58 AUSTRALIAN SKY & TELESCOPE May | June 2018
A
sk your astronomically curious
friends to name a famous
feature on another planet, and
the top choice will almost certainly be
Saturn’s rings. A strong runner-up will
likely be Jupiter’s Great Red Spot. Yet
while most casual telescope users have
gazed upon those glorious rings many
times, far fewer have ever spied Jupiter’s
iconic GRS. So for those of you longing
for your first glimpse of it — or just
wanting another look — read on!
This year Jupiter comes to opposition
on May 9. That’s when this giant planet
will appear its biggest (45 arcseconds
across) and brightest (magnitude –2.5),
and it’ll be nearly as big and bright for
several weeks before or after this date.
Even though it’s not an entirely
favourable opposition — in September
2010, Jupiter swelled to 50 arcseconds
and magnitude –2.9 (nearly 50%
brighter) — on the plus side, the planet’s
southern declination (–16°) this year
means the planet will be nice and high
in the sky for southern observers.
The GRS itself — an enormous, high-
pressure (anticyclonic) ‘storm’ rotating
every 6 days — has undergone a modest
metamorphosis of late. If you haven’t
looked for the spot in the past decade,
you’ll be surprised by how much it’s
changed in the intervening years.Shrinking and shifting
The GRS has gradually become smaller
since telescopic observers made the first
reliable measurements of its size in the
1880s. A century ago, for example, the
Great Red Spot was 2½ times longer
than it is now.
However, as Amy Simon (NASA
Goddard Space Flight Center) and her
colleagues detail in a forthcoming
Astronomical Journal article, the rate of
that shrinking accelerated by about 50%
beginning about 1979, the year thatSpotting Jupiter’s famous spot
Timing is everything when it comes to glimpsing the iconic Great Red Spot. Steady seeing helps too.
Voyagers 1 and 2 made their historic
flybys of Jupiter.
Today the GRS’s longitudinal length is
just 13.7°, or about 15,500 km. The width
has shrunk too, down to just under 10°
in latitude (12,000 km). It’s still bigger
than Earth — but not by much.
Meanwhile, the spot doesn’t stay
put. It’s sandwiched at latitude 22½°
south between a strong, westward-
flowing jet stream to its north and an
equally strong eastward jet to its south.
The GRS spins counterclockwise like
a giant ball bearing rolling between
them, creating an oversize obstacle that
deflects the jets as they flow past.
A little background: Jupiter rotates
differentially. Its deep interior spins once
every 9.925 hours (9h 55 m 30 s), at what’s
called its System III rotation period. But
the cloudtops within 10° of the equator
zip around 5 minutes faster (System I)
— while at the GRS’s latitude, the mean
rotation period (System II) is about 11
seconds slower.
Observers had long thought that
the GRS spun around Jupiter at the System
II rate, but it’s actually lagging behind
and gradually drifting westward with
respect to all the cloud features around it.
Simon and her team find that the GRS’s
westward drift has also accelerated in
recent years, and they’re struggling
to understand the steering forces that
could be causing it to move faster.
“The biggest factor is the surrounding
winds,” she explains, perhaps due to
very slight changes in the latitudes of
the adjacent zonal jets. But the effect is
hard to measure since the GRS deflects
those same winds. And the spot can be
buffeted by disturbances that churn
up the jets and other effects. “We have
hints,” Simon says, “but nothing has
been conclusive.”
The good news in all of this
morphing is that the Red Spot hasSThe seeing was nearly perfect on May 19,
2017, when veteran planetary photographer
Christopher Go captured the Great Red Spot
marching across Jupiter’s disk. The date was
six weeks after the planet’s opposition, and he
recorded the images with a 35.6-cm telescope
over a span of 52 minutes. South is up.JUPITER’S 2018 OPPOSITION by J. Kelly Beatty