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    Re: Getting to know a sextant.
    From: Frank Reed
    Date: 2017 Feb 25, 14:22 -0800

    Tony Oz, you wrote:
    "One can easily calculate the current angle values from the current NA. But the book goes on and gives the table (see p-290) with per-month star-Polaris angles. Some of those angles do change noticeably though the year. What is this? A parallax by Earth's orbiting the Sun, right? Since I'd like to make an estimation of the excentrisity in my sextant - where can I get the modern parallax data (per-month) of those stars?"

    You answered your own question :). Use the current Nautical Almanac. There are many online equivalents. For example, there is the USNO web app. There is a link for this right at the top of every NavList page under "Data". For your reference, here it is again:
    Under the same menu, I have a link for my own equivalent data source. The changes in angles during the year, by the way, are primarily due to aberration --you can measure the speed of light with your sextant by observing these small changes in angular distance. The Earth's orbital speed relative to the speed of light is 1/10,000. Convert that to minutes of arc by multiplying by 3438. You get 0.34 minutes. That's the maximum back and forth change in position for a star due to aberration during the year.

    Also I recommend you find some planetarium software similar to Stellarium for offline use. I think I remember that you use a very old installation of Windows, right? Windows XP even?? If so, that will be problematic. Have you considered adding a Linux system to your hard drive? Then you could install much more capable software without having to pay up for a newer Windows. Its much easier adding Linux than it once was.

    Frank Reed

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