Astronomy

1°

Path of Ceres

May 1

26

21

16

6

April 1

11

CANCER

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Sneaking past the Crab’s claws
Venus passes between two bright clusters (^)
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WWW.ASTRONOMY.COM 43
The largest body in the belt
of objects between Mars and
Jupiter is 1 Ceres. When astron-
omer Giuseppe Piazzi discov-
ered it in 1801, he thought he
had found the “missing planet”
that everyone thought lurked
in the unusually large gap
between the two planets.
But as many more objects
turned up in this region in the
19th century, scientists decided
that they couldn’t all be plan-
ets, and they were reclassified
as asteroids. That’s where
things stood until 2006, when
the International Astronomical
Union (IAU) proclaimed that
Ceres, Pluto, and a handful of
other objects in the outer solar
system should be considered
intermediate objects. The IAU
decided to call this new cate-
gory “dwarf planets.”
Whatever class it belongs to,
Ceres is pretty easy to find in
April. Almost any telescope will
pick up its 8th-magnitude glow
from the suburbs. It moves
slowly eastward relative to the
background stars of northern
Cancer the Crab, a region that
lies high in the south in early
evening. Use 4th-magnitude
Iota (ι) Cancri as your starting
point and then follow the star
patterns in the chart below. If
you can’t figure out which
point of light is Ceres, sketch
the field with a half-dozen of
the brightest stars. When you
return to the same field a night
or two later, the object that
moved will be Ceres.
LOCATINGASTEROIDS
Ceres plots a course through Cancer
sight. The large tilt assures
nice views of ring structure.
While Saturn’s appearance
changes slowly in April, Mars
seems in a hurry to deliver
on its finest apparition in
15 years. The Red Planet
reaches magnitude –0.3 by
the end of April, some 75 per-
cent brighter than when the
month began. During the same
period, its diameter swells
from 8.4" to 11.0", an increase
of 30 percent. Still, this is but a
prelude to what lies ahead. At
its long-awaited peak in late
July, Mars will shine 10 times
brighter and more than double
in size from its appearance in
late April.
But this doesn’t mean you
should skip viewing the planet
in April. Now is the time to
practice your observing skills
and imaging techniques. A
trained eye will see far more
detail on the rocky world’s
surface. Just be sure to view
after the planet climbs high
enough to be clear of the
worst atmospheric turbulence
near the horizon. From mid-
northern latitudes, this means
waiting until the hour before
twilight begins.
If you observe Mars at the
same time every morning —
which is almost a necessity this
month with the planet best
positioned just before dawn
— you’ll see only a slight
change in what’s visible on
successive nights. That’s
because Mars rotates once
every 24 hours, 37 minutes,
so the longitude at the center
of the martian disk shifts only
about 10° every 24 hours.
(The planet’s phase is the one
constant; the disk remains
88 percent lit all month.)
Here’s a rundown on the
main features visible along the
planet’s central meridian at
5 a.m. EDT this month. The
elongated neighbors Mare
Cimmerium and Mare
Sirenum take center stage
during April’s first week. On
April 11 and 12, look for the
dark spot Solis Lacus. But
April saves its finest features
for month’s end: On the 29th
and 30th, the planet’s most
prominent dark area (Syrtis
Major) and bright area
(Hellas) both lie near the
disk’s center.
Early risers can catch
Neptune in binoculars dur-
ing April’s final week. The
magnitude 7.9 planet glows
faintly and stands just 5° high
in the east at dawn’s first light.
If you’re up for a challenge,
hunt for it between the 4th-
magnitude stars Lambda (λ)
and Phi (φ) Aquarii. Neptune
lies 1.5° west-southwest of Phi
on the 30th. If atmospheric
conditions prevent you from
seeing these stars, wait for
better circumstances in May.
Mercury also makes a
brief appearance before dawn
in late April, but it won’t be
much easier to see. The inner-
most planet reaches greatest
elongation on the 29th, when
it lies 27° west of the Sun but
climbs only 4° above the east-
ern horizon 30 minutes before
the Sun rises. Glowing at mag-
nitude 0.4, it will be hard to
pick out in bright twilight.
Uranus passes behind the
Sun from our vantage point
April 18 and remains hidden
in our star’s glare all month. It
will return to view before
dawn in late May.
Martin Ratcliffe provides plane-
tarium development for Sky-Skan,
Inc., from his home in Wichita,
Kansas. Meteorologist Alister
Ling works for Environment
Canada in Edmonton, Alberta.
The Pleiades and Hyades star clusters flank the brilliant planet in late
April, much as they did when this photo was taken in April 1999. ALAN DYER
The 8th-magnitude dwarf planet Ceres should be easy to locate
this month as it treks through the backdrop of northern Cancer.