Astronomy - USA (2020-08)

August 12, 2 A.M.

Looking northeast

TAURUS

10°

CAMELOPARDALIS

AURIGA

Capella

PERSEUS

Radiant

Pleiades

Aldebaran

ARIES

Moon

Perseid meteor shower

WWW.ASTRONOMY.COM 37

RISING MOON I No glory for Galileo

WE HAVE DRESSED the Moon’s face in the

honor roll of the great scientists and philosophers

of human history. The bigger the name, the more

impressive a feature, generally, with the best cra-

ters used for the pre-telescopic age thinkers.

Galileo must have rubbed the lunar cartogra-

phers the wrong way, because his crater is half

the size and less prominent than the one named

for his friend and student, Reiner. Appropriately,

though, they appear near each other as isolated

islands in the large western “sea” of Oceanus

Procellarum. Look due west of Copernicus, the

dominating large crater with prominent

rays near the middle of the Moon's

disk. You’ll first find a smaller rayed

crater, Kepler; go farther west still to

find Galilaei.

Spanning 9 miles in diameter,

Galilaei is a classic sharp-rimmed crater,

young in that it formed well after the

huge lava upwellings that characterized the ear-

lier parts of the Moon’s existence. As evening on

the 2nd arrives in North America, the Sun has

already been shining for 20 hours over Galilaei,

high enough in the sky that bright albedo fea-

tures catch the eye. Use a filter to reduce the

intensity of light coming through the eyepiece.

A most curious splotchy white feature, Reiner

Gamma, lies between Galilaei and Reiner to the

east. Apollo missions confirmed this is not topo-

graphic in origin but, like other white splotches

on the farside, is quite magnetic. To date, there

Jupiter’s eastern limb and

begins to transit at 11:24 P. M.

EDT. Io’s shadow joins in

44 minutes later.

Io and Ganymede repeat the

event on August 21. Ganymede

begins transiting at 10:59 P. M.

EDT, and Io follows shortly

after, at 1:11 A.M. EDT. Soon

after 2 A.M., three events occur

in relatively quick succession.

Io’s shadow moves onto the

disk at 2:03 A.M., Ganymede

leaves the opposite limb at

2:19 A.M., and Ganymede’s

shadow appears on the eastern

limb at 2:31 A.M. (a l l t i mes

EDT). Ganymede’s shadow is

so large it takes seven minutes

to become fully visible.

The pair return August 29

in the early hours. This time Io

has caught up with Ganymede

and the pair begin transits

within 28 minutes of each other,

starting with Ganymede at

2:31 A.M. EDT. The western half

of the U.S. will see Io overtake

Ganymede to leave the disk first

at 5:15 A.M. EDT. Ganymede

follows 35 minutes later.

Swing your scope 8° east of

Jupiter to find Saturn. Through

a scope its disk is clearly visible,

spanning 18" while the long

axis of the rings stretches 41"

— almost the same as

Jupiter’s girth, but Saturn

lies twice as far away.

Saturn’s north polar axis is

tilted nearly 22° toward us,

revealing the northern

Galilaei

N

E

is no widespread agreement as to what it is or

how it formed — a lasting lunar mystery!

Come back on the 31st to see some gorgeous

hills, rilles, and ridges. Note the super long, sinu-

ous wrinkle ridge snaking through Galilaei and

its junior, A, to the north. The craters break up

the ridge, solid evidence that the impacts came

after the lava of Procellarum solidified. Just to

the northeast lies a fantastic field of volcanic

domes called the Marius Hills. Can you see the

thin Rima Galilaei rille wriggling along the hills’

western flank?

— Continued on page 42

VENUS shares the morning sky

with a waning crescent Moon

on August 15. You’ll find the

planet 4° due south of our

satellite in the east.

OBSERVING

HIGHLIGHT

METEOR WATCH I Dodging the Moon

THE FAMOUS PERSEID meteor

shower is active between July 17

and August 24, and peaks on

August 12. The predawn hours

are the best for viewing the

shower, but this year a Last

Quarter Moon in Aries adds a lot

of light to the sky. Consequently,

low rates of only the brightest

meteors will be observable.

Watch for glowing persistent

trains left by these brighter

shower members. Meteors streak

into our atmosphere at 36 miles

per second and burn up in a fiery

burst of light, vaporizing the

original particle.

The Perseid meteors are the

result of trails of debris left over

from Comet 109P/Swift-Tuttle,

which completes one full orbit

of the Sun every 133 years.

PERSEID METEORS

Active dates: July 17–August 24

Peak: August 12

Moon at peak: Last Quarter

Maximum rate at peak:

100 meteors/hour

Nine-mile-wide Galilaei lies within

Oceanus Procellarum. To the west are

Kepler’s bright rays. CO NSO LI DATED LU NAR ATL A S/UA/

LPL; INSET: NASA/GSFC/ASU

Although the Moon will brighten the sky,

you’re still likely to catch the brightest Perseid

meteors when the shower peaks August 12.

Galilaei

Reiner Gamma

A

Kepler