Nigerian skies, in 2017 – Part 5
Be the ZHR as it may, meteor shower ratings can be very helpful guides, especially if you are planning a community, classroom or club outing.
Another conceptual asset is the “radiant”. This is the direction, in the sky, from which most of the apparitions appear to emanate.
“Because… shower particles are… traveling in parallel paths, at the same velocity,” AMS explains, “they will…appear to radiate from a single point… [due to] the effect of perspective”.
Almost all meteor showers get their names from the constellation of stars their radiant is focused on—which serves as a reference point for observing the display.
Accordingly, the International Astronomical Union attaches “id” or “ids” (meaning “derived from”) to the names of background constellations, as designations for meteor showers.
Thus the “Taurids” appear to originate in the constellation Taurus the Bull, the “Orionids” in Orion the Hunter and so on.
Not incidentally, the closer the radiant is to overhead, the more meteors you’ll see. Keep in mind though, that the meteors are flashing in our atmosphere, rather than among the stars.
With that caveat, I now begin a composite review of the meteor showers, visible from Nigeria’s very privileged vantage point, at Earth’s equator.
My selections are based largely on publications of the American Meteor Society and the International Meteor Organization, along with supplementary sources, which I will cite.
AMS has compiled a diversified list that includes nine major meteor showers (ZHR of 10 or above) and 17 minor displays (ZHR below 10).
The review emphasizes showers with an equatorial radiant—i.e., whose meteors appear to come from constellations on the celestial equator. These displays are visually accessible to us.
Wikipedia lists 15 such background constellations: Pisces; Cetus; Taurus; Eridanus; Orion; Monoceros; Canis Minor; Hydra; Sextans; Leo; Virgo; Serpens; Ophiuchus; Aquila and Aquarius.
We’ll start with the Quadrantids shower, which commenced December 28 and closed out January 12th (nine days after its peak output).
At ZHR 120, this major will open the main viewing season next year. But remember: ZHR is an abstraction. In IMO’s estimate, one can only expect to see around 25 meteors per hour.
Still, that qualifies the Quadrantids for elite status, as one of the “Big Three”—along with the highly productive Perseids and Geminids exhibitions.
This light-heavy-weight stands out in other ways, as well. It has an asteroid and a comet as parents, for instance, and a name that is not derived from an existing constellation!
There is an interlude, after late-February, during which two minor showers—the Alpha Centaurids and Gamma Normids—reportedly dominate the cosmic stage.
Like the Quadrantids, Centaurids has already bowed out—peaking at an hourly rate of 3-5 meteors February 8th, then leaving Gamma Normids to carry on, in March (11th thru 21st).
Comet watcher Gary W. Kronk, counsels that both are Southern Hemisphere displays. Even so, a few equatorial strays from Gamma Normids’ 3-5 meteor spike may be visible on the 16th.
“Only the weakest minor meteor showers,” Kronk adds, “are active…April through June. But late July and early August is “one of the best times of the year to observe meteors”.
He notes further, that several minor meteor showers emanate from the neighbouring constellations of Capricornus and Aquarius—four of which peak July 28 to August 13.
But, as Lunsford points out, “all of these sources produce low activity and rarely exceeds 5 meteors per hour to the unaided eye”.
Yet there are exceptions. Some minor showers can produce pleasant surprises; and the Lyrids, a middle-weight major, are usually reliable.