http://www.skyandtelescope.com.au 17As early as the1900s, observers clearly saw
thattheGRSwasshrinking.Theirmeasurements
often varied, sometimes due to how much the spot
blended into the cloudy background around it, but
overall a consistent trend emerged. Many years of
measurements showed that the spot’s length was
decreasing in longitude by about 0.14°per year. More
recently, high-resolution measurements have refined
this rate to an even more rapid 0.19°per year. Thus,
the GRS has been steadily shrinking for more than a
century. It currently spans about 14°in longitude, less
thanhalfitssizeacenturyago.
This begs the question: could the GRS have
been observed even earlier by telescopic observers
—and,ifso,wasitevenlargerthen?Someofthe
earliest telescopic observations by Gian Domenico
Cassini and Robert Hooke in the mid-1660s note a
“permanent Spot” on Jupiter, which many have since
interpreted as being the GRS. It’s not clear from the
original papers themselves if this is the case, however,
because the descriptions are somewhat vague as to
size, colour and location. Also, observers provided
only intermittent reports of such a feature during the
two centuries that followed.
Moreover, the spot was of interest not so much
foritscolourandsizebutbecauseastronomerscould
useittodeterminetherotationrateofJupitertoa
remarkably accurate 9 hours 56 minutes. Beyond
that,itapparentlywarrantedlittlementionother
than as a “small spot in the biggest of Jupiter’s three
belts” spanning about 12°, as shown in sketches over
those years.
Notably,iftherecenttrendinsizeweretoholdtrue
farther back in time, the GRS would have been 30°to
40 ° larger in 1665 than in 1878 and thus occupied most
of a hemisphere! This clearly would not be a small
distinct spot seen rotating across the planet’s disk —
with that size, it could have certainly stood out and
even been seen through the low-resolution telescopes
of that era.
Conceivably, the spot reported by Cassini and Hooke
might have dissipated and been replaced by the current
GRS over the intervening years. Another possibility
is that the storm experiences episodes of growth,
after which it becomes smaller. For example, despite
evidence that the spot clearly was shrinking from the
1870s onward, occasional reports (for example, during
1915–25) gave a larger size — though only by a few
degrees of longitude.
It’s also hard to tell whether the larger
measurements corresponded to growth of the actual
oval of the GRS or to an extended region around it. As
many readers will know, the storm itself is sometimes
very hard to distinguish from the surrounding
clouds. These reports sometimes described the edges
as pointy, rather than round, which gives us a clue.
Hurricane-like vortices typically have rounded edges,
even when very oblong, while the region around the
GRS, shaped and bounded by regional winds, can
have more angular shapes. Sketches of Jupiter, when
compared with modern, high-resolution images, show
that, indeed, sometimes the pointed features are not
actually part of the storm itself.
This discrepancy shows one downside of using
colour to delineate storm size. The Great Red Spot’sHISTORICAL
HINTS Early
reports of a
spot on Jupiter
from Robert
Hooke and Gian
Domenico Cassini
appear in the
Royal Society’s
Philosophical
Transac tions
( 1 665–66).
Historians
generally (but
not universally)
believe these
observers saw
something other
than the Great
Red Spot.SHRINKING
WITH TIME
At left is a
representation of
how the Great Red
Spot might have
appeared in 1890,
albeit at higher
telescopic resolution
than was available
then, compared with
an actual image
from 2015.DAMIAN PEACHGRS in 1890 GRS in 2015