Sky & Telescope - May 2019 - 48

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

16h 40m

16h 30m

16h 20m

16h 10m

Star magnitudes

-14°

OPHIUCHUS

The biggest asteroid and
brightest dwarf planet is at
its best for 2019.

F

or the ancient Romans, spring
was the season of Ceres, the goddess of cereals and agriculture. So it's
appropriate that we devote at least
some of our May nights to the celestial
object named after the deity of seed
and growth. Asteroid 1 Ceres is well
placed for observation all month and
reaches opposition on the night of May
28-29, when it moves from Ophiuchus
into Scorpius.
Ceres, the ﬁrst discovered asteroid,
was found by the Italian astronomer
Giuseppe Piazzi, who spotted the
mysterious object on January 1, 1801,
while making observations from the
Royal Observatory of Palermo in order
to correct those published by Frances
Wollaston in 1789. The new star, with
its feeble 8th-magnitude light and a
color similar to that of Jupiter, puzzled
Piazzi and his assistants, Niccolò
Cacciatore and Niccolò Carioti. After
he spent several nights chasing the
interloper through the sky with a 5-foot
alt-azimuth circle and 4-inch achromatic refractor, Piazzi felt comfortable
enough to share his discovery with his
colleagues, but not so comfortable as
to make a claim as to the object's true
nature. Was it a planet or a comet? He
was reluctant to take a public stand,
though he did conﬁde to astronomer
friends that he thought "it might be
something better than a comet."
Indeed, it was something better
than a comet. At 940 km (585 miles)
across, Ceres is the largest object in

48

M AY 2 0 1 9 * S K Y & T E L E S C O P E

-16°

SCORPIUS
q

5

8

11

4
5
6
7
8
9

Path of Ce
res
14

17

20

-18°

23

26

r

29 June 1

4

7

10

13

16

i
-20°

s

`
t1
t2

t

pu Asteroid 1 Ceres stands
approximately 20º above the
south-southeastern horizon on
the night of May 28-29. Look
for a line of 5 stars with magnitudes ranging from 7.2 to 9.4
northeast of Chi Ophiuchi. The
line points to a 9th-magnitude
star that lies close to the asteroid's path. Ceres will be less
than ½º from that star on May
28th at 10 p.m. local daylightsaving time. The chart above
shows Ceres' path, with tick
marks indicating the asteroid's
position at 0h UT every three
days.

17h 00m

d

c

16h 30m

16h 00m

M107

-15°

SCO
q Path of Ce
res
r
`
i
s
t2 1
t
t
M80
OPHIUCHUS
l

the asteroid belt, and the only asteroid
to also be classiﬁed as a dwarf planet.
Unlike the smaller company it keeps,
Ceres is differentiated, or layered. It has
a dense rocky core at its center, a 2- to
3-meter-thick icy crust of carbonate
or argillaceous water ice at its surface,
and an icy mantle in between. Ceres is
approximately 25% ice, making it the
second-most water-rich body in the
inner solar system (Earth holds the
H2O record). And although Ceres lacks
an atmosphere, scientists have detected

-10°

m

e
LIBRA
-20°

b
-25°

occasional signs of water vapor near the
asteroid's surface.
Ceres is the smallest dwarf planet,
but because its orbit lies relatively close
to Earth - not out beyond Neptune -
it's also the brightest. On the night of
opposition, Ceres shines at magnitude
7.0, just out of reach of the naked eye
but an easy target for small scopes and
binoculars. This isn't quite Ceres at its
brightest - at more favorable oppositions, it can brighten to 6.7 or so - but
it still makes for a noticeable light in

N ASA / JPL- CA LTECH / UCL A / MPS / DLR / IDA

Ceres at
Opposition

3
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