Astronomy

10°

TAURUS

AURIGA

GEMINI ORION

URSA MAJOR

Aldebaran

Radiant

Capella

Castor

Venus

Pollux

Procyon

Betelgeuse

Rigel

September 1, 4 A.M.

Looking east-northeast

Aurigid meteor shower

Mare Fecunditatis at Full Moon

WWW.ASTRONOMY.COM 37

METEORWATCH

September is a quiet month on

the meteor calendar. The best of

the minor showers is the Aurigids,

which peaks September 1.

Although observers have noticed

occasional bursts in the past

century, astronomers don’t antici-

pate any this year. The best views

should come shortly before dawn,

once the waxing gibbous Moon

has set and the radiant climbs

high. Under a dark sky, observers

can expect to see an average of six

meteors per hour emanating from

the constellation Auriga.

The tiny dust particles that give

rise to meteors when they burn up

in Earth’s atmosphere also perme-

ate the inner solar system. Their

combined glow shows up as the

zodiacal light in the predawn

Auriga’s claim

to meteor fame

— Continued on page 42

While it takes Titan 16 days
to orbit Saturn, five dimmer
moons complete their circuits
in less than five days. Tethys,
Dione, and Rhea glow at
10th magnitude and show up
through 4-inch scopes on any
clear night.
To capture 12th-magnitude
Enceladus or 13th-magnitude
Mimas, you’ll need at least a
6-inch instrument with good
optics. The former revolves
around Saturn in 33 hours
while the latter takes 23 hours.
Both lie near the rings’ edge
and often get lost in the glare.
Your best bet is to look for
them September 17, when
they reach greatest western
elongation within two hours
of each other.
Coincidentally, the 17th
is also the best evening to
look for Iapetus, which takes
79 days to orbit Saturn. If you
draw a line from Saturn to
Titan and then extend it an
equal distance, Iapetus will
be right there. A 4-inch
scope reveals this moon’s
11t h-mag nit ude g low.
While you gaze upon the
ringed planet this month, the
Cassini spacecraft is wrapping

RISINGMOON

To the eyes of a beginning lunar

observer, the terrific ray system

radiating from the crater Tycho

is perhaps the Full Moon’s most

impressive feature. These rays

— bright material ejected dur-

ing the relatively recent impact

that gouged out Tycho — stand

out best within two days of the

September 5/6 Full Moon.

Let’s follow one of Tycho’s

prominent rays that angles

northeast and eventually cuts

across Mare Fecunditatis (Sea

of Fertility). This ray first passes

through Mare Nectaris (Sea of

Nectar), where it nearly pierces

the small crater Rosse.

After crossing highland ter-

rain, the ray seems to skip over

the crater Goclenius. It then con-

tinues across the rest of Mare

Fecunditatis. Some scientists sus-

pect that the greater elevations

of the highlands and crater rims

produced “ray shadows” where

Tycho’s ejecta could not fall.

Two other ray systems cross

the Sea of Fertility. On the east-

ern shore, numerous short rays

spread out from the crater

Langrenus like spokes on a bicy-

cle wheel. And in the mare’s

north-central section, you can’t

miss the dual rays shooting out

of the crater Messier A.

Rays of light in a fertile sea

N

E

Mare Fecunditatis

The incessant weathering of

the solar wind darkens these

lighter streaks over time, but

they likely won’t fade com-

pletely from view for hundreds

of millions of years.

A

Messier

Langrenus

Goclenius

Mare Nectaris

hours of September’s final two

weeks. With the Moon then out

of the way, viewers at dark sites

can see this cone-shaped glow

above the eastern horizon. The

zodiacal light aligns with the

ecliptic, so Venus serves as a

guide for finding it.

up its extraordinary 13-year

mission exploring the Saturn

system. The probe makes its

final two orbits in early

September, diving through

the 1,500-mile-wide gap

between the planet’s cloud

tops and the inner edge of the

The Moon sets well before dawn

September 1, gifting observers

with nice views of this month’s

premier display of “shooting stars.”

Neptune reaches its 2017 peak September 4/5, when the planet

glows at magnitude 7.8 and spans 2.4" through a telescope.

OBSERVING

HIGHLIGHT

Aurigid meteors

Active dates: Aug. 28–Sept. 5

Peak: September 1

Moon at peak: Waxing gibbous

Maximum rate at peak:

6 meteors/hour

Bright rays from the impact that created the large crater Tycho splashed

across the distant Sea of Fertility. CO NSO LI DATED LU NAR ATL A S/UA/LPL; INSET: NASA/GSFC/ASU