Sky and Telescope - March 2016 - 19

GRS in 1890

HISTORICAL HINTS Early reports of a spot on Jupiter from
Robert Hooke and Gian Domenico Cassini appear in the Royal
Society's Philosophical Transactions (1665-66). Historians
generally (but not universally) believe these observers saw
something other than the Great Red Spot.

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 Sky & Telescope
readers 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 color
to delineate storm size. The Great Red Spot's internal
structure consists of a high-velocity collar surrounding a
somewhat stagnant core. A cloud in this collar can complete a full circuit around the spot's perimeter in about
3 days. Some have suggested that using the location of
GRS in 2015

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, as compared
with an actual
image from 2015.

DAMIAN PEACH

As early as the 1900s, observers clearly saw that the
GRS was shrinking. Their measurements 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 than half its size a century ago.
This begs the question: could the GRS have been
observed even earlier by telescopic observers - and, if
so, was it even larger then? Some of the 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, color, 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 for
its color and size but because astronomers could use it
to determine the rotation rate of Jupiter to a remarkably
accurate 9 hours 56 minutes. Beyond that, it apparently
warranted little mention other than as a "small spot in
the biggest of Jupiter's three belts" spanning about 12°,
as shown in sketches over those years.
Notably, if the recent trend in size were to hold true
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 enormous 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

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Sky and Telescope - March 2016

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