RISINGMOON
A fresh perspective on the Moon
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
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
— Continued on page 42
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.
METEORWATCH
Halley’s debris reaches
a fiery demise
Furnerius and Mare Australe
Furnerius
N
Mare Australe
E
This pair of features near the Moon’s southeastern limb reveals our
changing viewpoint during May. CONSOLIDATED LUNAR ATLAS/UA/LPL; INSET: NASA/GSFC/ASU
Eta Aquariid meteor shower
Altair
CYGNUS
AQUIL A
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
OBSERVING
HIGHLIGHT
Enif
CAPRIC ORNUS
AQUARIUS
PEGASUS
Radiant
d
10°
May 6, 4 A.M.
Looking east
A gibbous Moon competes with
May’s best shower, reducing
the number of meteors visible
in the predawn sky May 6.
Hemisphere could see twice
as many because the radiant
— the point in Aquarius where
the meteors originate —
Eta Aquariid meteors
Active dates: April 19–May 28
Peak: May 6
Moon at peak: Waning gibbous
Maximum rate at peak:
50 meteors/hour
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.
Jupiter reaches its 2018 peak May 8, when the gas giant shines
at magnitude –2.5 and spans 44.8" through a telescope.
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