Astronomy

Beautiful noctilucent clouds

Aristillus and Autolycus

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METEORWATCH

June offers no major meteor

showers, but keep watch for the

few minor ones as well as the nor-

mal flow of sporadic meteors.

Perhaps the best minor shower

radiates from the constellation

Ophiuchus and peaks the morning

of June 20. The Ophiuchids could

deliver up to 5 meteors per hour

after the First Quarter Moon sets

around 1 A.M. local daylight time.

Meteors arise when dust parti-

cles slam into Earth’s atmosphere

and burn up through friction.

Similar dust helps create gorgeous

noctilucent (night-glowing)

clouds. These silver-blue clouds

form when ice crystals freeze onto

dust particles about 50 miles

above Earth’s surface, some five to

10 times higher than cirrus clouds.

On a quest for

twilight clouds

— Continued on page 42

greatest elongation from the
Sun in mid-August, it will
appear only half as high.
Blame the ecliptic — the
apparent path of the Sun and
planets across the sky —
which makes a steeper angle
to the western horizon after
sunset in spring.
Your best telescopic views
of Venus come in twilight
because the planet’s glare is
almost overwhelming in a
dark sky. On June 1, it appears
13" across and 80 percent lit.
By the 30th, the planet spans
16" and the Sun illuminates
70 percent of its disk.
Despite the inner planets’
charms, June belongs to the
solar system’s outer worlds.
Jupiter rides high in the south
at dusk, a brilliant object set
against the backdrop of Libra
the Scales. It shines at magni-
tude –2.5 in early June and
fades only to magnitude –2.3
by month’s end.
The giant planet reached
opposition and peak visibility

RISINGMOON

As the solstice approaches, the

waxing crescent Moon appears

higher each evening. Different

wonders pop into view along

the terminator that separates

day from night as it advances

westward across Luna’s face.

By First Quarter Moon on

June 20, our satellite rides half-

way up the sky during twilight.

Look for the rugged lunar

Apennines thrusting diagonally

into the sunlit domain north of

the equator. In the plains along

the terminator a bit north of

these mountains lie two striking

young craters: Autolycus and

Aristillus. They formed a couple

of billion years ago, after the

Late Heavy Bombardment fin-

ished pummeling the solar

system’s inner worlds.

The low Sun angle at First

Quarter highlights the debris

aprons that splattered from

these impact sites. The larger

impactor that created Aristillus

excavated a lot more material —

notice the many streaks and

ridges that radiate from this cra-

ter. Both craters’ high walls pre-

vent sunlight from reaching

their central peaks and floors. If

you return June 21, the higher

Sun reveals Aristillus’ multiple

central peaks, but begins to con-

ceal its apron’s roughness.

Compare the characteristics

of these “youthful” scars with

the much larger and older

impact craters Hipparchus and

Albategnius along the termina-

tor just south of the lunar equa-

tor. Their degraded features, the

Youthful impacts leave pristine scars

N

E

Aristillus

result of incessant pounding

from smaller impacts over time,

attest to their greater age. Their

central peaks are lower, their

walls are rounded and pock-

marked with dozens of smaller

craters, and their debris aprons

are smoothed out.

Autolycus

They occur most often in

early summer from latitudes

between 50° and 60°. Look for

them during twilight an hour

or two after the Sun sets (or

before the Sun rises).

June’s long twilight provides northern viewers with perfect conditions

for seeing these highly reflective, high-altitude clouds. NEIL ENGLISH

Saturn peaks June 27, shining at magnitude 0.0 and spanning

18.4" with rings extending 41.7" when seen through a telescope.

OBSERVING

HIGHLIGHT

These sharply defined craters stand out as the Sun rises over them at

First Quarter Moon on June 20. CO NSO LI DATED LU NAR ATL A S/UA/LPL; INSET: NASA/GSFC/ASU