2019-12-01_Astronomy

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RISING MOON I The young Moon’s lonesome sea

ON DECEMBER’S FIRST EVENING, Mare

Crisium (Sea of Crises) dominates our view of the

waxing crescent Moon. The oval sea lies slightly

north of the lunar equator and is the only fully

illuminated mare at this phase.

Just “inland” from Crisium’s northern shore

lies the 81-mile-wide crater Cleomedes. Named

after a Greek astronomer who historians believe

lived in the first century B.C., Cleomedes’ lava-

covered floor sports the hint of an off-center

central peak and two craters between 6 and

7 miles in diameter. Increase the magnifica-

tion to see Rima Cleomedes, a long, thin

rille (channel) that cuts across the main

crater’s northern half. Be patient for

moments of good seeing, when Earth’s

atmosphere steadies. These smaller fea-

tures disappear under the higher Sun that

comes the following few evenings.

You might be just as captivated watching the

Sun set over this terrain in the evenings after

December 12’s Full Moon. The reversed lighting

practically turns this into undiscovered country.

The Moon cycles through a complete set of

phases every 29.5 days, so it returns to a waxing

crescent in the evenings leading up to New

Year’s. On December 28, the same evening Luna

This month offers observers two

chances to view fine features on a

waxing crescent Moon. CONSOLIDATED LUNAR

ATL A S/UA/LPL. INSET: NASA/GSFC/ASU

shows the planet’s 12"-diameter

disk and 88-percent-lit phase.

Each night, Venus climbs a

bit higher while Saturn sinks a

little lower. The two are des-

tined for a close encounter

December 10, when Venus

passes 1.8° south of its

companion.

Saturn continues to lose

altitude after this conjunction

and becomes lost in the Sun’s

glow before Christmas. Plan

to catch your final telescopic

views of the ringed world in

early December. On the 1st,

the planet’s disk measures 15"

across while the rings span 35"

and tilt 24° to our line of sight.

Once Jupiter and Saturn

succumb to bright twilight,

Venus rules the evening sky. On

December 18 and 19, it passes

within 1° of the 9th-magnitude

globular cluster M75 on

Sagittarius’ eastern edge. Venus

crosses into Capricornus on the

latter date and spends the rest

of the month among the Sea

Goat’s stars. A slim crescent

Moon slides 2° below the planet

on the 28th, when the pair

remain visible for more than

two hours after sundown.

Once Venus sets, turn your

attention to Neptune. The out-

ermost major planet glows at

magnitude 7.9 among the back-

ground stars of Aquarius the

Water-bearer. That’s dim

enough that you’ll need bin-

oculars or a telescope to spot it.

Mare Crisium and Cleomedes

Cleomedes

N

E

appears 2° below brilliant Venus, both Mare

Crisium and Cleomedes are half-lit.

When the Moon appears as a slender crescent,

earthshine illuminates its “dark side.” It’s a good

time to pick out features normally associated with

the Full Moon. Look for Tycho’s rays, the bright

crater Aristarchus, the debris aprons surrounding

Copernicus and Kepler, and the darker seas.

— Continued on page 42

VENUS slides 1.8° due south

of SATURN during the early

evening hours December 10.

OBSERVING

HIGHLIGHT

METEOR WATCH I Will the Bear steal the Twins’ show?

DECEMBER BRINGS two notable

meteor showers. The Geminids —

the best annual shower — peak

the morning of December 14.

Unfortunately, the Full Moon arrives

just two days earlier. The waning

gibbous Moon on the 14th lies in

Gemini, the constellation from which

the meteors radiate. Bright moon-

light will drastically cut the number

of meteors, though observers with

a good sky who shield themselves

from the Moon might still see up to

20 meteors per hour before dawn.

By the time the Ursids peak the

night of December 22/23, the Moon

has waned to a thin crescent and no

longer interferes. Although the

shower typically produces only

about 10 meteors per hour, major

outbursts occurred in 1945 and

1986 with smaller enhancements

reported as recently as 2011 and

2014. Meteor experts predict a surge
      of up to 30 meteors per hour could
      happen this year, so it might be
      worth braving the cold for a look.

10°

December 23, 1 A.M.

Looking north

URSA

MAJOR

CASSIOPEIA

CEPHEUSURSA

MINOR

DRACO

Polaris

Kochab

Radiant

Ursid meteor shower

A surge of up to 30 meteors

per hour could light up

the sky when this often-

overlooked shower peaks

the night of December 22/23.

URSID METEORS

Active dates: Dec. 17–26

Peak: December 22/23

Moon at peak: Waning crescent

Maximum rate at peak:

10 meteors/hour

Mare

Crisium