An Interesting Observation Made During the 2017 Ursid Meteor Shower


Ursid Meteor Shower Meteors

These three meteors were recorded within a twenty minute period the morning of 12/24/2017 (local time), some 45¾ hours after the shower's expected peak, and only just barely within its usually tabulated date range (12/17-12/24). Universal Time (UT) is shown, and all three photos are reproduced at the same scale. The first two are from exposures of the same field, the third from a field only 3.6° S and 2.0° W of it.

The first meteor is shown as a negative because then the faintest parts of the trail are more easily visible all the way to its eventual fadeout in the lower right corner. It's total extent is 4¾°. The other two tracks are bounded on one end by the edge of the photo's frame -- top and bottom, respectively.

The middle meteor is a late Ursid shower meteor, as the angle of its direction of motion relative to due N is correct (6¾° E or counter-clockwise of vertical), given the locations of the meteor (for M58: Dec = +11°49.1', RA = 12h37.7m) and the shower's radiant on the celestial sphere (Dec = +75°36', RA = 14h20m, which is the mean of six values I found online from previous studies of the shower). This angle is independent of the time of observation, as the two points are fixed relative to one another in an equatorial coordinate system. The distance between them is 65°.

The other two meteors were nearly if not exactly parallel to one another, at an angle of 18.0° E of N. There's a ±0.05-0.10° uncertaintly on both this angle and the divergence from perfect parallelism, and it's probably closer to the higher end of the range.

Because of the lack of divergence there's no way to calculate a meaningful hypothetical second radiant from these two meteors, and it wouldn't be advisable to base a putative radiant on only two meteors in the first place. From their 18.0° angle relative to N one can only say it's farther E and/or S of the Ursid radiant, if it exists.

The list at Meteor Activity Outlook for December 23-29, 2017 suggests the Quadrantids as being the only radiant in roughly the correct direction, but it's off by ~10° (27 5/8° vs. 18°) and it's also way too early by about 10 days.

This is not really an area of astronomy I've ever worked in or know much about, so I'm not sure exactly what to make of these two meteors, but thought I'd throw what observations I have out there for others to be aware of and maybe make use of. The original photos are in color, and there's no indication for the two mystery meteors of the greenish color typical of manmade space junk burning up -- if anything the colors are orangeish-red -- but I can't totally discount the possibility.

The sun's ecliptic longitude at the mean time of the two observations shown here (12:29½ UT) was 272.642° (epoch 2000.0) or 272.896° (epoch of observations). The next time the sun will be at this longitude, 12/24/2018, will be before local noon here; in two years (12/24/2019) the local time will be slightly before civil twilight ends. Thus, I encourage observers W of here, in Hawaii or Asia (and possibly parts of Europe), to look for confirming meteors the next time or two around the sun, since it won't be until December 2020 that I'll be able to do so under dark skies.



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