case I ever do get out to bluewater; and it only cost me about $300 plus shipping. Fred Street -- Minneapolis S/V Oceanis (1979 C&C Landfall 38) -- on the hard in Bayfield, WI :^( On Jan 26, 2013, at 8:44 PM, Richard Davis <rdavis2...@me.com> wrote: Here is a nice metal sextant. http://www.landfallnavigation.com/-nhu01.html?cmp=pricegrabber <http://www.landfallnavigation.com/-nhu01.html?cmp=pricegrabber&pg=-nhu01&ut m_source=-nhu01&utm_medium=shopping+engine&utm_campaign=pricegrabber> &pg=-nhu01&utm_source=-nhu01&utm_medium=shopping%2Bengine&utm_campaign=price grabber _______________________________________________ This List is provided by the C&C Photo Album http://www.cncphotoalbum.com CnC-List@cnc-list.com _______________________________________________ This List is provided by the C&C Photo Album http://www.cncphotoalbum.com CnC-List@cnc-list.com _____ No virus found in this message. Checked by AVG - www.avg.com Version: 2012.0.2221 / Virus Database: 2639/5561 - Release Date: 01/27/13
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