A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Back sights
From: George Huxtable
Date: 2010 Mar 19, 15:28 -0000
From: George Huxtable
Date: 2010 Mar 19, 15:28 -0000
Brad has the right idea about backsights. I imagine that's just the way the index adjustment was made, at sea. There are a few things to bear in mind. It's a rather indirect procedure, in that it relies on first measuring the altitude of a near-zenith star. That depends on both the index-index error adjustment in the forward direction, then the 0º to 90º scale calibration being error-free, then a precise observation of the star in the forward direction, then another precise observation in the back direction, from the same part of the horizon. But on the other hand, as there was never a telescope involved in back observations, they would never be very precise, at the best of times. The disc confined within 5º of the zenith only offers a tiny fraction of the total sky, which is unlikely to offer a bright star at a particular time, so it may be necessary to wait until one turns up. Because one is shooting at a moving target, a star, careful timing, or better, careful interleaving, of forward and back observations would be called for. If a star at culmination is chosen, then its angle, from the North point of the horizon, would be unchanging, and it would be best to alternate fore and back observations from a due-North spot on the horizon, chosen with reference to another star near that spot, the observer facing alternately North and South. Then, to add a bit more data, another pair of altitudes could be measured with respect to the South point of the horizon. A normal quadrant / octant, for forward observation, uses some sort of telescope or at least a peep-tube to put the eye in the right place. In the back mode, there's no such guidance. So I wonder how sensitive the observation is to error, from the observer's eye being out of the plane? But I have never ever handled such an instrument, or attempted a back sight, so I'm guessing, a bit. George contact George Huxtable, at firstname.lastname@example.org or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "Brad Morris"
To: Sent: Friday, March 19, 2010 1:24 PM Subject: [NavList] Re: Back sights | Hi George | | Thank you for the information about backsights. That was precisely what I was after! | | You wrote: | The big problem with the backsight was this: With the familiar geometry of a foresight, the index error of the instrument was quickly obtained by aligning an object with itself. That was not possible with a backsight. It might be possible to check one end of its scale, by aligning the fore horizon with the aft horizon, if they could both be seen together, but that observation would include twice the dip, an unpredictable quantity. Some index mirrors had a special facet ground exactly 90º from the main surface, to aid such alignment, which could be as good as was the precision of that set angle. But otherwise, I imagine that mariners accepted any backsight index-error as it came, without checking. On land, given appropriate distant landmarks, I can imagine ways of doing the job with some difficuly. However, there's no sign that Lewis and Clark ever verified the error in their backsights. If any reader can suggest ways of dealing with such offset error, on land or sea, I am ready to learn. | | | I think I may have the answer to the index error for backsights. Consider NAV1268 at the National Maritime Museum in Greenwich. It is a backsight Octant. As is typical of Octants, it is a vernier type, with the arc from -2 degrees to +101 degrees. Due to the length of the vernier itself, however, the octant can only measure to 95 degrees. The measurement beyond 90 degrees is the key. Using a FORESIGHT, measure the altitude of a star whose apparent altitude is greater than 85 degrees. Why greater than 85 degrees? There is a doubled region between 85 degrees and 95 degrees, in which we can measure the altitude of a star with EITHER a foresight or a backsight observation. Since it is possible with either method, we must perform the observation with BOTH methods. In knowing what the altitude is with a foresight observation, we therefore know what the vernier must read for a backsight observation, given the same star. Set the octant's vernier to the arc for a backsight observation and adjust the backsight horizon mirror until the altitude is correct. Viola! | | Best Regards | Brad | | | | "Confidentiality and Privilege Notice | The information transmitted by this electronic mail (and any attachments) is being sent by or on behalf of Tactronics; it is intended for the exclusive use of the addressee named above and may constitute information that is privileged or confidential or otherwise legally exempt from disclosure. If you are not the addressee or an employee or agent responsible for delivering this message to same, you are not authorized to retain, read, copy or disseminate this electronic mail (or any attachments) or any part thereof. If you have received this electronic mail (and any attachments) in error, please call us immediately and send written confirmation that same has been deleted from your system. Thank you." |