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    Re: The Usps Aries Calculator
    From: Frank Reed
    Date: 2010 Nov 8, 15:55 -0800

    Alan, you wrote:
    "Having run a couple of LHA's on it, the LHA I got with it was about 15 Degrees greater than the calculated LHA Aries."

    Whenever you encounter a 15 degree difference, you should immediately ask yourself two questions: did I forget to correct for DST? or did I use the wrong time zone? Fifteen degrees corresponds to one hour's worth of the Earth's rotation. It almost has to be one of those two errors.

    If you find a discrepancy that's more on the order of a dozen or two dozen minutes of time or a few degrees in angle, then it's likely that the Zone Time to Local Time correction is off, possibly done backwards. Here you can get it right by thinking --thinking outside the black box, you might say :). The stars rise from the east, set towards the west. So suppose you work out how the sky should appear from the center of your time zone. In your present case, for example, you said you're in Pittsburgh. That's close to 80 degrees West. The center of the Eastern Time zone is at 75 W, running very close to Philadelphia. Those five degrees difference in longitude correspond to 20 minutes in time (15 degrees per hour).

    So let's suppose you want to set a star finder, whether the navigators' H.O.2102-D or a common planisphere star finder, to show the sky for Pittsburgh. You can set it for Eastern Time easily enough, but then you will have to adjust by 20 minutes to show the sky in Pittsburgh. Which way? Well, Pittsburgh is west of Philadelphia, and the stars rise from the east, so the stars "get there" (to Pittsburgh) twenty minutes later. That means that if I've set a star finder for 7pm (center of time zone, Philadelphia's sky), then I won't see the sky like that from Pittsburgh until 7:20pm.

    While there's a fair amount of calculation and technical jargon involved in setting up the navigators' Rude Star Finder, the common planisphere couldn't be simpler: you rotate the disk until the time and the date line up. Any child can do it --literally! The only tricky operation, usually ignored, is the correction for LMT from Zone Time (like the 20 minutes from Philadelphia to Pittsburgh above). Common planispheres don't include azimuth-altitude overlays, but these can be printed easily on an ordinary inkjet printer.

    A really nice innovation in the common planisphere was invented and marketed by David Chandler starting almost twenty years ago. His planispheres are double-sided. One side shows the view facing north. When you flip it over, without any change in adjustment, you have the view facing south. This avoids the high distortion near the edge usually found in planispheres. If you're trying to learn the star patterns and constellations, and not merely calculate their positions, these double-sided planispheres are a great tool. The constellations have the right shapes. You can find Chandler's "Night Sky" planispheres for various latitudes at amazon.com by searching for "Night Sky Small Star Finder" (also available in Large size).


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