If he's right - and I have no reason to doubt that he is, I'm sure I won't be alone in my disappointment. We get our hopes up on lots of comets, but it has been hard not to get caught up in the media hype over this one. But, it ain't over 'til it's over. And even his pessimistic prediction of expected performance beats what we've seen in the last decade or so. Holmes not withstanding... Darrell Spencer -----Original Message----- From: az-observing-bounce@xxxxxxxxxxxxx [mailto:az-observing-bounce@xxxxxxxxxxxxx] On Behalf Of Tom Polakis Sent: Thursday, May 30, 2013 10:19 PM To: AZ-Observing@xxxxxxxxxxxxx Subject: [AZ-Observing] John Bortle's Outlook for Comet ISON Today, comet expert John Bortle posted a good prediction for the future behavior of the "comet of the century" on the comets-ml mailing list Bortle is typically pessimistic in his predictions, but not without good reasons in this case. ============== In looking at the most recent photometric data, it would appear that C/ISON's absolute magnitude (assuming a change in heliocentric brightness in step with the typical 8.3 log r) is close to magnitude 7.5 currently. Under normal circumstances this would make ISON similar to many modestly bright (i.e. "average") comets seen in the past. However, with this comet's exceedingly small perihelion distance the ultimate situation is less clear. Comet Ikeya-Seki of 1965, with an absolute magnitude of 6.5 , appears to represent the faintest major sungrazer/sun-skirting comet to have survived perihelion essentially intact. The non-surviving sungrazing comets of 1880 and 1887 were "thought" to have absolute magnitudes of around 7.0-7.5 , although this might be open to question. Then there is Lovejoy's recent comet that violated all the rules and was intrinsically quite faint, ultimately classified as a member of the non-survival group. So...where will C/ISON fall? This is really difficult to predict at the moment. However, I would like to offer the following tentative prognostication. Comet ISON will develop more slowly in the autumn morning sky than had been initially hoped for. It will not actually attain naked eye brightness until just a week, or two, before perihelion passage. By then it will already be descending into the morning twilight. On perihelion day the comet may attain -6 very briefly (hours) and be visually detected near the Sun using great caution, then immediately begin to fade rapidly. As the comet retreats from the Sun its head will be brighter than magnitude +2 or +3 for just a few days, but it will be beginning to unfurl an extraordinary long straight tail of considerable surface brightness, at least initially. This tail could reach 30 or perhaps even 45 degrees in length a week post-T, but its visibility with the naked eye will rapidly wane. The tail's impressive visual show will last perhaps no more than a week to ten days in total. Thereafter, like C/Lovejoy, ISON's photographic tail will continue to lengthen and fade for some time. Concurrent to this the comet's head will likely dissipate more-or-less in the same manner as did Lovejoy's, with Comet ISON becoming a "Headless Wonder" by mid December, or very soon thereafter. I would anticipate that any further critical predictions of this comet's future behavior will have to wait until at least early September. J.Bortle -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list. This communication, including any attachments, may contain information that is proprietary, privileged, confidential or legally exempt from disclosure. If you are not a named addressee, you are hereby notified that you are not authorized to read, print, retain a copy of or disseminate any portion of this communication without the consent of the sender and that doing so may be unlawful. If you have received this communication in error, please immediately notify the sender via return e-mail and delete it from your system. -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.