Astronomy - February 2014

Catch a sporadic meteor

Peeking around the Moon’s eastern edge

WWW.ASTRONOMY.COM 37

of events. First, the satellite
crosses in front of, or transits,
Jupiter. Soon thereafter, the
moon’s shadow transits the
jovian cloud tops. (The time
delay depends on the moon’s
distance from Jupiter and the
Sun-Jupiter-Earth viewing
angle.) Half an orbit later,
the moon disappears behind
Jupiter’s western limb. The
sequence ends when the moon
leaves Jupiter’s shadow at some
distance off the planet’s east-
ern limb. As the eclipse ends,
the moon brightens slowly as
sunlight gradually returns to
its surface.
February features plenty of
interesting satellite events. At
11:36 p.m. EST February 3, Io
begins to transit Jupiter’s disk.
The moon’s shadow starts its
own journey across the plan-
et’s face at 12:19 a.m. The
transit ends at 1:52 a.m., and
the shadow lifts back into
space 42 minutes later.
By the next evening, speedy
Io has progressed halfway

RISINGMOON

METEORWATC H

The first few evenings of Febru-

ary offer great views of rugged

craters along the waxing cres-

cent Moon’s terminator, the line

that divides sunlight from dark-

ness on the lunar surface. But

after enjoying the sights for a

few minutes, you might start to

wonder if something is different

about the scene. Then it dawns

on you — there’s a lot more ter-

rain than usual between Mare

Crisium and the limb.

Mare Marginis and Mare

Smythii appear as huge dark

splotches instead of thin chan-

nels. Early February offers the

best perspective (or libration)

we can get on the Moon’s east-

ern zone. Our satellite’s rotation

and revolution combine to let

us see a bit beyond what our

normal face-on view provides.

These two large dark zones

are ponds of lava that welled up

from below and froze billions of

years ago. The northern one is

Mare Marginis (the Border Sea),

and the southern one is Mare

Smythii, whose name honors

British amateur astronomer

Admiral Smyth. The white wall

on the limb beyond Smythii con-

sists of a series of crater rims, the

largest belonging to Hirayama,

Purkyne, and Babcock.

In between the two maria is

Neper, a prominent crater with

a lava-filled basin and central

mountain. On Marginis’ north-

ern flank, the rugged walls of

Goddard ring a flat lava lake that

flooded the impact site higher

than the crater’s central peak.

For a few evenings, the lumpy

rim of Al-Biruni pokes above the

limb just beyond Goddard.

Each night after First Quarter

phase February 6, the Moon’s

orbit and spin combine to give

the impression that the eastern

part of Luna is rolling away from

us. Watch for Goddard’s rim and

Neper’s peak to stick up above

the limb when they are in pro-

file. By Full Moon on February

14, all the features past Crisium

have spun beyond the limb. This

sequence of views brought by a

favorable libration repeats the

next couple of months, but less

prominently than in February.

February is typically a quiet month

for meteor observers. No major

showers occur, and the only minor

one (the Alpha Centaurids) lies

deep in the southern sky. But the

occasional sporadic meteor can

appear at any time. Experienced

observers can expect to see half

a dozen sporadics per hour from

a dark-sky site.

As with all meteor viewing, the

highest rates occur in the early

morning hours. Before dawn, an

observer lies on the part of Earth

that faces in the direction of our

planet’s orbital motion. And in the

same way a car driving through a

snowstorm picks up more snow-

flakes on its front windshield,

Earth sweeps up more meteors

on its leading edge.

Searching beyond the normal lunar limb

You don’t need a

shower to get wet

— Continued on page 42

Most meteor observers watch the annual showers, but dark skies can

bring exceptional views of random flaring dust particles. TONY ROWELL

A favorable libration brings Mare Smythii and Mare Marginis into view in

early February. NASA/CLEMENTINE/ARIZONA STATE UNIVERSITY; INSET: NASA/GSFC/ASU

Northern Hemisphere observers at dark sites should look for the

zodiacal light after evening twilight from February 16 to March 1.

OBSERVING

HIGHLIGHT

N

E

Goddard

Mare Marginis

Neper

Mare Smythii