Astronomy

10°

Radiant

July 30, 2 A.M.

Looking south-southeast

CETUS

AQUARIUS

CAPRICORNUS

GRUS

Moon

Fomalhaut

Southern Delta Aquariid meteor shower

Schiller and Hainzel

N

E

Schiller

Hainzel

WWW.ASTRONOMY.COM 37

METEORWATCH

Although the Southern Delta

Aquariid meteor shower doesn’t

reach the heights of its more

famous cousins, it stands out for

its longevity. The shower lasts

from mid-July to mid-August,

and that’s a good thing this year

because it peaks July 30, just

three days after Full Moon. Your

best bet is to watch under the

dark skies before dawn prior to

July 25.

Or, if you live in the Eastern

Hemisphere, keep an eye out for

meteors during the total lunar

eclipse the night of July 27/28.

The eclipse brings nearly two

hours of darkness and a grand

stage for viewing totality, Mars,

and a handful of “shooting stars.”

Fighting off a Full

Moon fever

— Continued on page 42

Southern Delta Aquariid

meteors

Active dates: July 12–Aug. 23

Peak: July 30

Moon at peak: Waning gibbous

Maximum rate at peak:

25 meteors/hour

viewing the planet’s four
bright moons. Io, Europa,
Ganymede, and Callisto orbit
Jupiter with periods ranging
from 1.8 to 16.7 days, and
their relative positions change
noticeably hour by hour.
Scan east from Jupiter
and you’ll find Saturn set
against the rich Milky Way of
Sagittarius. In early July, the
ringed planet lies 4.7° due east
of the Trifid Nebula (M20),
with the even brighter Lagoon
Nebula (M8) 1° south of the
Trifid. Open cluster M25
resides 4.0° northeast of
Saturn and globular cluster
M22 sits 3.5° southeast of the
ringed world. Saturn drifts
slowly westward during July,
closing in on the Trifid and
Lagoon. By the 31st, the planet
stands 2.7° east of M20.
Despite this wonderful
backdrop, the best views of
Saturn come through a tele-
scope. The planet reached its
peak at opposition in late June,

RISINGMOON

The striking form of Schiller

stands out in the deep south-

west of a waxing gibbous

Moon. This shoeprint-shaped

impact crater lights up July 23,

when its mostly flat floor and

exceptionally elliptical shape

catch the eye. Of course, even

circular craters located close to

the Moon’s limb appear oval, an

effect called foreshortening. But

Schiller seems squashed twice

as much as any other impact

feature in the area. The shadows

appear quite long the evening

of the 23rd, which helps exag-

gerate the terrain’s height.

Early lunar observers were

left scratching their heads trying

to explain Schiller’s shape. A few

decades ago, however, scientists

studying high-velocity impacts

in laboratories on Earth showed

that single projectiles striking a

surface at a grazing angle can

produce some unusual craters,

including ones that look like

Schiller. (Messier A in the Moon’s

southeast is the poster child for

such low-angle impacts.) Long

after the impact gouged out

Schiller, lava welled up through

crustal fractures and created its

smooth floor.

Scan to the crater’s northeast

and you can’t miss the elliptical

crater Hainzel. A close look

reveals this feature to be the

result of three separate impacts.

The roughly circular crater on

the northwestern side shows a

classic central peak. The south-

eastern structure formed later —

the timing becomes clear when

you notice that the floor of the

overlapping area matches its

A pair of oval impact scars

texture and albedo. The south-

ern component was first on the

scene. Not only do the other

two craters overlay it, but its rim

is also softer, a result of wear

from long-term bombardment.

A nearly Full Moon lessens the

impact of this shower in 2018,

though meteor observers

should still watch for an

occasional bright streak.

Mars shines brighter (magnitude –2.8) and looms larger (24.3"

across) through a telescope in late July than it has in 15 years.

OBSERVING

HIGHLIGHT

These elongated craters stand out near Luna’s southwestern limb on the

July 23 waxing gibbous Moon. CO NSO LI DATED LU NAR ATL A S/UA/LPL; INSET: NASA/GSFC/ASU