Astronomy

ORION

URSA

MAJOR

AURIGA

GEMINI

TAURUS

LEO

Procyon Rigel

Betelgeuse

Aldebaran

Capella

Radiant

Castor

Pollux

December 14, 1 A.M.

Looking high in the south

10°

Regulus

Geminid meteor shower

WWW.ASTRONOMY.COM 37

METEORWATCH

The Geminid meteor shower peaks

the night of December 13/14. The

waxing crescent Moon sets by

11 P.M. local time on the 13th, and

you should have excellent views

from then until twilight starts to

paint the sky. The meteors appear

to radiate from a point in northern

Gemini. This region stands more

than halfway to the zenith at

11 P.M. and passes nearly overhead

between 1 and 2 A.M.

The Geminid shower ranks as

one of the three richest showers of

the year. But it benefits from the

fact that its radiant climbs over-

head from mid-northern latitudes,

so observers under dark skies have

a chance to see the maximum pos-

sible rate of 1 2 0 meteors per hour.

Don’t stare at the radiant, how-

ever. Any meteors you see there

Luna leaves the

Geminids alone

— Continued on page 42

Geminid meteors

Active dates: Dec. 4– 17

Peak: December 14

Moon at peak: Waxing crescent

Maximum rate at peak:

120 meteors/hour

Mars and Neptune to pass
closer under a dark sky.
Although the worlds
appear together these
December evenings, Neptune
lies 28 times farther from
Earth. The ice giant’s large
size only partially compen-
sates for its greater distance.
When viewed through a tele-
scope, Neptune appears 2.3"
across while Mars spans 9".
You won’t see any detail
on Neptune beyond its blue-
gray color. Mars is a different
story. Sunlight illuminates
86 percent of its Earth-facing
hemisphere, so its disk appears
distinctly gibbous. And most
scopes should reveal one or
two dark markings.
Following the conjunction,
Mars’ eastward motion carries
it within 0.3° of 4th-magnitude
Phi (φ) Aqr on December 12. A
First Quarter Moon passes 4°
south of the planet on the
14th. Mars crosses the invisible
border between Aquarius and
Pisces on the 21st and ends the
year a few degrees southeast of

Marius Hills

RISINGMOON

It’s easy to see evidence of volca-

nic activity on the Moon: Simply

look at the large dark “seas” that

populate the nearside. These

maria are huge expanses of now-

solidified lava that erupted from

beneath the surface more than

3 billion years ago.

Besides the maria, Luna’s best

volcanic features may be a field

of domes known as the Marius

Hills. This coarse, sandpaper-like

terrain lies near the craters

Marius and Reiner in the vast

Oceanus Procellarum of the

Moon’s western hemisphere.

The hills see first light the eve-

ning of December 19, about

three days before Full phase.

You can’t help but notice

Tycho’s spectacular ray system

and brilliant Aristarchus at this

phase. Take some time to enjoy

these features, but then boost

your telescope’s magnification

and shift your concentration to

the far west. The Marius Hills lie

just north of the equator along

the terminator — the line that

divides lunar day from night.

Although prominent on the 19th,

the hills become harder to see

on the 2 0th. And by the 2 1s t, the

Sun’s higher elevation has wiped

out the shadows necessary to

observe this textured terrain.

How did these hills develop?

Astronomers think the structures

formed through multiple epi-

sodes of volcanic activity. A few

hundred steep-sided cone volca-

noes first erupted on the scene

A land where volcanoes once ruled

when a huge bubble of magma

welled up from below. Less-

violent eruptions then built the

dome structures. Finally, huge

volumes of lava oozed out of

cracks and vents, filling much of

the surrounding basin. NASA

selected the Marius Hills as a tar-

get during the Apollo program,

but that mission was canceled.

N

E

Marius

Marius Hills

Reiner

Oceanus

Procellarum

will have short trails because

you’re seeing them coming

almost head-on. Instead,

look for the longer trails you’ll

see roughly 30° to 6 0° away

from Gemini.

After the Moon sets the evening

of December 13, observers under

a dark sky could see up to two

meteors per minute. ALL ILLUSTRATIONS:

ASTRONOMY: ROEN KELLY

Mars passes 2.2' from Neptune on December 7. The two won’t

appear closer in a dark sky until the 23rd century.

OBSERVING

HIGHLIGHT

This large area of volcanic hills stands out as the Sun rises over Oceanus

Procellarum on December 19. CO NSO LI DATED LU NAR ATL A S/UA/LPL; INSET: NASA/GSFC/ASU