Ursids (URS) Αρκτίδες - Μεγιστο 22 Δεκεμβριου '08

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Ursids (URS) - Αρκτίδες

 

Active: December 17 — 26

Maximum: December 22 07h30m UT (λsol = 270°7)

ZHR = 10 (occasionally variable up to 50)

Radiant: α = 217° δ = +76°

Radiant drift: see Table 6

vinf = 33 km/s; r = 3.0

TFC: α = 348° δ = +75° and α = 131° δ = +66° (β > 40° N)

α = 063° δ = +84° and α = 156° δ = +64° (β 30° to 40° N)

 

A very poorly-observed northern hemisphere shower, but one which has produced at least two major outbursts in the past 70 years, in 1945 and 1986. Several other rate enhancements, recently in 1988, 1994, 2000 and 2006 (when ZHRs were around 30 for two hours or more), have been reported too. Other similar events could easily have been missed due to poor weather or too few observers active. All forms of observation can be used for the shower, since many of its meteors are faint, but with so little work carried out on the stream, it is impossible to be precise in making statements about it. The radio maximum in 1996 occurred around λsol = 270°8, for instance, which might suggest a slightly later maximum time in 2008 of December 22, ~ 10h UT, while the 2000 enhancement was seen surprisingly strongly (EZHR ~ 90) by video at λsol = 270°78 (equivalent to 2008 December 22, 09h30m UT), although the visual enhancement then was much less, ZHR ~ 30. Whether the relative proximity of the shower's parent comet, 8P/Tuttle, at perihelion on January 26 this year, will have any influence on the shower this December, is unclear, though no such direct linkage has been apparent previously. The Ursid radiant is circumpolar from most northern sites (thus fails to rise for most southern ones), though it culminates after daybreak, and is highest in the sky later in the night. The waning crescent Moon should cause few difficulties for observing the maximum on this occasion, and if any recur as suggested, the peak timings should favour northerly sites between North America westwards across the Pacific Ocean to east Asia, including Japan and China.