Astronomy - USA (2020-08)

South Polar Region

North Polar Region

South South Temperate Zone

South South Temperate Belt

South Temperate Zone

South Temperate Belt

South Tropical Zone

South Equatorial Belt

Equatorial Zone South

Equatorial Band

Equatorial Zone North

North Equatorial Belt

North Tropical Zone

North Temperate Belt

North Temperate Zone

North North Temperate Zone

North North Temperate Belt

Maxwell

Gap

Colombo

Gap

B Ring

D Ring

C Ring

Cassini

Division

A Ring

Encke

Division

F Ring

Atlas

Ring

Janus/

Epimetheus Ring

G Ring

E Ring

Pallene Ring

58 ASTRONOMY • AUGUST 2020

diminished in size since the

1800s and may be on its way

to nonexistence, although its

recent shearing behavior may

also be linked to a natural

weather phenomenon: the

interaction of winds between

the GRS (an anticyclone) and

a cyclone that has come close

to it.

In August 2019, Australian

amateur astronomer Phil

Miles detected a white storm

erupting in the planet’s South

Equatorial Belt (SEB, just pre-

ceding the GRS). By February

2020, the disturbance had

coursed through the normally

dark belt, creating a bright

interior zone.

Such disturbances are

uncommon but not unusual.

Jupiter’s dark bands change

over irregular timescales.

Occasionally they can even

fade away entirely, as the SEB

last did in 2010. The SEB dis-

appears when whitish clouds

form on top of it. This type of

event is associated with exten-

sive changes to other bands,

spots, and colors in Jupiter’s

atmosphere. Interestingly,

astronomers anticipate a sig-

nificant color change in the

planet’s Equatorial Zone in

the next year or two.

Jupiter’s Equatorial Zone

normally appears white due

to extensive high cloud cover.

Every six or seven years

(though not always), the

zone’s high ammonia clouds

clear out, changing the

region’s color to ochre. The

disturbance usually lasts

between 12 and 18 months,

allowing telescopic observers

a rare chance to see details

deeper in the planet’s atmo-

sphere, normally hidden by

those high clouds. We are

currently in that window of

opportunity, as the next

clearing has been predicted to

occur between 2019 and 2021.

Saturn and the

midnight hour

Saturn achieves opposition on

July 21 at 22h14m UT, when

the planet will shine at mag-

nitude 0.1 with an apparent

equatorial diameter of 19",

while the rings will measure

42" across. If you monitor

Saturn’s rings through a

telescope in the weeks before

opposition, you’ll find them

shining roughly as bright as

the planet’s globe. For a cou-

ple of days around opposition,

the rings will temporarily

intensify in apparent bright-

ness, outshining the globe

before dimming back to their

normal appearance. It’s one of

the most visually fascinating

occurrences of any planet.

This peculiar change is

called the Seeliger effect, in

honor of German astronomer

Hugo von Seeliger (1849–

1924), who first noticed it in

1887. In addition to this
      shadow hiding, the Cassini
      spacecraft revealed that
      coherent backscattering

— sunlight interacting with

the collective particles in the

planet’s rings — contributes

significantly to the phenom-

enon, as the many irregular

bits of rock and dust combine

to produce more intense light.

This light scatters back to our

eyes and makes the rings

seem to brighten.

A return of Saturn’s

White Spots?

Overall, Saturn presents a less

dynamic range of details on

its globe than does Jupiter,

mainly displaying vary-

ing belts and zones without

the finer details. This is

why most observers tend to

focus their attention on the

rings. Episodically, however,

Saturn’s globe erupts with

stunning detail. The planet’s

2020 apparition may be one

of those years that brings us

some visual surprises.

Chief among them may be

the return of the Great White

Spot — a fantastic high-

altitude storm that surfaces

roughly every 20 to 30 years.

The greatest storms can be

as large as Earth, with tails

that wrap entirely around

the planet over the course of

months as regions of Saturn’s

globe transform into a tempest

of dynamic change. Six such

storms have been observed

SATURN CLOUD FEATURES

SATURN RING FEATURES

This diagram shows the common

nomenclature for Saturn’s

various belts, zones, and ring

sections. ASTRONOMY: ROEN KELLY