<p>From this month's NAMN Notes:</p><p> </p><p>1. The Perseids - The Public’s Shower...<br /><br />If you walk up to the man or woman on the street and ask them about<br />meteors and if they’ve seen any, about half will look at you dumbfounded<br />with no idea what you are talking about. Most of the rest will mention<br />something about “those meteors in the summer”. The Perseids are the most<br />accessible shower to the general public and the casual meteor watcher.<br />The weather in the northern hemisphere is warm, though the skies are<br />often hazy. Fortunately, the Perseids are generally bright and swift, so<br />can punch through the less than ideal skies. This has likely been going<br />on for nearly two millennia, since the earliest records of this shower<br />go back to 36 AD.<br /><br />The parent, 109P/Swift-Tuttle, was the third comet discovered in 1862,<br />and in fact as a result of that discovery, Giovanni Schiaparelli made<br />the first association between comets and meteor showers. At that time,<br />calculations of the comet’s orbit (with a lot of uncertainty) indicated<br />a period of about 120 years, meaning the next return was expected in<br />1980. When it failed to appear on schedule, other possible orbits were<br />considered leading to the correct period of ~ 133 years. For a good feel<br />of the history of the comet I suggest reading Gary Kronk’s excellent<br />page on the subject at:<br /><br />
http://cometography.com/109p.html<br /><br />Earlier returns have been found (using the correct orbital period) going<br />back as far as 6 BC. The comet has been around for thousands of years,<br />and its orbit is stable enough that it will be around thousands of years<br />into the future.<br /><br />The orbit is highly inclined (113 degrees), eccentric, and retrograde,<br />orbiting the sun in the opposite direction from 98% of the material in<br />the solar system. The aphelion is well outside of that of Neptune’s<br />orbit, at 51 AU. That’s about as far out as Pluto gets, near the outer<br />edge of the Kuiper Belt. The perihelion (closest approach to the sun) is<br />just inside of Earth’s orbit. These factors combine to make the Perseids<br />a northern shower with swift meteors of about 59 km/sec when they hit<br />the atmosphere. It is the largest periodic comet nucleus to visit the<br />inner solar system.<br /><br />The Perseids no longer produce the exceptional rates that occurred near<br />the 1992 perihelion, but still are reliable and bright, and so put on a<br />good show when skies permit. The rate rises slowly from mid July until<br />the peak period, then falls more rapidly afterward, ending by August<br />26th. As for the peak itself, I have examined last year’s IMO on-the-fly<br />data (no full analysis has been completed) and it shows that high<br />activity occurred for more than 2 full days. One way to measure a peak<br />is what is called Full Width Half Maximum (FWHM), the period when the<br />rate exceeds half the peak value. Last year that occurred from Solar<br />Longitude 139.3 to 141.5. This year that would be from 20 UT August 11th<br />through 02 UT on the 14th.<br /><br />The IMO lists the peak at Solar Longitude (SL) 140.0 to 140.1 or 12-14<br />UT on the 12th. If one examines last year’s results though, at:<br /><br />
http://www.imo.net/live/perseids2007/<br /><br />that particular time appears to be in a lull between peaks earlier and<br />later. There is evidence of a narrow peak about SL 139.5, a plateau of<br />high activity from 139.7 to 139.9, and later peaks at 140.5 and 141.4.<br />The peak at 140.5 could be a return of one of the extra ones that were<br />last seen in 1999 from recent debris. The point here is that you should<br />not focus on the narrow 2 hour window of the predicted peak, but rather<br />observe well before and after. That’s good news for North America, since<br />the predicted time is not very favorable, occurring during the day of<br />the 12th.<br /><br />One possible explanation for this is the filaments of old trails ejected<br />hundreds to thousands of years ago. Peter Jenniskens and Jeremie<br />Vaubillion calculated 3 filaments would arrive at SL 139.4, 139.45, and<br />139.81 last year; this was the time when the plateau mentioned above<br />occurred. The same trails (from 1479 and 513 AD) are also expected to<br />intersect the earth again this year from 139.6 to 139.83, so if those<br />calculations are valid, we might expect enhanced activity between 02 and<br />07 UT on the 12th. That is 10 PM EDT (7PM PDT) on the 11th until 3AM EDT<br />(midnight PDT) on the 12th. That’s a few hours before the predicted IMO<br />peak, and almost perfect for North America.<br /><br />An interesting fact I did not realize, but discovered while preparing<br />this month’s notes, is that the day after the peak, the radiant actually<br />moves out of Perseus into the constellation Camelopardalis. Imagine,<br />after all these years of following meteors, there are always things to<br />be learned!<br /><br />During the peak nights the moon will be setting after midnight, so the<br />darkest skies will be between then and dawn. That’s OK, because due to<br />the radiant rising higher during the morning hours, the best rates will<br />occur near dawn anyway. However, even before then, the brightness of the<br />Perseid meteors and the low elevation of the moon should allow plenty to<br />be seen. Just be sure, as usual, to accurately estimate the Limiting<br />Magnitude of your sky if you are submitting scientific data. The NAMN<br />has a tool to help with that at the following URL:<br /><br />
http://www.namnmeteors.org/lm_calc.html<br /><br />Here are the radiant locations for selected mornings during August.<br />Between these dates you can interpolate. Before and during the peak the<br />radiant lies in Perseus, as you would expect. You can also see a<br />graphical representation of this by accessing the IMO 2008 meteor shower<br />calendar:<br /><br />
http://www.imo.net/calendar/2008</p><p> </p><p>To read the whole thing (it's kind of techno geeky for the meteotoricist) Look at August 2008 here:<br /><br />
http://www.namnmeteors.org/namnnotes2008.html<br /><br />It should be posted shortly....<br /></p> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>