Re: Magnitude estimates on Heavens-Above

From: Rik Hill (rhill@LPL.Arizona.EDU)
Date: Mon Apr 23 2001 - 12:42:42 PDT

  • Next message: Allen Thomson: "Re: Magnitude estimates on Heavens-Above"

    One magnitude is a factor of 2.512. 
    What I meant when I indicated I did not think this a serious error is
    that a simple orientation change in a satellite (whether due to its
    intrinsic orientation, or your position) could make it change
    dramatically. Take a look at the information for iridium satellites and
    looks what happens with a slight change in your position on the earth.
    Now I'm no expert in satellites, having just come to this type of
    observing in the last few months, but it seems from what I've been
    seeing that several magnitudes change can easily occur just from one
    solar panel catching the light just right. On Halloween night we saw a
    passage of ISS that was predicted at something like -2 mag. As usual, I
    had a telescope set up in front of my house to show the sky, in this
    case the moon, to the passing ghosts and goblins...and their parents.
    When it came over it flashed, in a matter of a couple seconds, to
    brighter than -4 mag.  and I estimated it at -6 as we could see
    shadows. Lots of witnesses, in fact all the trick-or-treaters stopped
    to watch with moms and dads in tow. It had a metalic blue color too.
    But this flash, like and Iridium flash, lasted only a few seconds.
    So do we call the inability for Heavens-Above to predict this flash 
    a serious error? I wouldn't. With the frequent changes in attitude of
    these things I'm amazed Chris Peat does as well as he does. 
    I am working on getting video of ISS + STS and I will pursue any
    opportunity regardless of 'predicted' brightness. 
    Now if you are _constantly_ experiencing differences like these you
    may want to be sure of the coordinates you are using and/or the offset
    from the database position from H-A. I just changed the site I was using
    for my predictions and it did make a noticable difference. 
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