Leonid Trail Encounter Parameters

Forecasts of Leonid outbursts and explanations for past displays, based on determining that the Earth encounters specific trails, have been published elsewhere (e.g. [8,14-18]). In Table 1 the nodal positions of trail centres are listed for encounters in years near the present. To generate these data, the point along the trail must be found that reaches the descending node in mid-November; the effect of perturbations on the nodal position of particles at that point can then be determined. As the time of nodal passage is generally quite a smooth function of Aao for the youngest trails, there is usually no problem in finding the desired Aa0- This technique has been applied to many streams, e.g., the June Bootids [19] and Draconids [20].

It is possible to identify details of the perturbation history of particles in Table 1. For example, fI of particles in the 4-revolution trail at Aa0 = +0.14 increases by between 1866 and 2001, and this happens mainly during two moderately distant approaches (~1 AU) to Jupiter in 1898 and 1901, before and after perihelion. At those times, the comet is ~2 AU ahead of the particles, so that the geometry of the approaches is different and fi only changes by half as much. The rD of those particles changes a few times during the 4 revolutions, sometimes by ~0.01 AU during the course of a single, moderately distant, planetary approach. It is therefore rather fortuitous that this part of the trail is brought

Table 1

Encounters with young Leonid trails during years around present return of 55P/Tempel-Tuttle. Listed are age of trail in revolutions, point along trail parameterised by Aao, longitude of nodal position given as date when Earth reaches that longitude, and difference in heliocentric distance (AU) between Earth and descending node of trail.




te - rD



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