Ceres meets the ArcherA partial eclipse for southern Africa (^)
GET DAILY UPDATES ON YOUR NIGHT SKY AT http://www.Astronomy.com/skythisweek.
N
E
SAGITTARIUS
SAO
211624
SAO
211729 Sept 1
Path of Ceres
11
21
Oct 1
0.25°
NASA/JPL-CALTECH/UCLA/MPS/DLR/IDA
WWW.ASTRONOMY.COM 43
Typically, the farther out in the
solar system an object lies, the
more leisurely it moves rela-
tive to the background stars.
Because orbital motion slows
dramatically in the outer reach-
es of our planetary system,
it’s rare to see a nearby object
trudge along at an outer body’s
pace. But that’s exactly what
seems to happen in September
when asteroid 1 Ceres (275 mil-
lion miles from the Sun) travers-
es the same amount of sky as
Uranus (1.86 billion miles away).
Ceres still orbits the Sun at a
faster speed than its cousin,
but our line of sight from Earth
makes the asteroid navigate a
loop that extends barely 1°.
The upshot is that Ceres
remains in the same field of
view through a typical 4- or
6-inch telescope at low power.
It lies in a sparse region of
Sagittarius some 20° east of
that constellation’s Teapot
asterism. The best time for
hunting the asteroid is mid-
evening, when the Archer
climbs highest in the south.
Ceres fades from magnitude
8.2 to 8.7 during September.
You’ll find two noticeably
brighter stars in the field —
magnitude 7.1 SAO 211624 and
magnitude 7.4 SAO 211729.
Sketch the five or six brightest
stars, and then come back a
few nights later to locate the
object that moved slightly —
this is Ceres. The task is easiest
around September 16 and 17
when the asteroid passes a
nearly identical 8th-magnitude
background star.
LOCATINGASTEROIDS
A dwarf planet’s looping stroll
east. The seventh planet, which
will reach opposition in mid-
October, trails about two hours
behind its more distant cousin.
Uranus resides in a faint part
of Pisces (although, let’s face
it, all of Pisces is faint), in the
same binocular field as mag-
nitude 5.2 Zeta (ζ) Piscium.
You will find Zeta 17° east-
southeast of Algenib (Gamma
[γ] Pegasi), the star that forms
the southeastern corner of the
Great Square of Pegasus.
Once you locate Zeta
through binoculars, Uranus
should be a snap. On Septem-
ber 1, the planet lies 0.5° (the
diameter of a Full Moon) due
south of Zeta and about half
that distance west-northwest
of 88 Psc, a magnitude 6.0 star.
Uranus shines at an intermedi-
ate magnitude of 5.7. The plan-
et’s westward motion relative
to the starry background dur-
ing September carries it to a
position 1.2° southwest of Zeta
by month’s end.
If you target Uranus
through a telescope, you’ll
immediately notice its non-
stellar appearance. The world’s
disk spans 3.7" and has an
obvious blue-green hue.
The relatively subtle plea-
sures of the overnight hours
quickly give way to a dazzling
display of three bright planets
before dawn. Venus leads the
way, rising before twilight
commences and appearing far
brighter than either Mars or
Jupiter. Venus reaches its
greatest brilliancy of the year
September 21 when it shines
at magnitude –4.8.
Fortunately, the ecliptic
stands nearly straight up from
the eastern horizon before
dawn at this time of year, so
angular distance from the Sun
translates into altitude. How
much difference does this
make? Consider that Mercury
lies 27° east of the Sun at
greatest elongation September
4 but appears only 3° high a
half-hour after sunset. On the
same day, Venus stands 28°
west of the Sun and climbs
16° above the eastern horizon
30 minutes before sunrise.
The beginning of Septem-
ber finds Venus in southern
Cancer and Mars 9° to its
north. A waning crescent
Moon passes between the two
worlds on the 10th; Jupiter
adds to the spectacle when it
rises an hour before the Sun.
Venus crosses from Cancer
into Leo on September 24,
coincidentally the same day
Mars passes 0.8° due north
of the Lion’s luminary, 1st-
magnitude Regulus. The Red
Planet shines at magnitude 1.8
and offers a lovely color con-
trast with the slightly brighter
blue-white star. Jupiter gleams
at magnitude –1.7 just 10°
below Mars. The gap between
these two worlds tightens in
late September; the pair will
meet in mid-October.
The stunning naked-eye
and binocular views of the
three planets will garner lots
of attention, but it’s also worth
targeting Venus through a tele-
scope. On the 1st, its beautiful
9-percent-lit crescent measures
52" from cusp to cusp. By
month’s end, the planet’s
diameter shrinks to 33" and
the Sun illuminates 34 percent
of the disk.
Although Mars is too small
(4" across) to show appreciable
detail through a telescope,
Jupiter spans a more impres-
sive 31". You’ll have a narrow
window of visibility between
the time it rises and the onset
of bright twilight to view its
dynamic cloud tops and four
bright moons.
Observers across the southern tier of African countries and parts of
Antarctica can watch the Moon pass in front of the Sun on September 13.
While the Dawn spacecraft probes Ceres from orbit, Earth-based viewers
can follow the asteroid’s path through eastern Sagittarius (inset).
HINRICH BÄSEMANN