Sky & Telescope - October 2019 - 48

OCTOBER 2019 OBSERVING
Celestial Calendar by S. N. Johnson-Roehr

October 21
11:30 pm

October
Is For
Orionids

Pleiades

TA U R U S

Capella

AURIGA

Even though observing

Aldebaran

10°

conditions aren't ideal, you
may see a few fireballs this
month.

T

wice a year, Earth travels through
the trails of dust that Halley's
Comet leaves behind when it passes
through the inner solar system every
76 years. In May, Earth encounters the
particle streams that mark the comet's
multiple inbound paths; weather
permitting, we witness the interaction
as the Eta Aquariid meteor shower. In
October, when Earth hits Halley's outbound paths, it's time for the Orionids.
The earliest record of a shower that
can credibly be identiﬁed as the Orionids comes from the annals of the Sui
dynasty in China, where historians
noted "hundreds of meteors scattered in
all directions" on the night of September 23 AD 585. Yet the location of the
shower's radiant - the point in the sky
from which a shower's meteors appear
to emanate - wasn't determined until
the 1860s, when astronomer Alexander
Herschel included his observations of
"luminous meteors" that originated
near Nu (ν) Orionis in several annual
reports for the British Association for
the Advancement of Science.
Connecting the Orionids to their
parent comet took more than a century
longer, however. Chinese astronomers
recorded observations of Halley's Comet
as early as 239 BC, but the relationship between comet and shower wasn't
clearly deﬁned until 1983, when B. A.

48

OCTOBE R 2 019 * SK Y & TELESCOPE

ORION
GEMINI

Orionid
Radiant

Castor

Betelgeuse
Rigel

Pollux

Looking East

The radiant for the Orionid meteor shower is in Orion's raised club, which
stands highest in the south before dawn.

McIntosh (National Research Council
Canada) and A. Hajduk (Astronomical
Institute of the Slovak Academy of Sciences) co-authored a paper on Halley's
meteor stream.
Look for the Orionids to begin
appearing in early October, with a
predicted peak on the night of October
21-22. The shower has a zenithal hourly
rate of 20, which means that under a
dark, moonless sky you'd typically see
about 20 meteors in an hour if the
radiant was directly overhead. For the
Orionids, the radiant is northeast of
Betelgeuse, which rises about 10:30 p.m.
daylight-saving time on the 21st. The
radiant reaches a comfortable altitude

by 11:30 p.m., but the waning crescent
Moon, about 40% lit, rises just an hour
after that. Typically, meteor viewing is
better in the hours between midnight
and dawn, but this year, it might pay to
start watching in mid-to-late evening.
Even after the Moon clears the
horizon, all is not lost. Meteors are
more visible when you watch the sky
45°-90° away from the radiant, so if
you look to the north and west, both
the climbing radiant and Moon will be
at your back. And though the peak falls
on October 21-22, Orionids appear
through November 7th or so. The slimming Moon will be less of a factor in
the week following the predicted peak.
Sky & Telescope - October 2019

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