Re: Complicated Question about sky geometry

From: Björn Gimle (
Date: Sun Sep 09 2012 - 16:10:53 UTC

  • Next message: Russell Eberst: "2012SEP8-9.OBS"

    Yes there is - any app with a suitable projection (not standard polar
    or RA/Dec rectangular grid).
    Rob Matson's SkyMap has these two, and a stereographic alt/az.
    This has the property to show correct angles from the map centre, and
    "straight" lines/tracks (great circles) across the centre are straight
    lines. Equidistant points from the centre are circles.
    >From that centre point, it's easy to locate the point 10 degrees below.
    In Skymap you can quickly switch projection to read the RA/dec of the
    center point
    (about 17:06 +18:10) and the az/alt (about 212.9 / 61.9)
    Choosing a latitude of N 18:10 and a time or longitude where siderial
    time is 17:06 and plotting alt/az with zenith in the centre you can
    easily verify this with the azimuth lines exactly opposite, and since
    both stars will be at 49 deg elevation their distance is 82 deg.
    You can also use spherical trigonometry to compute the midpoint coordinates.
    (Another way is to create fake observations at Altair and Arcturus of
    a very low altitude orbit, and plot that satellite track)
    Some demo maps, dated 2012-09-09, are at the very bottom of page
    2012/9/8 joseph daniel <>
     So , that's what I did in Satflare in it's Skychart , as I described
    in that post
    (see - Any idea which of these objects I may have seen) .
     However , you cannot draw a straight line at that elevation (49 degrees for
    Altair & 35 for Arcturus) in a Skychart . The line between those 2
    points obviously
    must be curved downward .
     My question is ... how curved ? Is there an app that would allow me to select
    2 points or 2 stars in a skychart and then it would draw the proper curved line
    that would represent the straight line that I drew in that light sky ?
     If not , is there a workaround approach to picking a midway point that would be
    at the proper elevation (not the improper elevation that I initially assumed ) ?
    Björn Gimle, COSPAR 5919
    59.2576 N, 18.6172 E, 23 m
    Phone: +46 (0)8 571 43 312
    Mobile: +46 (0) 704 385 486
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