Astronomy

AQUILA

DRACO

HERCULES

OPHIUCHUS

LY R A

CYGNUS

April 22, 2 A.M.

Looking east

Vega

Deneb

Altair

Radiant

10°

Lyrid meteor shower

WWW.ASTRONOMY.COM 37

METEORWATCH

The first of spring’s major meteor

showers peaks before dawn

April 22. The Lyrids benefit this year

from dark skies — the First Quarter

Moon sets shortly before 2 A.M.

local daylight time. That nicely coin-

cides with when the radiant, the

point in the constellation Lyra from

which the meteors appear to origi-

nate, climbs 40° high for observers

at mid-northern latitudes.

Meteor rates typically reach 15

to 20 per hour shortly before dawn,

when Lyra passes nearly overhead.

It’s a great shower to reignite your

love of meteors after a frigid winter,

with every streak of light hinting at

the warmer nights of summer and

the prolific showers to come.

A nice view of Lyra’s

spring show

— Continued on page 42

Lyrid meteor s

Active dates: April 14–30

Peak: April 22

Moon at peak: First Quarter

Maximum rate at peak:

18 meteors/hour

horizon. From mid-northern
latitudes in early April, the
planet stands 30° or higher
from about 2 to 5 a.m. local
daylight time. (It reaches the
same altitude two hours ear-
lier by month’s end.)
Jupiter’s dynamic atmo-
sphere ranks among the top
sights in the solar system.
Even beginners have no trou-
ble spotting the planet’s two
dark equatorial belts that
straddle the equator. More
subtle clouds, spots, and belts
reside on the edges of these
features as well as closer to the
poles. All of them rotate with
Jupiter’s 10-hour period, so
they present a constantly
changing scene.
Our view from Earth
always shows the jovian equa-
torial regions best. But once
every six years or so, we get
a slightly enhanced view of
one pole or the other as the
planet’s axis tilts a bit in our
direction. This tilt reaches a
maximum in April, when the
south pole tips 3.4° toward
Earth and we get a better-
than-normal look at Jupiter’s
south polar region.

Tycho and Clavius

RISINGMOON

The craters Tycho and Clavius rank among

the favorites of most Moon observers. The

two lie in the Moon’s southern hemisphere

and stand out in a magnificent play of light

and shadow when the Sun rises over them

April 24. Under this low Sun angle, Tycho’s

normally brilliant ray system is largely hid-

den from view. But you still should be able

to identify the crater from the signs of its

youth: the prominent bowl, sharp rim, and

well-defined central peak. A large majority

of features in the crater-crowded southern

highlands look nothing like it.

Clavius is the huge feature even closer

to the southern limb. Its debris-filled floor

and softer rim are telltale indicators of the

relentless bombardment it has suffered

over the past few billion years. Take a

close look inside the shallow bowl to see

a neat curving chain of more modest

impact features. Over the course of an

hour, track the shadows as they retreat

and spy still-smaller craters.

As the Moon waxes toward Full on the

29th, Tycho’s rays become increasingly

obvious. Also note the darker ring around

its raised rim. Heat from the blast that cre-

ated Tycho caused the surrounding sur-

face to partially melt and lose its lighter

shade. Although Clavius remains promi-

nent at Full Moon because it is so big and

distinct, many of the nearby smaller cra-

ters that were obvious on the 24th practi-

cally disappear under the higher Sun.

Young and old share the spotlight

Youthful Tycho stands slightly north of

ancient Clavius near the Moon’s southern

limb. CO NSO LI DATED LU NAR ATL A S/UA/LPL; INSET: NASA/GSFC/ASU

N

E

Tyc h o

Clavius

The modest tilt shows up

more clearly in the behavior of

the four bright Galilean moons.

Because they lie in Jupiter’s

equatorial plane, their orbital

motions carry them farther

north or south of the planet’s

center than usual. The effect

shows up most clearly with the

two outer moons, Ganymede

and Callisto.

The First Quarter Moon sets

by 2 A.M. local daylight time

April 22, leaving nearly three

hours of dark skies to view

April’s best meteor display.

Mars passes just 1.3° south of Saturn before dawn breaks April 2,

providing a colorful treat for early risers.

OBSERVING

HIGHLIGHT