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Re: [comets-ml] Re: Garradd tonight

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  • jbortle@aol.com
    With the recent postings concerning the possibility of a sharp brightness drop and physical changes in Comet Garradd, the following may be useful to visual
    Message 1 of 11 , Sep 30, 2011
      With the recent postings concerning the possibility of a sharp brightness
      drop and physical changes in Comet Garradd, the following may be useful to
      visual observers.

      In the case of major (high intrinsic brightness) comets, documented
      examples of such objects abruptly declining in brightness in a single day, with
      an accompanying dramatic reduction in coma diameter, are essentially
      nonexistent.

      Unlike the situation with brightness outbursts, significant real fading
      events typically evolve over the course of multiple days, rather than hours.
      The very rare exceptions to this rule are mainly seen with a few
      intrinsically very faint periodic comets following massive photometric outbursts,
      although even then change is not accompanied by any rapid decrease in coma
      size.

      In fact, the only major comet in the past 50 years that I can recall at
      the moment as having actually experienced a strikingly abrupt (virtually
      overnight) intrinsic brightness decline not associated its changing
      heliocentric distance was the famous Comet Kohoutek in January of 1973, about a week
      or so following perihelion passage. And even in that instance no significant
      alteration in coma diameter was reported.

      Thus, I think it very wise for anyone apparently seeing such an event take
      place to be immediately highly doubtful of its actual reality. Right away
      consider just what outside situation could be responsible for skewing your
      observations in some way. Things like changing sky conditions, varying
      extinction if the comet is low and variable light pollution conditions at a
      site are far more likely culprits than any actual physical alteration in the
      comet. Comets displaying relatively large, faint, outer coma regions are
      highly sensitive to any potential obscuration from the least amount of haze,
      or high humidity, in the atmosphere. Light pollution is the comet observer's
      worst enemy and even modest alterations in it can skew one's magnitude
      estimates dramatically. Even a last trace of twilight, or a congested star
      field can prove troublesome.

      By all means always record just what you see, but at the same time don't be
      afraid to question any seeming oddities with your observations, too.

      J.Bortle

      [Non-text portions of this message have been removed]
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