Astronomy

SOUTHERN

SKY

MARTIN GEORGE describes the solar system’s changing landscape

as it appears in Earth’s southern sky.

November 2017: Saturn’s last hurrah

On each evening in early

November, the familiar shape

of Scorpius the Scorpion

appears to dive toward the

western horizon. To its right

lies the less-recognizable

constellation Ophiuchus the

Serpent-bearer. But this month,

Ophiuchus bears a bright inter-

loper: the ringed planet Saturn.

The solar system world shines

at magnitude 0.5 just to the

right of a short line of stars

anchored by 3rd-magnitude

Theta (θ) Ophiuchi.

Although Saturn draws

closer to the Sun and dips lower

in the evening sky with each

passing day, it remains a tempt-

ing target for observers in early

November. A telescope reveals

the planet’s f lattened disk,

which measures 15" across the

equator, and a glorious ring

system that spans 35" and tilts

27° to our line of sight. Also

keep an eye out for Saturn’s

family of moons. Any telescope

shows 8th-magnitude Titan,

and three 10th-magnitude sat-

ellites come into view through

10-centimeter and larger

instruments.

Look closer to the horizon

and you should be able to pick

out Mercury. The innermost

planet will be hard to spot in

the twilight glow in early

November, but it gets progres-

sively easier as it pulls away

from the Sun. By the time it

reaches greatest elongation

November 23/24, it lies 22° east

of our star and stands 11° high

in the west-southwest 45 min-

utes after sunset. Mercury then

shines at magnitude –0.4 and

shows up easily in the twilight.

A few days later, on the 28th,

Mercury slides 3° south (to the

upper left) of fainter Saturn.

Mercury is an interesting

telescopic object throughout

November, particularly later

in the month as its disk grows

larger and its illuminated hemi-

sphere turns away from Earth.

At greatest elongation, the

planet appears 6.6" across and

63 percent lit. On the month’s

final evening, Mercury’s disk

spans 7.8" and shows a distinct

crescent phase, just 40 percent

illuminated.

The other bright planets

don’t fare as well. Although

Mars gradually pulls away from

the Sun during November, it

remains fairly low in the east

before dawn. By the 30th, it

climbs nearly 15° high an hour

before sunrise and should

appear conspicuous among

the background stars of Virgo.

Mars then lies 3° north (to the

lower left) of the Maiden’s

brightest star, 1st-magnitude

Spica. At magnitude 1.7, the

Red Planet shines half as bright

as the blue-white star, though

the color contrast between the

two should be obvious. A tele-

scope won’t show any detail on

the planet’s 4"-diameter disk.

Jupiter returns to the

morning sky in the latter half

of November. Look for the

giant planet well to the lower

right of the Mars-Spica pair.

By the 30th, Jupiter climbs 10°

high a half-hour before sunrise.

You shouldn’t have any prob-

lem identifying it because it

shines so brightly, at magnitude

–1.7. There’s not much point in

targeting the planet through

your telescope, however — its

low altitude means you have to

look through thick layers of

Earth’s turbulent atmosphere

and thus won’t see much detail.

The brilliance of Venus will

go unappreciated by observers

at mid-southern latitudes. On

November 1, the inner planet

rises just 40 minutes before the

Sun and is essentially lost in the

bright glow of twilight. If you

live closer to the equator, how-

ever, you might want to give

the magnitude –3.9 object a

shot. If you do, target Venus the

morning of the 13th when it

passes just 0.3° north of Jupiter.

The starry sky

Late on November evenings,

a beautiful string of stars

stretches across the sky from

Aldebaran in the northeast to

the Southern Cross and the

pointer stars nearly due south.

In comparison, the sky to

the north looks barren, espe-

cially about halfway from the

horizon to the zenith. The

region is not completely devoid

of brighter stars, however. One

of the most conspicuous is Beta

(β) Ceti, also known as Diphda.

Diphda is the brightest star

in the constellation Cetus the

Whale, though some skywatch-

ers refer to it as a sea monster.

Whichever shape you prefer,

Diphda marks the beast’s tail.

The star’s name comes from

an ancient Arabic title meaning

“second frog,” while the “first

frog” happens to be Fomalhaut

in Piscis Austrinus. Diphda

came second because it follows

the first frog across the sky as

Earth rotates beneath it.

Although the International

Astronomical Union formally

accepts the name Diphda, it has

also long been known as Deneb

Kaitos, which means “tail of

the whale.” However, its full

Arabic name described it as

the “tail of the whale to the

south.” To add to the confu-

sion, another star, Iota (ι) Ceti,

was also once described as the

whale’s tail, though its full

name meant “tail of the whale

to the north.” Iota lies 11°

north-northwest of Beta.

Diphda is a yellow-orange

giant star of spectral type K0.

It possesses about three times

the mass of the Sun and is in

the latter stages of its life.

Surprisingly, unlike most such

stars, it is a relatively strong

X-ray source. Astronomers sus-

pect that the X-rays come from

Diphda’s superheated corona,

which gets warmed by the star’s

magnetic field.

It may seem surprising that

magnitude 2.04 Diphda has the

Bayer designation of Beta when

the constellation’s Alpha (α)

star, Menkar, comes in signifi-

cantly fainter at magnitude

2.54. There is some evidence

that Diphda brightened notice-

ably in the 17th century, how-

ever. Leading astronomical

historian Agnes Clerke docu-

mented this brightening in the

1800s, and Jean-Baptiste Biot

also noted it in his Tra i t é

d’Astronomie Physique (Treatise

on Physical Astronomy).

There have been other

reports of temporary brighten-

ing as well. Observers in Perth,

We ster n Au st r a l ia , c a ref u l ly

followed one such episode in

1923. Needless to say, Diphda
      seems far more intriguing than
      your typical giant star.