Astronomy

5°

LEO

Mercury

Venus

Regulus

Moon

Mars

September 18, 45 minutes before sunrise

Looking east

HYDRA

5°

DELPHINUS

CYGNUS

SAGITTA

AQUILA

Altair

Deneb

a

Path of Florence

Sept 1

2

3

4

5

6

E

N

Catch Florence as it skims past Earth

The Moon crashes a predawn planet party (^)
GET DAILY UPDATES ON YOUR NIGHT SKY AT http://www.Astronomy.com/skythisweek.
WWW.ASTRONOMY.COM 43
Except for the Moon and a
handful of comets, it’s rare to
see a reasonably bright object
move against the background
stars in real time. But occasion-
ally we get lucky and receive
advance notice of a 10th-
magnitude or brighter near-
Earth asteroid destined to whiz
across the sky. September’s
first week gives observers the
chance to see such a space rock
hurtle across the evening sky.
Asteroid 3122 Florence —
named after British nursing pio-
neer Florence Nightingale — is
perfectly placed for evening
viewing from the Northern
Hemisphere. It comes closest to
Earth on September 1, when it
sweeps within 4.4 million miles
of our planet and appears
among the background stars of
eastern Delphinus. By the next
evening, it has shifted into
northern Delphinus. Florence
moves at some 9° per day,
which translates into 22" per
minute. (In comparison, Jupiter
currently spans 32".)
Florence glows at 9th mag-
nitude on September’s first
three nights and dims to 10th
magnitude the following three
nights. On the 5th, it sweeps 1°
west of 2nd-magnitude Gamma
(γ) Cygni, the central star of
Cygnus the Swan. But by then
its motion has slowed to nearly
half of what it was at the begin-
ning of the month.
Astronomers classify
Florence as an Amor asteroid,
which means it comes close to
but does not actually cross
Earth’s orbit. If you scaled our
planet’s orbit to the size of a
dinner plate, Florence would
come within 0.2 inch of its outer
edge. At its farthest, Florence
reaches halfway to Jupiter. The
asteroid orbits the Sun once
every 2.35 years, but this is its
closest approach to Earth since
1890, and it won’t be as close
again until after 2500.
LOCATINGASTEROIDS
See an asteroid move while you watch
sky more than two hours
before the Sun and standing
high in the east as dawn
breaks. You won’t mistake the
planet for anything else — at
magnitude –3.9, it far outshines
every other point of light in the
sky. Don’t miss Venus passing
1° south of the magnificent
Beehive star cluster (M44) the
mornings of September 1 and

2. The pair makes a lovely bin-
   ocular sight and a stunning
   photographic composition.
   On September 10, Venus
   crosses from Cancer into Leo,
   where it remains for the rest of
   the month. The Lion serves as
   a backdrop for some impres-
   sive morning vistas because
   Mercury and Mars also reside
   in this constellation, though
   they hang closer to the horizon.
   Mercury reaches the peak
   of its best morning appearance
   of the year September 12. It
   then lies 18° west of the Sun
   and stands 11° high in the east
   a half-hour before sunrise.
   Shining at magnitude –0.4, it
   appears conspicuous in morn-
   ing twilight. Use binoculars to
   pick out magnitude 1.8 Mars
   3° to Mercury’s lower left.
   These planets pull closer
   over the next few days. On the
   16th, they come within 10' of
   each other and appear in the
   same low-power field through
   a telescope. (Mercury spans
   6.4" and is about two-thirds lit;
   Mars is 3.6" across and full.)

A waning crescent Moon

adds to the scene on the 17th.

The slim crescent hangs 6°

above Venus with Leo’s

brightest star, magnitude 1.4

Regulus, 3° below the planet.

Mars and Mercury huddle 11°

below Venus.

The solar system fireworks

peak September 18. A wafer-

thin crescent Moon then lies

below Venus and above the

Mercury-Mars pair, with just

1.5° separating the latter two

planets. While Venus and

Mars shine as brightly as they

did earlier in the month,

Mercury has improved to

magnitude –1.0.

The next two morn-

ings see Venus pass within

1° of Regulus, but that’s the

last September conjunc-

tion. Mercury quickly falls

back toward the Sun and

disappears from view in

the month’s final week.

Meanwhile, Mars climbs

higher before dawn. By the

30th, it approaches within

3° of Venus, setting the stage

for a close conjunction in

early October.

Leo forms the backdrop when a crescent Moon stands below dazzling
Venus and above Mercury and Mars the morning of September 18.

This fast-moving asteroid peaks at 9th magnitude when it passes

within 4.4 million miles of our planet during September’s first week.

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.