Sky & Telescope - July 2022 - 29

SUN: JORDI L. COY; SOLAR SPECTRUM: N. A. SHARP, NOAO / NSO / KITT PEAK FTS / AURA /
NSF; ABBOTT: HARRIS & EWING COLLECTION - LIBRARY OF CONGRESS / PUBLIC DOMAIN
Unlocking the Sun's secrets is an ongoing
challenge that got its start in the 19th century.
W
ith satellite missions continuously streaming solar
data from space, it's hard to imagine a time when it
took years of effort to wring out even the smallest
bits of dependable information about the Sun. Yet, despite all
the progress, basic questions a century old remain unresolved.
How much total power does the Sun produce, and is it
truly constant? Answers remain elusive. Understanding how
our nearest star works now has renewed importance since a
firm value for the solar output (total solar irradiance, or TSI)
is a vital component in climate modeling and global energy
balance calculations.
For most of the 19th century, almost nothing was known
about how the Sun worked. No one knew the causes of
sunspots, faculae, prominences, or anything about the Sun's
interior. Even the most obvious questions were hair-pulling
puzzles. How was the Sun's heat generated and sustained? Is
it stable or variable? How long will it last? And most obvious
of all, how hot is it? In 1876, the Paris Academy of Sciences
offered a prize to anyone who could determine the Sun's surface
temperature. Up for grabs were thousands of francs and
international prestige. The game was afoot.
Getting Warmer
An early, promising approach was to simply focus the rays of
the Sun on a container of water (or other liquid) and time
how long it took to heat up. A hot Sun would warm the liquid
more quickly than a cool Sun. English astronomer John Herschel
tested the concept in 1837 while at the Cape of Good
Hope in South Africa. Then, in that same year, experiments
by physicist Claude Pouillet showed that 1.76 grams of water
under the French Sun warmed 1°C in one minute. (This can
also be expressed as 1.76 calories per square centimeter per
minute.) He named this value the solar constant.
It was a beginning, but data only make sense in context.
Only repeated experiments over long periods would reveal
if the solar constant were truly a constant. But there were
two immediate problems. The first is Earth's atmosphere.
We humans exist at the bottom of a moving ocean of atmospheric
gases. How did that affect the results? And what about
different parts of the world, altitudes, weather conditions,
and times - how would they influence the outcomes? A second
major problem was that, until the late 19th century, there
was no way to mathematically relate Earthly measurements of
heat to a reliable value for the Sun's surface temperature.
Tackling even the first of these problems was a steep challenge.
Early investigators assumed that the task was simply
RISING STAR Although the Sun is relatively close to Earth, a basic
understanding of our star eluded astronomers for most of the 19th
century. Even obvious features, like the sunspots shown in this striking
sunrise photo, defied explanation. Despite advances in technology, the
Sun continues to hold on to many of its secrets.
SOLAR PIONEER Charles Greeley Abbot appears here sometime between
1913 and 1917 with his silver-disk pyrheliometer, a device used to
measure solar irradiance. In contrast to the bolometer, which measures
radiation intensities at different wavelengths, the pyrheliometer measures
the total amount of solar radiation.
sk yand tele scope .o r g * JULY 2022 29
to determine the degree to which sunlight was diminished
before it reached their detectors. Estimates of total atmospheric
absorption ranged wildly from about 30% to more
than twice that, leading to estimates of the Sun's surface
temperature that were all over the map. Something was
plainly wrong.
The key turned out to be in the Sun's rainbow of colors. As
early as 1814, German lens maker and physicist Joseph von
Fraunhofer had been examining sunlight using the spectroscope
he'd invented, but at the time no one knew how (or
even if) spectroscopic information could reveal the temperature
of the Sun.
Fraunhofer's solar spectrum presents an intriguing forest
of dark lines of unequal intensity. As shown on page
32 (lefthand graph), plotting intensity against wavelength
results in a rounded curve interrupted by occasional sharp
dips. When sunlight passes through Earth's atmosphere,
absorption features (the so-called telluric lines) appear superimposed
upon the spectrum, most prominently from the
presence of atmospheric water vapor, carbon dioxide, oxygen,
and ozone. Revealing the solar constant meant being able to
distinguish the telluric lines from those inherent in the Sun's
atmosphere. To do this, a transmission coefficient for Earth's
atmosphere had to be calculated at each wavelength. It was
a daunting task. Yet the timing was fortuitous for the future
of solar research - the problem was perfectly teed-up for two
pioneering solar scientists at the Smithsonian Astrophysical
Observatory (SAO) to have a crack at it.
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Sky & Telescope - July 2022

Table of Contents for the Digital Edition of Sky & Telescope - July 2022
