Sky and Telescope - December 2016 - 49

Learn how to use detailed star charts to ﬁnd the very faintest
things with your telescope: skypub.com/charts.

urge readers to do meteor counts for
several nights before and after a shower's
peak, to obtain a longer record of what
the shower is up to - especially because
fewer people observe away from the peak.
But this year, the last time you'll get even
one moonless dark-sky hour is before
dawn on December 11th. Then you're
mooned out until after nightfall on the
16th (for mid-northern latitudes).
So let's turn some needed attention
to December's other signiﬁcant meteor
shower: the Ursa Minorids, which are
brieﬂy active three days before Christmas. The Ursids' nominal peak lasts
just 12 hours, supposedly how long the
shower remains above half maximum
strength. This year the peak should come
around 9h UT December 22nd, good
timing for North America, especially the
West: 4 a.m. EST, 1 a.m. PST.
The shower's radiant is near Kochab
at the bowl of the Little Dipper. This spot
is circumpolar, just 14°° from the north
celestial pole, so the Ursids are active all
night (and all day too). But the radiant
stands highest just before dawn, and this
year the waning Moon, one day past third
quarter, rises around 1 a.m. local time on

Alan MacRobert

the morning of December 22nd. Earlier,
you're about equally well oﬀ all the way
from nightfall on the 21st until moonrise;
the radiant changes altitude little during
this time. The higher a shower's radiant,
the more meteors appear all over the sky.
Don't expect much. The Ursids are
typically reported to produce 5 to 15 meteors visible per hour under a very dark sky.
The International Meteor Organization
(IMO) lists its peak zenithal hourly rate as
just 10. And that's under theoretical ideal
conditions, which includes the radiant
being overhead - and for this shower,
that doesn't happen outside the Arctic!
For meteor watchers around latitude 40°
north, the Ursid radiant remains about
30° high from dusk until 1 a.m., so the
rate you see even under very dark-sky
conditions will be about half the zenithal
hourly rate that's actually occurring.
But surprises happen. Outbursts with
ZHRs of up to 50 or 100 or more were
observed in 1945, 1986, and 1993. Longtime meteor observer Robert Lundford
witnessed the 1993 outburst. "It caught
me totally oﬀ guard," he wrote. "My data
sheet ﬁlled with over 100 entries, and
I had to scribble some 50 more on the

blank side of the sheet." Several lesser
enhancements have also been reported
over the years, and, says the IMO, "Other
events could have been missed easily."
That's because the Ursids are very
poorly observed. Maybe it's the cold,
maybe it's the Geminids stealing the
glory, maybe it's their sparseness, or
people being busy just before Christmas.
So if you'd like to collect some data that
meteor scientists really want - and if
an hour lying back in a lawn chair and
gazing into the stars appeals you to even
if the action is likely to be quiet - go to
www.imo.net/visual to learn the standardized methods for carrying out a scientiﬁc
meteor count and reporting it in.
If you're more ambitious you can also
record the meteors' magnitudes, or plot
their paths on a special star map in case
some even lesser shower with a diﬀerent
radiant proves to be active too.
The Ursids originate from Comet 8P/
Tuttle, which has an orbital period of 13.6
years. The shower was apparently discovered more than a century ago by the great
meteor observer William F. Denning,
but coordinated studies of it didn't begin
until after the outburst of 1945.

Quick Uranus Spotter: Can You Detect the "Secret Naked-Eye Planet"?
From November through
January, Uranus forms a triangle less than 2° long with
Zeta (ζ) and 88 Piscium.
Uranus is magnitude 5.7
or 5.8, Zeta is 5.2, and 88
is 6.0. Binoculars show the
pattern easily - and here's
a nice chance to try for
Uranus with your unaided
eye from a dark site.
This ﬁeld is in the southern cord of the two Pisces
ﬁshes. The brightest two
stars of Aries point 18°
down to 4.2-magnitude
Epsilon (ε) Piscium, crossing Eta along the way.

1h 30m

1h 20m

1h 10m

1h 00m

-10°

Path

of U

Mar 1

PISCES

ran

us

Feb 1

ε

Jan 1

+8°

ζ

Nov 1
Dec 1
88

+

+6°

80

Star magnitudes
4 5 6 7 8

Sk yandTelescope.com December 2016

49
http://www.imo.net/visual http://www.SkyandTelescope.com
Sky and Telescope - December 2016

Table of Contents for the Digital Edition of Sky and Telescope - December 2016
