Pearly noctilucent clouds
WWW.ASTRONOMY.COM 37METEORWATCH
Despite the lack of major meteor
showers in June, sporadic meteors
continue to light up the sky. These
random flashes arise when tiny
grains of dust slam into Earth’s
atmosphere and friction with air
molecules incinerates them. Under
a dark sky, observers typically see
a half-dozen or so of these random
meteors per hour.
Similarly fine meteoritic dust
plays a role in producing summer’s
gorgeous noctilucent clouds.
These silver-blue clouds form
about 50 miles up (10 times higher
than cirrus), where Earth’s atmo-
sphere is coldest, when ice crystalsEarly summer’s
twilight clouds
— Continued on page 42all night. Opposition brings
Jupiter closest to Earth, so it
shines at its brightest for the
year: magnitude –2.6. It brings
a touch of brilliance to the
otherwise faint constellation
Ophiuchus the Serpent-bearer.
Jupiter’s proximity also
makes it appear larger than
at any other time in 2019. Its
46"-diameter disk should dis-
play a wealth of detail through
telescopes of all sizes. A paral-
lel pair of relatively dark belts,
one on either side of a brighter
zone that coincides with the
planet’s equator, dominates
the view. In moments of
steady seeing, look for a whole
series of alternating belts and
zones as well as the dusky
north and south polar regions.
You also might notice several
dark and white spots lurking
near the boundaries between
the belts and zones. The Great
Ptolemaeus, Alphonsus, and Arzachel
RISINGMOON
Three magnificent large craters
beckon observers the evening
of June 10, when the Moon is
less than a day past First Quarter
phase and appears slightly more
than half-lit. The northernmost
and largest of the three craters,
Ptolemaeus, lies just south of
the lunar equator. It sports a
rugged rim that casts long shad-
ows onto the relatively smooth
floor. A small crater northeast of
Ptolemaeus’ center affords a
good test of your telescope’s
optics and seeing conditions.
Like most large impact fea-
tures, Ptolemaeus probably pos-
sesses a complex central peak
— but you won’t see it no mat-
ter how hard you look. Lunar sci-
entists suspect that it lies buried
under deposits sprayed out dur-
ing the excavation of giant Mare
Imbrium to the northwest. You
might also glimpse a few subtle
depressions, telltale signs ofancient craters hidden under a
blanket of debris.
The smaller impact that cre-
ated Alphonsus immediately to
the south of Ptolemaeus pro-
duces more distinct features and
a higher central peak. Return to
this feature every hour or so and
note how quickly the spire’s
shadow retreats under the rising
Sun. The unusual ridge that
bisects Alphonsus lines up with
other linear features that point
back toward Imbrium.
The youngest of the three
craters is Arzachel to the south.
Its rim and inner walls appear
sharper than those of its north-
ern neighbors, which suffered
many more millennia of impacts.
Don’t hesitate to return
to this region on subsequent
nights. Although the higher
Sun masks topographic relief,
it reveals surface composition.
Notice the handful of darkerA terrific trio of conspicuous craters
The First Quarter Moon boasts three large and spectacular craters just
south of the lunar equator. CO NSO LI DATED LU NAR ATL A S/UA/LPL; INSET: NASA/GSFC/ASUgray spots on Alphonsus’ floor.
These are deposits of ash
gently sprayed out during vol-
canic eruptions. Spectroscopicstudies show the composition
of the ash is similar to that of
the lava that welled up to flood
the large lunar maria.NEPtolemaeuscondense on dust particles.
They typically appear in early
summer from latitudes
between 50° and 60° north.Search for them in twilight an
hour or two after sunset, when
our star still illuminates these
high-altitude clouds.AlphonsusArzachelJune’s extended twilight offers ideal conditions for northern skygazers
to see these highly reflective, high-altitude clouds. NASAJupiter reaches its 2019 peak June 10, when the gas giant planet
shines at magnitude –2.6 and spans 46" through a telescope.OBSERVING
HIGHLIGHT