Plotting Observed Meteors
These advanced observations require specially designed gnomic star charts, which allow the plotting of meteor paths using straight lines. These observation is disadvantaged, in that they require the observer to take his/her eyes off the sky in order to plot meteor paths immediately after the event. Gnomic charts are used to plot meteors in the field, but care must be taken when they are used to identify radiants, since distortions may arise when plots are made close to the edge of the maps.
Before commencing an observing session, the charts representing the observer's field of view are selected. There are 10 charts altogether, but not all are used at once by a single observer. The observer may wish to place the charts on a flat folder and use a pencil and ruler to plot observed paths. The direction of meteor flight is indicated on the charts by an arrow in the leading direction and the magnitude/colour/train characteristic is recorded (in the shorthand described above) alongside the plotted path.
Depending on the meteor activity at the time, more than one hour of observation may be recorded on the same chart, as long as it is clear what time and date interval each belongs to. For example, label each trail with a letter for the date/hour of observation, for example "hour A" might mean "1989 July 12 from 20h-21h AEST". A meteor seen in this hour may be labelled for instance "2ytA" using the above convention for a second magnitude yellow meteor with a train. Another hour may then be defined as "hour B" and meteors seen in this time period are then labelled with an ending B, and so on.
After meteors on a set of charts are recorded they may be classed as shower or sporadic meteors. Known radiants are plotted on the maps using the Southern Hemisphere Meteor Stream List, and meteors are allocated to categories. This technique allows the identification of new meteor radiants, and refinement of the positions of old ones.
Radiants, by plotting, are identified as follows. The plotted paths of meteors are projected, without drawing, in the opposite direction to their flight, by a distance of 1.5 times the path length. From this point, the meteor is projected by a further 8.5 times its path length, with a line being drawn (see below). This makes for a projection of 10 times the original path length. Meteors projected backwards which fall onto two or more charts are plotted carefully, and the overlap between maps taken into careful consideration.
Projection of Meteor Paths for plotting
After projection of all meteors is complete the following rules are used to identify a radiant. The intersection of a number of meteors in a diameter of 10mm (20mm for meteors close to the edge of a chart) on the gnomic charts constitutes a radiant. The number of intersecting projected meteor paths needed to define a radiant depends on the number of meteors plotted:
If < 18 meteors plotted: 3 or more intersecting projected paths define a radiant.
If 18-35 meteors plotted: 4 or more intersecting projected paths define a radiant.
If > 35 meteors plotted: 5 or more intersecting projected paths define a radiant.
Streams listed in the Southern Hemisphere Meteor Stream List are also plotted with a radiant size of 5mm to 10mm. Projected paths intersecting these radiants constitute shower meteors belonging to those streams. Note that some radiants are more diffuse than other radiants on the sky. Old showers tend to have radiant sizes greater than younger showers. Special consideration should be given to radiant sizes if these are given. Plotting a given radiant size on a gnomic chart must also be done carefully, to avoid misrepresenting the radiant size on the map due to miscalculations in scale.
Meteors which do not belong to a radiant by intersection or by plotting known radiants are termed SPORADIC and are listed under this classification.
Projected paths intersecting more than one stream are divided between those streams and recorded as fractions.
NOTE:The intersection of the actual path of a meteor with a radiant DOES NOT define a shower meteor. It is the intersection of the projected paths which are important.
Also note that the radiant positions given in the Stream List are given for the middle of the active period or the date of maximum. The radiant co-ordinates drift by 0.6° eastward, parallel to the ecliptic, per day. Allowance for this drift is essential.
Double Station Plotting.
This is the most valuable form of visual meteor observation. It involves the plotting of observed meteors in the same way as described under the 'Meteor Plotting' section, but by two observers observing simultaneously. Observers are spaced between 50km to 150km apart, and observe areas of sky such that the triangle formed by the 2 observers and the meteor does not involve angles less than about 15°. Observers should not face in exactly the same direction or exactly towards each other, since this produces a critical case when evaluating formulae for triangulation. Make sure to note down:
(a) Site Latitudes, Longitudes, and straight-line separation in kilometers
(b) Azimuth of eastern site to western site.
Three stations can also be used, in which case the observations will determine the path of a meteor through space.
Meteors are then plotted as usual, but the time of each meteor is recorded to the nearest minute or better to avoid any ambiguity. Plotted meteors may be given numbers, and their data recorded on a special sheet containing columns to record all information. Such a form is enclosed and may be used for double station work. Meteor heights can then be calculated from this data, to an accuracy governed by the plotting accuracy.
The accuracy of a plotted meteor may be recorded in your own shorthand form, which must then be well explained, including the error margin corresponding to each shorthand symbol. For example a plot accuracy of "good" could correspond to an error of 2°, and be labeled as "g". Observers must aim to keep their plotting as accurate as possible. The height of the meteor is usually determined at its end point, and it may be of value to plot the end points of meteors on charts where the scale is better than that of the gnomic charts, in order to accurately calculate the parallax between the two sites. Observers must note the exact time of each observed meteor, and it is wise to synchronize accurate timepieces in order to avoid any ambiguity.