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Triangulation of a Geminid Meteor

A copy of the original paper is available Here

A group of Society members decided to observe and photograph the 1996 Geminids on Friday 13 December, 1996. There was a band of clear, cold weather across the London area during the evening, though intending observers 50 miles to the north-west or to the south-east were clouded out. In the event 3 observers exposed films from 2100 to 2400 UT, and all were rewarded with a good display of meteors, some brightening, it was estimated, to -6 magnitude.

All observers used 5 minute exposures, beginning exactly on the hour and at multiples of 5 minutes thereafter. The observers were organised in pairs, each pair pointing their undriven cameras at the same point 100km above the surface. This is the same procedure that was used for the successful Perseid triangulation in August 1993 (see "The Astronomer", April 1994, 30, 360).

Inspection of the film revealed 2 images that evidently corresponded to the same meteor, one taken from a site near Sidcup and the other near Chelsfield. The Chelsfield photograph only caught the last part of the trail, the start being off-frame. Both observers had used 35 mm Ilford HP5 Plus and a 50 mm f1.8 lens. Curiously, these observers had not been working as a pair: the partner of the Sidcup observer was clouded out at the last moment: the partner of the Chelsfield observer saw and timed the meteor, but it missed his camera's field of view. Unfortunately neither successful photographer had used a light interrupter (chopper), though (typically) the third photographer had! The meteor was timed at 2256:17 UT and its brightness was estimated to be -6 magnitude at peak.

As with the 1993 Perseid, both images were measured using the Zeiss Plate Measuring machine at the Crayford Manor House. Two points were measured on each trail, which would uniquely define the meteor's trajectory through the upper atmosphere. Unfortunately the 2 observers were closer together than one would have wished - about 8.4 km - and their positions turned out to be almost coplanar with the meteor trail. This meant that that the distance from either observer to the trail could not be determined very precisely, particularly the distance to the brightening (which only one film had recorded anyway).

Nevertheless, the calculations yielded the following details:

(1) Distance from Sidcup to brightening: 100 km (approx)
Distance from Chelsfield to edge of field : 70 km (approx)
Distance from Sidcup to extinction: 58 km
Distance from Chelsfield to extinction: 59 km

(2) Distance along path from brightening to extinction: 40 km (approx)

(3) Brightening was about 80 km above the surface.

(4) Extinction was 49 km above the surface.
(5) The slope of the trajectory was about 50 degrees from the horizontal, and the direction was roughly westwards along the Thames estuary, with extinction occurring over Canvey Island.

(6) The direction of origin was estimated to be RA 7h 26m, Dec +31.5 (2000), close to the Geminid radiant given in Norton's Star Atlas 2000.0 as RA 7h 31m, Dec +32 deg.

Fortuitously, the low altitude of this object, compared to the expected height of 100 km, permitted it to be captured by 2 observers who were not partners, but whose fields of view chanced to intersect at a lower altitude.

As a cross-check, the position in space of the point of extinction was calculated on the assumption that it was the same on both images. (Generally a dubious assumption, but probably justifyable here because both observers had used the same type of film and lens, and seemed to be about the same distance from the trail.) This gave an extinction at about 50 km altitude, just as before.

Summary


The observing session on 13 December showed a good rate with many bright meteors, and was enjoyed by all those who took part. The object that was
triangulated appeared to come from close to the Geminid radiant, and penetrated to a much lower altitude than many other streams. This will be taken into account when planning future Geminid triangulation projects.


JJH, February 1997.

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