Post by stewartw on Jan 25, 2015 14:16:29 GMT
Hi All,
On Tuesday 19th January 2015 reports started to appear in various media outlets that late the previous evening Graham Harkness, a Terenure based photographers, had fortuitously imaged a fireball while taking images of the night sky. The initial report at www.breakingnews.ie/discover/amateur-photographer-captures-once-in-a-lifetime-fireball-photo-over-dalkey-658692.html provides additional details, specifically that the image was taken at around 10.30pm from Dylan’s (a mis-spelling of Dillon’s) Park, Dalkey during a 30s exposure with a 14mm lens coupled to a Canon 6D camera.
The wording in the news report suggests that Graham did not notice the fireball at the time but only spotted it during a subsequent review of the image. Graham’s image made its way to the www.breakingnews.ie website and eventually to David Moore at www.astronomy.ie who in turn confirmed that it was a fireball. Additional reports are linked from astronomy.ie/mediaarchive.php
A high resolution copy of the image is available at pix.ie/harkio/3718045/size/5472/in/album/502556
So, was it really a fireball? A review of the image raises an important question: There is a cloud bank in the distance that is blocking out light from stars close to the horizon. A review of weather satellite imagery (see page 4 of the attached file) shows this linear cloud bank running north-east to south-west along the centre of the Irish Sea. The “fireball” appears to be passing in front of the cloud bank suggesting that it is relatively close at a range of approximately 100km. Taking account of the elevation above the horizon, this equates to an altitude of under 10km. Fireballs typically become visible at altitudes of between 110km and 80km and seldom penetrate deeper than 40km. The estimate from David Moore has been that the fireball was much more distant, somewhere close to the border with England and Scotland, however this fails to explain how the fireball was visible through the intervening cloud bank.
If the fireball was more distant one would also expect to see some reddening of the fireball trail due to atmospheric extinction (the same reason the sun appear red during sunrise / sunset). However the trail appears white.
It is also worth noting that the night-time skies above us are continuously monitored by networks of high sensitivity video cameras including those of Armagh Observatory (http://star.arm.ac.uk/meteor-cam/) and NEMETODE (http://nemetode.org/). These systems detect many thousands of meteors every year. NEMETODE coverage of the northern part of the Irish Sea and across northern England is excellent and although skies were clear, nothing especially bright was detected.
So, if it was not a fireball, is there a plausible alternative explanation? One possibility comes to mind by reviewing other parts of the image and considering the location from which the picture was taken. To the lower right hand side of the image one can see the trail resulting from the collision avoidance beacons on an aircraft – the trail being the result of the fact that the camera shutter was open for 30s. The image itself was taken from a location that overlooks the flight paths of aircraft on final approach to landing at Dublin Airport.
Using a planetarium package such as Stellarium it is possible to simulate how the night sky would have looked from Dillon’s Park at the time the picture was taken. By carefully measuring the position of the trails (the “fireball” and the aforementioned aircraft) with respect to the stars it is then possible to accurately estimate the start and end azimuths (Az) and Elevations (Ev) of each trail. The aeroplane extends from Az 85 to 92 degrees while the “fireball” runs from Az 34 to 39 degrees. The elevations are trickier as there are no stars visible behind each trail (due to the cloud bank) and the wide angle lens (14mm) used will inevitably introduce some image distortions that lead to errors in the estimated elevation. The aeroplane trail is estimated to have an Ev of approximately 09 degrees and the “fireball” of approximately 05 degrees.
So, were there any aircraft in the vicinity that would have simultaneously appeared at the aforementioned Az and Ev to an observer in Dillon’s Park?
At 10.43pm Ryanair RYR996C_FR6394 was passing due east of Dillon’s park on a northbound trajectory. The azimuth and elevation is a good match for the aeroplane trail in the lower right of the image.
At precisely the same time Ryanair RYR92QG_FR2972 from Kaunas to Dublin was transitioning from a south-eastern to a south-western trajectory out over the Irish Sea at an azimuth and elevation that again are a good match for the “fireball” trail in the lower left of the image.
Of course aircraft in the right place don’t explain the differences in what was observed – the “aeroplane” trail in the lower right is very different to that of the “fireball” in the lower left.
A reasonable explanation is that when on final approach to landing, commercial aircraft switch on a front “headlight” to aid visibility both for themselves and for observers on the ground. If this headlight was switched on then an observer in Dillon’s Park would have seen this light mounted on Ryanair RYR92QG_FR2972 appear relatively faint as the aircraft flew on a south-east trajectory then dramatically increase in brightness as the aircraft swung round to the south-west at which point the headlight would be shining directly towards an observer in Dillon’s Park. This explains why the trail (due to the shutter being open for 30s) is initially faint then very bright towards the end.
The attached file (which is a .pdf of a spreadsheet used to perform the calculations) provides further details together with references and screenshots relating to the aircraft position / altitudes.
Of course all this assumes that the image was taken at or close to 10.43pm. Attempts have been made to ascertain precisely when the photograph was taken but thus far I have not been able to obtain anything more definitive than the “… around 10.30pm …“ mentioned in the initial reports.
In the absence of more definitive timing data I therefore suggest that as it can be proved that two aircraft were in the right place close to the reported time and taking account of the weaknesses in the “fireball” explanation discussed earlier, that the image is more likely to be that of Ryanair RYR92QG_FR2972 and not of a fireball.
With acknowledgements to NEMETODE observer Graham Roche who independently raised the possibility that what was observed was an aircraft.
Best regards
William Stewart
www.nemetode.org
Graham Harkness Fireball Analysis 20150125_01.pdf (456.04 KB)
On Tuesday 19th January 2015 reports started to appear in various media outlets that late the previous evening Graham Harkness, a Terenure based photographers, had fortuitously imaged a fireball while taking images of the night sky. The initial report at www.breakingnews.ie/discover/amateur-photographer-captures-once-in-a-lifetime-fireball-photo-over-dalkey-658692.html provides additional details, specifically that the image was taken at around 10.30pm from Dylan’s (a mis-spelling of Dillon’s) Park, Dalkey during a 30s exposure with a 14mm lens coupled to a Canon 6D camera.
The wording in the news report suggests that Graham did not notice the fireball at the time but only spotted it during a subsequent review of the image. Graham’s image made its way to the www.breakingnews.ie website and eventually to David Moore at www.astronomy.ie who in turn confirmed that it was a fireball. Additional reports are linked from astronomy.ie/mediaarchive.php
A high resolution copy of the image is available at pix.ie/harkio/3718045/size/5472/in/album/502556
So, was it really a fireball? A review of the image raises an important question: There is a cloud bank in the distance that is blocking out light from stars close to the horizon. A review of weather satellite imagery (see page 4 of the attached file) shows this linear cloud bank running north-east to south-west along the centre of the Irish Sea. The “fireball” appears to be passing in front of the cloud bank suggesting that it is relatively close at a range of approximately 100km. Taking account of the elevation above the horizon, this equates to an altitude of under 10km. Fireballs typically become visible at altitudes of between 110km and 80km and seldom penetrate deeper than 40km. The estimate from David Moore has been that the fireball was much more distant, somewhere close to the border with England and Scotland, however this fails to explain how the fireball was visible through the intervening cloud bank.
If the fireball was more distant one would also expect to see some reddening of the fireball trail due to atmospheric extinction (the same reason the sun appear red during sunrise / sunset). However the trail appears white.
It is also worth noting that the night-time skies above us are continuously monitored by networks of high sensitivity video cameras including those of Armagh Observatory (http://star.arm.ac.uk/meteor-cam/) and NEMETODE (http://nemetode.org/). These systems detect many thousands of meteors every year. NEMETODE coverage of the northern part of the Irish Sea and across northern England is excellent and although skies were clear, nothing especially bright was detected.
So, if it was not a fireball, is there a plausible alternative explanation? One possibility comes to mind by reviewing other parts of the image and considering the location from which the picture was taken. To the lower right hand side of the image one can see the trail resulting from the collision avoidance beacons on an aircraft – the trail being the result of the fact that the camera shutter was open for 30s. The image itself was taken from a location that overlooks the flight paths of aircraft on final approach to landing at Dublin Airport.
Using a planetarium package such as Stellarium it is possible to simulate how the night sky would have looked from Dillon’s Park at the time the picture was taken. By carefully measuring the position of the trails (the “fireball” and the aforementioned aircraft) with respect to the stars it is then possible to accurately estimate the start and end azimuths (Az) and Elevations (Ev) of each trail. The aeroplane extends from Az 85 to 92 degrees while the “fireball” runs from Az 34 to 39 degrees. The elevations are trickier as there are no stars visible behind each trail (due to the cloud bank) and the wide angle lens (14mm) used will inevitably introduce some image distortions that lead to errors in the estimated elevation. The aeroplane trail is estimated to have an Ev of approximately 09 degrees and the “fireball” of approximately 05 degrees.
So, were there any aircraft in the vicinity that would have simultaneously appeared at the aforementioned Az and Ev to an observer in Dillon’s Park?
At 10.43pm Ryanair RYR996C_FR6394 was passing due east of Dillon’s park on a northbound trajectory. The azimuth and elevation is a good match for the aeroplane trail in the lower right of the image.
At precisely the same time Ryanair RYR92QG_FR2972 from Kaunas to Dublin was transitioning from a south-eastern to a south-western trajectory out over the Irish Sea at an azimuth and elevation that again are a good match for the “fireball” trail in the lower left of the image.
Of course aircraft in the right place don’t explain the differences in what was observed – the “aeroplane” trail in the lower right is very different to that of the “fireball” in the lower left.
A reasonable explanation is that when on final approach to landing, commercial aircraft switch on a front “headlight” to aid visibility both for themselves and for observers on the ground. If this headlight was switched on then an observer in Dillon’s Park would have seen this light mounted on Ryanair RYR92QG_FR2972 appear relatively faint as the aircraft flew on a south-east trajectory then dramatically increase in brightness as the aircraft swung round to the south-west at which point the headlight would be shining directly towards an observer in Dillon’s Park. This explains why the trail (due to the shutter being open for 30s) is initially faint then very bright towards the end.
The attached file (which is a .pdf of a spreadsheet used to perform the calculations) provides further details together with references and screenshots relating to the aircraft position / altitudes.
Of course all this assumes that the image was taken at or close to 10.43pm. Attempts have been made to ascertain precisely when the photograph was taken but thus far I have not been able to obtain anything more definitive than the “… around 10.30pm …“ mentioned in the initial reports.
In the absence of more definitive timing data I therefore suggest that as it can be proved that two aircraft were in the right place close to the reported time and taking account of the weaknesses in the “fireball” explanation discussed earlier, that the image is more likely to be that of Ryanair RYR92QG_FR2972 and not of a fireball.
With acknowledgements to NEMETODE observer Graham Roche who independently raised the possibility that what was observed was an aircraft.
Best regards
William Stewart
www.nemetode.org
Graham Harkness Fireball Analysis 20150125_01.pdf (456.04 KB)