Astronomy - USA (2020-06)

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THE LAVA-FILLED GRIMALDI BASIN is eas-

ily identified on the western limb of the Moon

as the last patch of dark mare west of the huge

Oceanus Procellarum. Fully illuminated on

June 4, the highly battered crater rim is direct

evidence of its age. Several interesting sights

have also appeared next to Grimaldi, notably the

equally large and battered Riccioli closer to the

terminator. That crater has a much rougher floor

because it was a little too far from Procellarum’s

lava floods to get filled up like Grimaldi.

Use a dark filter on your eyepiece

to reduce the stark brightness levels

of this nearly Full Moon. If you don’t

have one, just crank up the power to

spread out the light and you’ll find it

a lot easier on the eye. Spend some

time here to note that Grimaldi’s basin

floor is not a uniform shade. See if you can

follow the indistinct rays of lighter rock back to

their source, ejected from a couple of major

impacts late in the Moon’s bombardment.

You can see Grimaldi in a completely differ-

ent light on Thursday morning, the 18th, when

the Moon is a gorgeous crescent floating above

Venus. While the rest of the face is bathed in

earthshine, Grimaldi is a snap to spot as a sharply

defined dark ellipse. The cycle almost repeats

one lunation later, with lunar sunrise recurring

on the evening of July 4.

hide subtle features if you take a
quick glance — relax your eye
and gaze for a minute or so to
become accustomed to the
majestic view.
The four Galilean moons
offer a continually varying dis-
play, appearing east and west of
the planet. Most interesting is
when the moons pass in front of
or behind the planet in occulta-
tions, eclipses, and transits.
Multiple events can occur
within minutes of each other.
The relative placements of
Jupiter, its shadow, the moons,
and their respective shadows
offer a unique appreciation of
the dynamic three-dimensional
nature of the entire system as
these events progress.
One multiple-moon event
occurs on the night of June
13/14, when Callisto is eclipsed
by Jupiter’s shadow beginning at
1:17 A.M. EDT, just as Io’s
shadow begins to transit. Note
that Callisto is visible at first
almost 1' west of Jupiter
(approximately two jovian disk
diameters away) and fades over
a 10-minute period. The follow-
ing night, Ganymede is eclipsed
on June 14/15 at 12:40 A.M. E D T,
while Europa and its shadow are
transiting the disk of the planet.
Pluto sits less than 2° south-
west of Jupiter’s location. At
magnitude 14.7, it is a difficult
challenge — within reach of
imaging but beyond visual
observers. On June 1, Pluto
lies a mere 4' due south of a

Grimaldi

Grimaldi

Riccioli

N

E

MERCURY reaches greatest

eastern elongation June 4,

when it will glow at magnitude

0.4 in the early evening sky.

OBSERVING

HIGHLIGHT

METEOR WATCH I Clear nights with daytime showers

NO MAJOR METEOR SHOWERS

occur in June. A few daytime show-

ers are active in the first week of the

month, of possible interest to radio

observers. These are the Arietids and

Zeta Perseids. Later in the month, the

Boötid meteor shower is active from

June 22 to July 7, peaking on June 27.

Boötes is visible most of the night,

although rates have been quiescent

except for a reported outburst in 1998.

The summer season sets up condi-

tions to produce noctilucent (night-

glowing) clouds, formed by ice crystals

on high-flying dust particles. Their

pearly white appearance remains sun-

lit long after regular clouds darken in

the deepening twilight because they

occur 50 miles up — more than 10

times the height of cirrus clouds. They

are best seen from latitudes between

55° and 70° north.

Noctilucent clouds

Delicate night-shining clouds float above Lake Saimaa, Finland. June is

an ideal time to spot this phenomenon. MIKA Y/FLICKR

9th-magnitude field star, HIP

97251. By June 20, Pluto wanders
       west near a grouping of about
       nine 10th- and 11th-magnitude
       field stars that form a small
       oval-shaped collection about

1/3° across. By June 29, Pluto

lies 41.3' due south of Jupiter.

Saturn is best viewed in the

two hours before dawn, less

than 6° east of Jupiter. It shines

at magnitude 0.4 in early June

and brightens by 0.2 magnitude

by June 30. The 18"-wide disk is

a fine target for small telescopes,

and the wider expanse of the

rings is a spectacle. The major

RISING MOON I Westward ho

Dark-floored Grimaldi sits east of Riccioli,

which lies partly in the terminator's shadow.

CO NSO LI DATED LU NAR ATL A S/UA/LPL. INSET: NASA/GSFC/ASU

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