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Murphy's Galactic Gizmo

  1. May 22, 2003 #1


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    http://fuse.pha.jhu.edu/support/tools/eqtogal.html [Broken]

    An astronomer named Murphy at Johns Hopkins University
    has provided an online calculator going from celestial to
    galactic coordinates

    We should try it out, and so, for instance

    In 1996 the COBE team published

    http://arxiv.org/PS_cache/astro-ph/pdf/9601/9601151.pdf [Broken]

    "The Dipole Observed in the COBE DMR Four-Year Data"

    In which they said that the Cosmic Microwave Background hotspot was located at

    (11 h 12 m, -7.22 degrees)

    And that it corresponded to a speed of 1.23 E-3
    (1.231 +/- 0.008 x 10-3 is what they actually said
    but for convenience I'm not giving confidence intervals)

    I was pleased to see that they expressed the solar system's speed relative to the CMB as the fraction of the speed of light just quoted, as well in the usual way.

    They also gave galactic coordinates for the hotspot, which can provide a check of Murphy's calculator.

    (11 h 12 m, -7.22 degrees) happens to be in the constellation Leo.

    I find that if I put those equatorial coords into the calculator it gives me (have to do this and get back...) galactic coords:

    (265.3 degrees, +48.2 degrees)

    This is pretty close to what the COBE team published for the galactic coordinates of the hotspot, which was:

    (264 degrees, +48 degrees)

    I rounded off their figures which had a couple of more decimal places and confidence intervals in the last two digits, which I dont show. So it is not surprising to get an answer thats a degree different---calculator itself is approximate---but anyway close enough.

    If the hotspot is in Leo the cold spot would be in Aquarius or thereabouts.

    If you want the speed of the sun's motion w/rt CMB to be in meters per second, as you should if you use metric units exclusively, then multiply their figure of 1.231 E-3
    by 299792458 meters per second. For my part I shall think of the the sun's speed relative to the expansion of the universe as
    1.23 thousandths of the speed of light.

    Leo has been in the sky a lot of evenings this spring
    Last edited by a moderator: May 1, 2017
  2. jcsd
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