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    Re: Line of Position Fix
    From: Gary Hubler
    Date: 2012 Nov 2, 11:01 -0700

    I commend you for learning CN on your own! At some point you may find a classroom course helpful where you can learn directly from an instructor and fellow students. Consider the Junior Navigation and Navigation courses from the United States Power Squardon, as they are relatively inexpensive and are available in many areas of the country. There are other classroom courses, but I mention USPS as I'm familiar with their CN courses.

    You've probably already found that nighttime doesn't work. You've got plenty of celestial bodies, but no horizon. Daytime generally gives you only the sun, so is not sufficient for a classical 2-body fix. Your best bet for a 2-body fix is during morning or evening twilight, where you will often have multiple bodies to choose from (i.e., stars, planets, moon). In a training course you will learn a lot about sight planning, but if you have not learned those procedures yet, just check the United States Naval Obervatory website (data services and astronomical application sections). There you can get the exact time of sunrise, sunset, and 'technical' twilight times(i.e., astronomical, nautical and civil twilights).

    Stars are great, and you will usually have plenty to choose from. There are 57 navigational stars, but only some of them will be available in your location and time of year. To see what is available in your location and at your planned observation time, you can again use the USNO website to pull up the relevant data so you can choice the appropriate stars. To make life easier while you are learning:
    1. Choose 1st magnitude stars--they are easier to locate and will be visible longer during twilight.
    2. Choose stars above 15 degrees and below 75 degrees altitude.
    3. Try to find two stars separated by 90 degrees of azimuth (45-135 is OK, but try to find stars at about 90 degrees separation).
    4. Make a 'run' of at least 3 sights on each body. As your speed increases, you may want a run of 6-sights on each body to give you lots of sextant time during the learning phase.
    5. If you obtain a GPS fix of your sight location and use that as your 'known position' when you reduce the sight, the intercept is a measure of your sight error. If you have the time, plot both your Hc vs. Time and Ho vs. Time on the same graph. This allows you to compare your sights (Ho) to what you 'should' get from the known location (Hc).
    6. If your sextant arc has no light, bring a small red flashlight along to read the arc during the darker phases of twilight.

    Of course, you can also bring in a planet or a moon sight to go along with a star sight depending on what's available.

    Around first and last quarter moons, you may have the opportunity to do a 2-body fix during the day using the moon and sun.
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