It is currently Fri Aug 22, 2014 1:42 pm


Post new topic Reply to topic  [ 7 posts ] 
Author Message
 Post subject: Interesting Meteor
PostPosted: Tue Dec 02, 2008 11:15 pm 
Offline

Joined: Sat Dec 11, 2004 8:18 pm
Posts: 5899
Location: Manchester
Dear al(L)
I almost forgot.
1st December 2008 at 21:26 UT give or take a minute I reckon, I noted a fine meteor, possible fireball but not thought to have been as bright as Venus when at its best (though that was not seen by me that night ). The meteor was seen about 25 degrees below Gamma Peg travelling "down", roughly in line with the upright side of the Square. It was travelling roughly in that same line (possibly angled slightly east-ish of that direction) and just right of Beta Cetus. It was "white", apparently not moving very fast, I saw it travelling about 15 degrees, I thought I saw the start but it may have been visible over a longer track than I saw, but probably initially then fainter and building up brightness, because the meteor left a visible straight grey trail in excess of 10 degrees long which lasted about 2 or three seconds.
Location 2 deg 17' west, Latitude 53deg 32' north.
Altitude about 80 metres.
Best of luck from Cliff


Top
 Profile Send private message  
 
 Post subject: Another sighting
PostPosted: Fri Jan 30, 2009 10:17 pm 
Offline

Joined: Fri Jan 30, 2009 5:16 pm
Posts: 1
I found this website the other day, so I thought I would report my first fireball sighting...

December 1, 2008 at 9:25pm (by my watch + or - 1 minute) from Chard, Somerset, 50.87N, 2.96W. Magnitude approximately -12. Colour blue-ish white. Took approximately 1 second to travel approximately 20 degrees of arc. I didn't identify the surrounding stars at the time (so shocked I was at what I had seen) but I guess it began somewhere near delta Draco and ended under (near to the horizon) delta Cygni. It left a "smoke" trail that lasted about 3 seconds.

Gordon


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Sat Jan 31, 2009 5:13 pm 
Offline

Joined: Fri Dec 01, 2006 12:30 pm
Posts: 17
Dear Gordon
I recently re-appeared (did I hear many others groaning in the background) having been briefly in oblivion.
My first reaction after perusing your meteor sighting report was, not a cat in hell's chance we both saw the same meteor on 2008\12\01. The only thing in common really seemed to be we both saw meteors at very similar times.
However, I decided not to take things too much for granted and give the matter a bit more more thought.
We are only 3 or four degrees apart and Draco and Cygnus (where you saw yours)are many degrees away from Pegasus (where I saw mine) in the night sky are many more degrees apart . However. the stars in the constellations are immense distances away but we are only a couple hundred miles apart and meteors we see even closer.
I have never done any meteor height measuring and suspect our two reports are not accurate enough to provide definitive results but a few quick pencil marks on the back of an envelope suggest that it might not be totally out of the question that the "two meteors we saw" are one and the same thing.
With regards our reported colours; you mention bluish-white; me white.
Well as far as i am concerned that isn't very much different. Furthermore my feeling is that I personally tend not to see very many subtle difference in colour of night sky objects.
Perhaps one of the biggest problems might be reconciling our apparently very different estimates of the two meteors apparent magnitudeI think you reckoned mag -12; me more like -3. I suppose there could be many reasons for those differnces. One thing being your meteor was high in the sky (and possibly seenat a darkish site), mine was not only low down but also in quite a light polluted sky. Incidentally those same factors might even account for our slight colour differences.
I may give the matter a bit more thought. At very least I might learn a bit more about meteors and observing them.
It was nice to see your report.
Best of luck from Cliff


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Tue Feb 03, 2009 7:35 pm 
Offline

Joined: Sun Aug 13, 2006 12:29 pm
Posts: 663
Location: Goosnargh, north of Preston, UK
CliffM wrote:
Perhaps one of the biggest problems might be reconciling our apparently very different estimates of the two meteors apparent magnitude I think you reckoned mag -12; me more like -3. I suppose there could be many reasons for those differences. One thing being your meteor was high in the sky (and possibly seen at a darkish site), mine was not only low down but also in quite a light polluted sky. Incidentally those same factors might even account for our slight colour differences.


Hi Cliff,

You're quite right that there are several good reasons why two observers, of the same meteor, report different magnitudes. The first, as you suggest, is due to distance between the observer and the meteor. There are also going to be differences of perception between individual observers. McKinley covers the former in "Meteor Science and Engineering".

To quote McKinley:

Quote:
The absolute visual magnitude Mv of a meteor is defined as the magnitude it would have if it were placed at the zenith at a standard height of 100km.... Two corrections to the apparent visual magnitude of a meteor are necessary to convert it to absolute magnitude. The first is the usual inverse square law of attenuation of luminosity with the distance R to the meteor. The second correction is due to atmospheric absorption of light. Since this absorption occurs almost entirely in the lower troposphere, it is independent of the meteor's height and is a function only of the meteor's zenith distance Z, which is the angle between the observer's vertical and his line of sight to the meteor.


The chart below shows the corrections to be applied in relation to distance. I'm sure the absorption correction is only to be used as a guide and the true figure will be very dependent on atmospheric conditions.

Image

McKinley goes on to caution...

Quote:
The atmospheric correction should be applied if, for example, a meteor far from the zenith is compared either directly with a star of known magnitude near the zenith or indirectly by comparison with a nearby star whose apparent brilliance is in turn referred to a known star overhead. The atmospheric correction should not be made if the meteor is compared only with a nearby star of known magnitude at the same zenith distance, as the absorption will affect both equally.


Best wishes,

_________________
David Entwistle


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Tue Feb 03, 2009 11:45 pm 
Offline

Joined: Sat Dec 11, 2004 8:18 pm
Posts: 5899
Location: Manchester
Dear David
Many thanks!
I am not really sure I interpreted the McKinley gen you kindly provided correctly. However, using the Graph I just guestimated the meteor has being say 400 km distance (horizontally ) from here which gives a convenient correction of about magnitude -3. I then applied the absorption correction of mag -1. The taken my original estimated meteor apparent magnitude of mag -3, and then applying the mag -4 correction, gives a total mag -7.
That is still a way below Andrews observation estimate but not totally out of the question I suppose me perhaps needing to apply another correction for the meteors low altitude (say 70 degrees and worse below the zenith)as seen from Manchester (?).
Best wishes from Cliff


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Wed Feb 04, 2009 7:26 pm 
Offline

Joined: Sat Jul 23, 2005 6:51 pm
Posts: 646
Gordon: Many thanks for your report on this fireball, the fourth I've received on it. Unfortunately, the details available haven't allowed me to estimate a particularly accurate trajectory for it, but my best-guess is it was likely high above southern Wales, which seems consistent with your sighting too.

CliffM: There's little doubt that all four observers saw the same meteor; the timing and rough directions all point to this, assuming typical meteoric heights of around 80-100 km. If not, there would have to have been more than one bright meteor in a similar part of the atmosphere within a couple of minutes, while only one was seen at each of the separate locations. That's not impossible of course, but it is stretching coincidence rather.

Colours are too subjective to be used to confirm or deny the point, and occasionally meteors don't seem to show the same colours to witnesses at different angles to the track anyway.

The apparent magnitude discrepancy too could be down to a number of causes, not just those in McKinley's text David drew attention to. Fireball brightness estimates are notoriously difficult to make for visual observers. Potential variations result from differences in visual perception and sky clarity, distance to the object, and how much attention the observer was paying to recording this aspect of the event, as well as how familiar the observer was with reference targets like the brighter planets and phases of the Moon. One of the other December 1-2 event's witnesses suggested it was about magnitude -10/-11, while another noted it lit up the ground nearby, which suggests a brightness probably in excess of magnitude -7/-8, for instance. Though you may have been the most distant of the four observers to the trail if my "southern Wales" guesstimate is right, it's likely some tropospheric murk quite low in your sky - or between you and the meteor at least - had more to do with it than anything else. McKinley's formulae rely on a "clean" atmosphere, and as David mentioned, only give a guide to what the extinction may have been once reality intervenes.

Further sightings of this fireball or any others - meteors of magnitude -3 and brighter - made from the British Isles or nearby would be welcomed by the Meteor Section. See the "Fireball Observing" webpage for details of what to send me, at: http://www.popastro.com/sections/meteor/fireball.htm .

Alastair McBeath,
Meteor Director, Society for Popular Astronomy.
E-mail: <meteor@popastro.com> (messages under 150 kB in size only, please)


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Wed Feb 04, 2009 8:22 pm 
Offline

Joined: Sat Dec 11, 2004 8:18 pm
Posts: 5899
Location: Manchester
Dear Alastair
Many thanks !
I think David did suggest the "Mckinley Graph and associated matters" are not necessarily very accurate. I am pleased that despite the limited information related with respect to the 2nd Dec fireball that you think the meteor occurred possibly over South Wales because that fits with my back of an envelope (I really did use an envelope related to a bill we got that day) as a possible locality for the meteor. Bearing in mind I have never attempted to guestimate a meteors location before (although I have seen several over the years).So I feel quite pleased.
Having said that I had guestimated the meteors rough distance (400 km) horizontally from me purely on the basis of seeing it low towards my south west and possibly being 50 to 100 km altitude and it also being roughly north of Gordon in Somerset.
I have to admit that I was initially concerned about the differences in Gordon's mag -12 apparent brightness for the meteor and my own mag -3. Although I knew that my own apparent brightness estimate was not definitive. However, I feel pretty sure Chard's Somerset skies are much less light poluted than here in Greater Manchester (which qualify limiting mag +4.2 overhead even on a decent night).
On top of all that I think it nice that a tiny piece of material perhaps after travelling through aspce for many aeons possibly never ever seen by any form of life should be seen by at least two people and has Alastair now tells us, most probably four people, in the final and possibly only bright few seconds of its existence.
Thanks again to Alastair, David and Gordon. If it wasnot for them I wouldnot have known half as much as I do about this little meteor.
Best of luck from Cliff


Top
 Profile Send private message  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 7 posts ] 

All times are UTC


Who is online

Users browsing this forum: No registered users and 0 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group