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The modification to the 2102-D that I posted last year added the date
and time scales so that it can be set like the usual planisphere.

http://www.fer3.com/arc/m2.aspx?i=107982&y=200904

gl

On 11/8/2010 3:55 PM, Frank Reed wrote:
>
> 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).
>
> -FER
>
>
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