WWW.ASTRONOMY.COM 43
LOCATING ASTEROIDS I
Pursue a pebble in a pitcher
ASTEROID 15 EUNOMIA SPLASHES across the starry back-
drop of Aquarius the Water-bearer in December. Eunomia spends
the month within a binocular field of Alpha (α) Aquarii, a magni-
tude 2.9 star that lies about 20° southwest of the southwestern
corner of the Great Square of Pegasus. The asteroid glows at 10th
magnitude, however, so you’ll need a small telescope to pick it up.
Eunomia slides 0.8° south of Alpha on the 17th. But the aster-
oid saves its best encounters for 2019’s final week. It then passes
through the Water Jar asterism, a group of 4th- and 5th-magnitude
stars comprising Gamma (γ), Pi (π), Zeta (ζ), and Eta (η) Aqr.
To find Eunomia, use the finder chart below to zero in on its
approximate location and then sketch the half-dozen or so bright-
est stars you see through your telescope. Come back a night or
two later, and the “star” that has shifted position is the asteroid.
Italian astronomer Annibale de Gasparis discovered Eunomia
in 1851. With an average diameter of about 165 miles, it ranks
among the dozen biggest objects in the asteroid belt. Scientists
suspect it is the remnant of a larger body destroyed in a collision.The Water Jar asterism in Aquarius forms a magnificent backdrop for
this 10th-magnitude asteroid during the latter part of December.1°kAQUARIUSPEGASUSNEPath of EunomiaDec 161116212631
d/ca_Eunomia grabs a drink of water
the backdrop of central Libra,
some 11° east of Mars. At mag-
nitude –0.6, the innermost
planet shines nearly 10 times
brighter than its neighbor.
The gap between these two
worlds grows as the month
progresses because Mars is
pulling away from the Sun
while Mercury is drawing
closer. You can follow Mercury
throughout December’s first
half. It rises an hour before the
Sun on the 15th, though it
stands only 6° high a half-hour
later. If you have an exception-
ally clear sky and an unob-
structed horizon toward the
southeast, you might spot the
1st-magnitude star Antares 5°
to its lower right.
The best views of Mercury
through a telescope come on
December’s first few mornings.
It then shows a 6"-diameter
disk with a gibbous phase.
The planet moves away from
Earth in the following weeks,
shrinking slightly as it waxes
toward full.
As Mars moves eastward
across Libra, it sets dead aim
on the 3rd-magnitude double
star Zubenelgenubi (Alpha
Libra). On the morning of
December 12, the planet
stands just 13' north of this
star. View the pair with bin-
oculars or a telescope to see
a nice color contrast between
Zubenelgenubi’s whitish glow
and Mars’ ruddy hue.
You’ll also want to check
out the attractive pairing of a
waning crescent Moon with
Mars on December 22 and 23.
The Moon stands 9° above
Mars on the 22nd and 6° to the
planet’s lower left on the 23rd.
Although Mars offers a
visual treat on December
mornings, it doesn’t look like
much through a telescope. The
planet’s 4"-diameter disk shows
no detail, and likely won’t for
several more months. But
things will improve next spring
as Mars heads toward an
exceptionally good opposition
in October 2020.The Moon has three promi-
nent encounters with solar sys-
tem objects in December: a
conjunction with Mars on the
22nd and 23rd, an even closer
pass by Venus on the 28th, and
a spectacular meeting with the
Sun on the 26th. The final one
brings an annular solar eclipse
to people along a narrow path
that touches parts of Saudi
Arabia, Qatar, the United Arab
Emirates, and Oman just after
sunrise. The track then crosses
southern India and northern
Sri Lanka in late morning;
Sumatra, Singapore, and
Borneo near midday; andBurning ring of fire
An annular solar eclipse December 26 should deliver superb views for those
in parts of southern Asia and Indonesia as well as all of Guam. JATIN RATHOD
Guam about an hour before
sunset. During an annular
eclipse, the Moon appears
slightly smaller than the Sun
and thus leaves a ring of sun-
light visible. Viewers need to
practice safe solar-viewing
techniques to avoid serious
eye damage.GET DAILY UPDATES ON YOUR NIGHT SKY AT
http://www.Astronomy.com/skythisweek.Martin Ratcliffe provides
planetarium development for
Sky-Skan, Inc., from his home
in Wichita, Kansas. Alister
Ling, who lives in Edmonton,
Alberta, has watched the skies
since 1975.