Sky and Telescope - April 2018 - 49

What's in a Name?
◗ We prefer to make our comets

CORONA
BOREALIS

April 22
About 10 p.m.

10°

DRACO

HERC ULES

Vega

Lyrid
radiant

LYRA

eneb

Facing Northeast
p Lyrid meteors may appear anywhere in the sky. If you can trace a meteor's track backward to a
point of apparent origin near Vega in Lyra, you'll know you've spotted a Lyrid.

But one never knows, and the new quarter Moon falls on April 22nd, so the
morning hours won't be disturbed by moonlight. The shower's radiant is in eastern Hercules, 8° southwest of brilliant Vega. This means that it's low at dusk for
observers at mid-northern latitudes, hovering just above the northeastern horizon.
Visibility, and thus meteor counts, should improve as the radiant climbs higher and
the Moon sets after midnight.

G REGG DINDER M A N / S&T

Moon Occults Nu Geminorum
ON THE NIGHT of April 20-21, the waxing crescent Moon, about 30% lit,
occults 4th-magnitude Nu (ν) Geminorum. Nu's a double, possibly multiple, star
system, with a blue giant B6-type primary. This is an easy split for small scopes.
A 90-mm reflector at 50× will separate the 8th-magnitude secondary from the
lucida without problems. But Nu Gem A and B themselves are both very tight
double (or even triple) stars, which leads to some uncertainty about the stars'
spectral classes.
The broad occultation path covers the southern half of North America and all
of Central America, but for those in the west, this is an evening twilight event.
Phoenix, for instance, sees the occultation begin just 45 minutes after sunset.
Conditions should be better toward the east, where it will be darker, but the
Moon will be lower. For Jacksonville, Nu's reappearance on the dark limb comes
with the Moon at an altitude of only 10°.
Timetables for the event for cities and towns along the path are available from
the International Occultation Timing Association (lunar-occultations.com/
iota/iotandx.htm), but here are some preliminary timings:
Phoenix, disappearance 7:45 p.m., reappearance 8:51 p.m. MST; El Paso, d. 8:55
p.m., r. 9:58 p.m. MDT; Dallas, d. 10:08 p.m., r. 10:57 p.m. CDT; Huntsville, d.
10:22 p.m., r. 10:47 p.m. CDT; Jacksonville, d. 11:23 p.m., r. 11:57 p.m. EDT.

more cozy by using their common names: Comet McNaught,
Comet Thatcher, Halley's Comet,
the Great Comet of 1811 . . . But
astronomers follow a different set
of rules when referring to these icy
travelers. In 1994, the International
Astronomical Union (IAU) codified
a system meant to eliminate confusion caused by duplicate names.
Let's look at Comet Thatcher's
official designation, C/1861 G1
(Thatcher), to explain the rules.
The most common prefixes for
comet designations are C and P. A
preceding P means you're looking
at a periodic comet (any comet
with an orbital period of 200 years
or less). The number attached to
the P reveals the order of discovery. For instance, Comet 1P/1882
Q1 (Halley) is so-called because
it was the first comet determined
to be periodic. These numbers are
assigned after a comet's second
perihelion passage, which (ideally)
confirms the object's periodicity.
A preceding C is used for nonperiodic comets, or comets with
periods longer than 200 years.
Comet Thatcher rounds the Sun
every 415 years, so its designation
begins with a C. If you see an X,
you'll know that the comet's orbit
couldn't be calculated. If you see
an I, you'll know you've got an
interstellar object (the IAU added
I to the system last year after the
discovery of 1I/2017 U1 ('Oumuamua) in October).
Thatcher's 1861 is straightforward - that's the year of discovery. The G tells us that the comet
was discovered in the first half of
April (January = A+B; February =
C+D; March = E+F; April = G+H;
and so on). The 1 lets us know that
this was the first comet discovered
in the G period. And, of course, we
wrap up the designation with the
last name of the discoverer or the
object's coziest call sign.
sk yandtelescope.com * A PR I L 2 018

49


http://www.lunar-occultations.com/iota/iotandx.htm http://www.lunar-occultations.com/iota/iotandx.htm http://www.skyandtelescope.com

Sky and Telescope - April 2018

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