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    Re: Star - Star Observations
    From: Frank Reed
    Date: 2010 Mar 13, 09:23 -0800

    George H, you wrote:
    "A posting from Frank made some valid points, but was a victim of Frank's familiar attempts to take the trig out of navigation. [...] Isn't it better to know a procedure which applies all the time, even if a bit of trig is involved?"

    Nope. It's better to know both, but if you can only do one, take the easy one. The rule I gave can be rendered down to an even simpler form (which I've already posted, but I'll do it again): accurate to +/-0.1 minutes of arc, the effect of refraction is a tenth of a minute of arc for every five degrees of measured distance when both stars are above 45 degrees altitude, no matter how they're oriented relative to each other in the sky.

    Want another easy rule? This same rule applies when both stars are at the same altitude AND above 15 degrees altitude AND the observed distance is 90 degrees or less. So let's say you're among some navigation enthusiasts interested in this business of checking arc error by star-star distances, and you hear someone repeat that old INCORRECT rule claiming that there is no refraction correction on star-star distances when both stars are at the same altitude. If you know this rule, then you can quickly tell them just how much error they will incur. You'll be able to say, "even at 45 degrees separation between the stars, the error would be close to 0.9 minutes of arc using that rule!" Of course, you would have been able to work this out by the very long spherical trigonometry approach, but unless you're carrying around a pre-programmed computing device, by the time you're done, it would be too late to make much of an impression.

    Easy rules are your friend. "Easy" is not a four-letter word.

    Oh wait...

    -FER


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