Sky & Telescope - July 2022 - 30

POUILLET'S PYRHELIOMETER: WORLD HISTORY ARCHIVE /
ALAMY STOCK PHOTO; BOLOGRAPH: THE ASTROPHYSICAL JOURNAL
Understanding the Sun
Kindred Spirits
SAO founder, visionary scientist, and
aviation enthusiast Samuel Pierpont
Langley had always been drawn to solar
studies. Determining the degree to
which our planet's atmosphere intercepts
solar rays would become Langley's
first great contribution to astronomy.
In 1881, he invented a device called a
bolometer to measure the Sun's heat at
different wavelengths. This was a crucial
innovation because, as plainly as the sky
is blue and sunsets are red, atmospheric
absorption and scattering vary greatly
with wavelength. Measuring sunlight
through the high air of Mount Whitney
in California, Langley created a solar
radiation curve based on 12 selected
wavelengths and derived a solar constant of 2.54 calories per
square centimeter per minute - higher than Pouillet's value
of 1.76. Langley's findings were rough, but his purpose was to
establish a proof of concept and demonstrate that his wavelength-by-wavelength
approach to atmospheric transmission
was a viable way to calculate the solar constant.
What Langley most needed next was a capable collaborator
to help take his work to the next level. It was his good fortune
to then meet an exceptionally talented New Englander with
unique skills for the task. Their paths converged in a basement
lab at Massachusetts Institute of Technology (MIT).
It's hard to imagine two more outwardly unlike people than
the urbane, immaculately dressed Samuel Pierpont Langley
and the rough-hewn farm boy Charles Greeley Abbot. Born in
1872 in Wilton, New Hampshire, Abbot grew up in a farmhouse
where all the rooms were freezing cold except in the
kitchen and the sitting room, which was warmed by a great
fireplace. Upstairs was a library of books left by his great uncle,
who was an inventor. On Saturdays, it was Abbot's delight to
MAPPING SUNLIGHT Langley published his first bolograph of the
Sun in an article titled " The 'Solar Constant' and Related Problems, "
which appeared in the March 1903 issue of the Astrophysical
Journal. He extrapolated the 2.54 calorie value of the solar
constant from the areas under the curves. Langley emphasized,
" This is but a provisional value, given to illustrate the method,
which, it is hoped, may be pursued later under more favorable
local conditions. "
A MEASURE OF THE SUN French astronomer
Claude Pouillet used a pyrheliometer in an
early attempt to measure the solar constant. The
device consisted of a dark plate (aimed sunward)
that heated a small volume of water, which was
measured by an internal thermometer.
" steal away to the arctic upstairs room "
and pore over dusty books about the workings
of clocks, watches, and the doubleacting
condensing steam engine of James
Watt. " To trace out the operation of such
things I was willing to shiver with cold for
hours, " he later wrote. Perhaps it's no surprise
that heat later became his life's work.
An inquisitive tinkerer, Abbot when he was
little used the kitchen stove as a torch to
solder pans for the household, and at age
13 he built a forge to fix farm implements.
Abbot's future calling was partly shaped by chance. One
day, some of his classmates at Phillips Academy in Andover,
Massachusetts, decided to go to Boston to take the entrance
examination for MIT. Abbot went along for the train ride, and
not wanting to wander alone in the city, decided on a whim
to take the examinations himself. He passed them easily,
enrolled, and graduated with high honors in physics.
It was during a postgraduate year at MIT that Abbot met the
dapper Langley, who happened to drop by the Institute in search
of an assistant. " Wouldn't you like to see my experiment? "
Abbot asked. " I should like it extremely, " replied Langley.
They chatted briefly, and no doubt sensing a kindred spirit,
Langley hired the amiable inventor. The boy, who not long
before was building bicycles and water mills, would soon be
making scientific instruments of great sensitivity.
Arriving at the SAO in 1895, Abbot's first task was to help
Langley map the Fraunhofer lines in the Sun's infrared spectrum.
Langley quickly recognized the young man's talent for
working with delicate instruments and soon engaged him as
30 JULY 2022 * SKY & TELESCOPE
Sky & Telescope - July 2022

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