Astronomy

Radiant

10°

May 6, 4 A.M.

Looking east

Enif

Altair

PEGASUS

AQUARIUS

CAPRICORNUS

AQUILA

CYGNUS

d

Eta Aquariid meteor shower

Furnerius and Mare Australe

WWW.ASTRONOMY.COM 37

METEORWATCH

Two meteor showers trace their ori-

gins to history’s most famous comet.

Both the Eta Aquariids in May and

the Orionids in October result from

Earth crossing the orbital path of

Comet 1P/Halley. As our planet plows

through dust particles shed by the

comet over many millennia, friction

with molecules in the upper atmo-

sphere incinerates the bits and cre-

ates “shooting stars.”

The Eta Aquariid shower peaks the

morning of May 6. Unfortunately, a

waning gibbous Moon shares the sky

and will drown out fainter meteors.

For the best views, find an otherwise

dark site and place yourself where a

tree or building blocks the Moon’s

direct light.

With clear weather, observers at

mid-northern latitudes might see up

to 10 meteors in the hour before

dawn. People in the Southern

Halley’s debris reaches

a fiery demise

— Continued on page 42

Eta Aquariid meteors

Active dates: April 19–May 28

Peak: May 6

Moon at peak: Waning gibbous

Maximum rate at peak:

50 meteors/hour

its appearance changes notice-
ably in as little as 10 minutes.
Combined with Jupiter’s gas-
eous nature, this rapid spin
causes the world’s equatorial
regions to bulge. Look care-
fully and you’ll notice that the
diameter through the poles is
6 percent smaller than across
its equator.
Small scopes also excel at
showing Jupiter’s four bright
moons. It can be a challenge
to identify them, but the
alignment on opposition
night makes it easy. On the
evening of May 8, Io, Europa,
and Callisto line up in that
order east of Jupiter while
Ganymede is the lone moon
west of the planet. With three
satellites on one side and one
on the other, you can quickly
tell which direction is which
— often a problem because
the orientation of the field
depends on the type of instru-
ment you use and when you
observe — and thus distin-
guish the moons.
Once each orbit, Io,
Europa, and Ganymede

RISINGMOON

The Moon’s nearside provides

more than a thousand features

within range of small telescopes.

But our satellite offers more than

just the craters, maria, and moun-

tain ranges visible at first glance.

Every month, Luna performs a

subtle dance with Earth that you

just have to look for.

Turn to the “Path of the

Planets” on p. 40–41 and locate

the blue curve showing the

Moon’s path and the orange arc

depicting the Sun’s motion. Notice

how the Moon swings below the

Sun and then rises above it, a

motion that stems from the tilt of

the Moon’s orbit relative to Earth’s.

As the two partners twirl

across the solar system’s dance

floor, the Moon’s face rises and

drops from our perspective. As a

waxing crescent May 18, it lies

somewhat below us, and the cra-

ter Furnerius looks like a bright

dimple on the face’s lower right.

As Luna rises up with each pass-

ing day, the dimple moves away

from the limb and we see more

“under the chin.”

By the 22nd, we catch the first

hint of a dark blemish — Mare

Australe — at the southeastern

limb. This sea, the edge of a large

farside basin, slowly reveals itself

until Full Moon on May 29. The

Moon’s motion is at its peak, and it

then slowly drops back down, but

the change near the limb occurs in

darkness and out of sight.

A fresh perspective on the Moon

N

E

Furnerius

Mare Australe

Hemisphere could see twice

as many because the radiant

— the point in Aquarius where

the meteors originate —

climbs much higher. It’s still a

far cry from the normal rate

(up to 50 per hour), but it’s the

best May has to offer.

A gibbous Moon competes with

May’s best shower, reducing

the number of meteors visible

in the predawn sky May 6.

Jupiter reaches its 2018 peak May 8, when the gas giant shines

at magnitude –2.5 and spans 44.8" through a telescope.

OBSERVING

HIGHLIGHT

This pair of features near the Moon’s southeastern limb reveals our

changing viewpoint during May. CO NSO LI DATED LU NAR ATL A S/UA/LPL; INSET: NASA/GSFC/ASU