10°CAPRICORNUSPEGASUSAQUARIUSMarsEnifdMay 5, 4 A.M.
Looking eastRadiantWWW.ASTRONOMY.COM 37RISING MOON I Cracks, craters, and domes
TODAY’S SEA OF TRANQUILLITY was once
quite untranquil. It began 4 billion years ago,
when a giant impact excavated a large basin.
Millions of years later, lava erupted through fis-
sures in the basin floor. The cracks tended to
be oriented along stress lines that were created
by other large impacts. During a second round
of upwelling lava, the terrain unevenly heaved
from below, causing the formation of a scarp or
fracture. In other places, lava tubes collapsed into
rilles. Nearby, some volcanoes made it to the sur-
face, but the failed ones created domes. Mini-
asteroids that slammed into the surface mil-
lennia later added the final touch of texture.
On May 26, roughly four days after New
Moon, the Sun rises on the fascinating
region of crater Cauchy, just north of the ter-
minator’s midpoint. The terminator is the line
that divides lunar day and night, but at such a
young age, the dark face of our sister Luna might
be modestly lit by the gray glow of earthshine,
which is sunlight reflected from Earth’s dayside.
Crater Cauchy itself is a modestly small,
7.5-mile-wide, simple impact feature with nice,
sharp edges that reveal its relative youth.
Immediately to Cauchy’s north is a prominent
rille named Rima Cauchy, where years of tiny
impacts have softened its edges. Within a couple
of Earth days, the higher angle of the Sun shines
light directly into the rille, wiping out the shad-
ows and making it all but invisible. South of
Cauchy is a prominent fault scarp called RupesDetails abound in and around Cauchy
Crater, a 7.5-mile-wide impact site
located in eastern Mare Tranquillitatis.
CO NSO LI DAT ED LUNAR AT L A S/UA/LPL. INSET: NASA/GSFC/ASUgiant rising above the southeast-
ern horizon as Altair in the
Summer Triangle reaches an
altitude of 20°. Saturn rises 20
minutes later, standing less than
5° from Jupiter.
Jupiter begins the month at
magnitude –2.3 and brightens
to –2.6 by May 31. Saturn glows
at magnitude 0.6 in early May
before brightening by 0.2 mag-
nitude by May 31. Both planets
slow their eastern track against
the background stars as the
month goes on, with Saturn
reaching its stationary point
May 11 and Jupiter May 14.
The beautiful pair remain
less than 5° apart all month,
straddling the border of
Sagittarius and Capricornus.
Check out the field of view with
binoculars — can you spot the
dim (magnitude 9.5) globular
cluster M75 forming an isosce-
les triangle with the planetary
duo? It lies less than 2° south of
a line between the two planets.
A waning gibbous Moon stands
3° south of Jupiter on the morn-
ing of May 12.
Thanks to Jupiter’s southerly
declination, it remains at a rela-
tively low altitude for the rest of
the year. It reaches its highest
elevation of about 30° above the
southern horizon (depend-
ing on your latitude) dur-
ing morning twilight,
when it’s located in eastern
Sagittarius. Jupiter is just
two months away from
Crater Cauchy and its companionsCauchy, which lies parallel to the rille. Well
known to lunar aficionados, the fault is second
only to the famous Straight Wall. When the Sun
rises over the region, the scarp casts a sharp
shadow westward, but the dark line disappears
a couple of Earth evenings later.
As an added bonus for this region, a pair of
lava domes sit at the edge of Mare Tranquillitatis.
Look for the small, light-dark pairing caused by
these hills protruding up in the sunlight. The east-
ern dome, cataloged as Omega, is likely a shield
volcano, while the western one, Tau, appears to
have been formed by uplifting from below.— Continued on page 42VENUS and MERCURY cross
paths this month, and they
stand just 1° apart an hour
after sunset May 21.OBSERVING
HIGHLIGHTMETEOR WATCH I Comet Halley returns to Earth’s sky
MAY’S MAIN METEOR SHOWER
is heavily affected by a nearly Full
Moon that remains visible most of
the night. The Eta Aquariids is one of
two showers associated with Halley’s
Comet, which has spent eons litter-
ing its path with debris that results
in the yearly shower (the Orionids in
October is the other).
The Aquariids sport a maximum
observable rate of about 10 meteors
per hour under perfectly dark skies.
However, with a bright Moon present,
you’ll be lucky to spot five streaks per
hour in most urban locations. The first
few days of May after the Moon sets
(4 A.M. on May 2) is a good time to
catch the few early shower members
streaking through the sky. A New
Moon on May 22 offers dark skies for
viewing sporadic meteors, as well as
the occasional straggler of the Eta
Aquariid shower.Eta Aquariid meteor showerETA AQUARIID METEORS
Active dates: April 19–May 28
Peak: May 5
Moon at peak: Waxing gibbous
Maximum rate at peak:
10 meteors/hourPartly due to the low altitude of the Eta
Aquariids’ radiant when viewed from
northern latitudes, the shower is not
considered one of the year’s best. But with
Mars hovering in the nearby sky this year,
it’s worth taking the time to view it.NERima CauchyTau
CauchyRupes
CauchyCrater CauchyOmega Cauchy