Sky & Telescope - May 2024 - 52

MAY 2024 OBSERVING
Exploring the Solar System by Thomas A. Dobbins
Observing Solar Granulation
T
he Scottish telescope maker James
Short was the first to describe
the finely mottled appearance of the
Sun's visible surface, or photosphere, in
1740. The greatest observer of the 18th
century, William Herschel, discerned
an overall pattern of tiny, bright points
with apparent diameters of one to two
arcseconds separated by narrow, dusky
spaces. During the early 19th century,
these minute features were often
described as " rice grains. " It wasn't
until 1864 that the eagle-eyed British
observer William Rutter Dawes coined
the term " granulations " for them.
In 1876, French astronomer Jules
Janssen embarked on a decades-long
program of high-resolution solar photography
using a 5-inch refractor at the
Meudon Observatory in suburban Paris.
The Sun's brilliance made exposures
52 MAY 2024 * SKY & TELESCOPE
of a fraction of a second even with the
era's slow emulsions, so it was possible
to freeze the seeing and overcome the
blurring effects of atmospheric turbulence
to some extent. In most of Janssen's
photographs, individual granules
resembled rounded cobblestones, but in
his sharpest images they appeared to be
angular polygons.
The resolution of Janssen's photographs
remained unsurpassed until
the 1950s, when Project Stratoscope
lofted a 12-inch telescope to an altitude
of 24 kilometers (80,000 feet) aboard
an unmanned ballon. Turbulence is
absent from this vantage point, where
atmospheric pressure is 25 times lower
than at Earth's surface and a constant
temperature of -50° Celsius (-58° Fahrenheit)
prevails. These flights obtained
photographs of unprecedented clarThe
Sun's surface is a rapidly changing, seething cauldron of delicate detail.
pResolving the convective cells of solar granulation
is well within the capabilities of amateur
equipment. This superb image was captured
with a 12-inch reflector equipped with a Baader
AstroSolar Safety solar filter and a high-speed
video camera last September.
ity that clearly resolved the polygonal
shapes of individual granules.
Similar structures form in pots of
boiling oatmeal or thick soup, so it's
hardly surprising that as early as 1874,
Allegheny Observatory astronomer
Samuel Pierpont Langley suggested that
solar granulation is produced by convection
and that the bright granules are
the tops of rising columns of hot gas
and the intervening dark spaces regions
where cooler gas is descending.
The validity of Langley's speculation
became apparent at the dawn of the
20th century, when the French physicist
DANIEL BROUSSEAU

Sky & Telescope - May 2024

Table of Contents for the Digital Edition of Sky & Telescope - May 2024

Contents
Sky & Telescope - May 2024 - Cover1
Sky & Telescope - May 2024 - Cover2
Sky & Telescope - May 2024 - 1
Sky & Telescope - May 2024 - Contents
Sky & Telescope - May 2024 - 3
Sky & Telescope - May 2024 - 4
Sky & Telescope - May 2024 - 5
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Sky & Telescope - May 2024 - Cover4
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