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    Re: What should I expect for a land based sight accuracy
    From: Frank Reed
    Date: 2014 Jan 10, 13:27 -0800

    Gary wrote:
    "Just use your GPS coordinates as your AP and then the intercepts will accurately show the quality of your sextant work. The easiest way to use your GPS position as the AP is to use the U.S. Naval Observatory website to do the sight reduction."

    I am also a big fan of the USNO web site, but just a reminder: there are innumerable software products that calculate accurate altitudes and azimuths for celestial objects. For example, there's Stellarium (http://www.stellarium.org/). You set your location to any AP you like (which, of course, for sight-testing purposes should be your actual location), and then set the time to the time of the sight. Then just click on the object and read off the altitude ("geometric", for standard sight reduction practice).

    Decades ago, the intercept method was harder to explain than it is today. All we're doing is simulating the sky from some known location. For one familiar example, each line of seven altitudes in Pub.249 v.1 specifies the appearance of the sky from a given location and time. That line of numbers is a "sky simulation". It's just as if we've run sky simulation software like Stellarium, set it for some location and time and then recorded a bunch of altitudes and azimuths of bright stars. It's a glorified, accurate planisphere. Then the "intercept method" simply compares those simulated altitudes with what we observe in the real world (after clearing the altitudes, of course). If my observation is one minute of arc greater, then I must be one nautical mile towards the azimuth of the body --as usual, as we all know. Or if I'm testing from a known location, then naturally the difference between observations and an exact simulation is a measure of my sight error.

    You also wrote:
    "Don't get lazy, however, continue to practice with other methods of sight reduction, such as HO 229, HO 249, log methods such as HO 208 and, of course, the Bygrave solution available here which has the advantage that you can work it from your GPS position as your AP"

    It all depends on one's interest. Suppose my interest is a real navigation backup in the event of jamming or spoofing of all satellite signals. That "disaster" does not mean that local electronics are suspect. We can still count on computers. There's nothing "wrong" with working sights digitally if that's the intended application --real navigation. On the other hand, a small boat ocean-sailor may be more interested in the complete failure of all electronics (but even modern calculators??). Another path around HO 2xx, etc. is historical interest. Greg has mentioned his work in historical re-enacting. He's focused on the late 18th/early 19th century period from the sound of it. Those twentieth century methods were in the far distant future.


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