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Directions in the sky

  1. May 23, 2003 #1


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    This spring was a good time to spot the direction that
    our little fleet of galaxies---the Local Group---is traveling in the sky.

    I calculated this for myself about 10 years back and recently had it confirmed when someone gave an arXiv reference
    in the "Astronomy" gamethread.

    The direction the fleet of galaxies is cruising is 20 degrees and over 250 kilometers a second different from the motion of the solarsystem relative to CMB. So it is nice to know both and not to confuse them.

    On May 15 at 9PM (other nights different times) here is how to find the course the fleet is "steering for".
    Roughly on the meridian to the south will be Corvus, a small diamond or rhombus of fairly bright stars. Just to the right or west of Corvus is a cup-shape of dim stars called Crater (greek for winecup or drinking-bowl). Our bearing is in Crater.
    The speed is 627 km/sec-------2.09E-3 or about two thousandths of the natural speed unit.

    Another way to find Corvus (and its neighbor Crater) on a spring evening is to spot the big dipper. Its handle points east then and starts a curve that goes to the bright star Arcturus and than continues curving south to the bright star Spica. West of Spica, you are looking south now, and about on meridian, is Corvus, with Crater just to the right of it. The rhombus shape of Corvus is reminiscent of an old cross-stick kid's paper kite.

    The coordinates of the Local Group's course are:

    11h 14m, -25 degrees.

    The authors of that arXiv link paper give it in galactic as

    276 +/-3, 33 +/-3.

    BTW last I saw our galaxy is the most massive in the Local Group. People used to say Andromeda, which is coming along behind us in the collective motion and likely to catch up, was the most massive but this view may be changing with better estimates of mass.

    Ive done some ocean sailing and have associations with being able to spot one's bearing on the horizon. Maybe you do to and this means something to you and maybe it doesnt.

    Oh, forgot to include the Solar System's speed and direction---it's known rather accurately from COBE data published 1996


    The direction is in the constellation Leo (about 20 degrees north from Crater) and the speed is 369.0 +/- 2.5 km/sec.

    The coordinates for the Solar System's motion (more precisely the CMB dipole or "hot spot") are

    11 h 12 m, -7 degrees

    the 1996 arXiv reference gives them with greater accuracy and with confidence intervals
    Last edited: May 23, 2003
  2. jcsd
  3. May 26, 2003 #2
    Uh, please pardon my intrusion, Marcus.
    This looked like a good place to post a link for free software that might be of interest to viewers of this forum and I needed a place to post it;


    As always, use this stuff at your own risk...
  4. May 26, 2003 #3


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    A Iranian site with free astromonical/planetarium software!
    do you know any Farsi names of constellations or any
    Persian or Arabic constellations that
    are different from ones familiar to us?

    Does an modern Iranian see "the big dipper" as a dipper or something else?

    No intrusion---free software that helps observe the heavens is very welcome
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