Astronomy - September 2015

Neptune near its peak

36 ASTRONOMY • SEPTEMBER 2015

Visible to the naked eye

Visible with binoculars

Visible with a telescope

MARTIN RATCLIFFE and ALISTER LING describe the

solar system’s changing landscape as it appears in Earth’s sky.

September 2015: Totality’s long embrace

SKYTHIS

MONTH

T

he first half of 2015 fea-

tured lots of evening

planets, but as autumn

begins, the focus shifts to

the predawn sky. Venus,

Mars, and Jupiter return to

prominence on September

mornings. Although the eve-

ning sky quiets some, Mercury

brief ly pokes above the hori-

zon and Saturn continues to

look spectacular. Meanwhile,

the overnight hours offer some

of the year’s best views of

Uranus and Neptune.

But September’s star attrac-

tion lies closer to home. The

Moon takes center stage three

times during its monthly tour

of the sky. In the first act,

Earth’s satellite passes in front

of 1st-magnitude Aldebaran

before dawn September 5.

Observers along a line that

runs from the western shore of

Lake Superior to Florida’s east

coast will see the star emerge

from behind the Last Quarter

Moon’s dark limb as the pair

rises. The farther north and

east of this line you live, the

night of September 27/28 when

it dives deep into Earth’s

shadow to create a spectacular

total lunar eclipse. Observers

across most of North America

will see all 72 minutes of total-

ity the evening of the 27th.

Viewers in most of Europe,

Africa, and the Middle East

will witness the eclipse before

dawn on the 28th. (See “Eclipse

of the Super Moon” on p. 56

for complete details.)

In contrast to lunar events

timed in minutes or hours,

planets typically remain on

display for weeks or months.

Case in point: Mercury, which

hangs low in the west after

sunset during September’s first

half. The planet reaches great-

est elongation September 4,

when it lies 27° east of the Sun.

Although that sounds pretty

impressive, Mercury remains a

horizon hugger for Northern

Hemisphere observers. The

higher the two objects will

appear. From New York City,

for example, they stand 11°

above the eastern horizon

when Aldebaran returns to

view at 12:40 a.m. EDT.

The Moon passes in front

of a much bigger and brighter

star September 13. The target

this time is the Sun, and Luna

obscures part of it for people

in southern Africa, southern

Madagascar, and parts of the

Indian Ocean and Antarctica.

The best sites on land for this

partial solar eclipse are

around Cape Town, South

Africa, where the event starts

shortly before sunrise and

peaks at 5h43m UT. The

Moon then blocks 30 percent

of the Sun’s surface area.

The Moon’s final and most

impressive act arrives the

ecliptic — the Sun’s apparent

path across the sky that the

planets follow closely —

makes a shallow angle to the

western horizon after sunset

around the autumnal equi-

nox. Most of Mercury’s elon-

gation translates into distance

along the horizon and not

altitude above it.

From 40° north latitude,

the inner planet stands just 3°

high a half-hour after sunset

on the 4th. Mercury shines

brightly (magnitude 0.1),

however, and you should be

able to see it in the twilight

glow through binoculars. A

telescope reveals its disk,

which spans 7" and appears

slightly more than half-lit. If

you have exceptionally clear

skies September 14, you might

spot Mercury 7° to the left of

a waxing crescent Moon.

Saturn is a much more

attractive target for evening

The Full Moon slides through the southern part of Earth’s dark umbral

shadow the night of September 27/28, mimicking this eclipse from April

14/15, 2014. RICHARD MCCOY

The bluish outer planet reached opposition in late August and remains a

tempting target during September. This amateur image also captures the

planet’s brightest moon, Triton, at upper left. RON WARNER

Martin Ratcliffe provides plane-

tarium development for Sky-Skan,

Inc., from his home in Wichita,

Kansas. Meteorologist Alister

Ling works for Environment

Canada in Edmonton, Alberta.