Sky & Telescope - July 2022 - 31

MT. WHITNEY OBSERVATORY: JUSTIN JOHNSEN / CC BY 2.0; PORTRAIT OF
LANGLEY: SMITHSONIAN INSTITUTION ARCHIVES; SUNSPOT DRAWING:
SAMUEL LANGLEY / CHARLES YOUNG / THE SUN / PUBLIC DOMAIN
a collaborator in the important effort to measure the intensity
of solar radiation. Over time, Langley's interests would
drift to the nascent field of aviation, leaving Abbot in charge
of solar work.
Abbot built two instruments for probing the Sun's heat.
The first, a pyrheliometer of the type used by Pouillet, was
an odd sort of thermometer with a blackened silver disk that
was aimed at the Sun. Tracking the Sun across the sky, the
device measured changes in solar radiation as sunlight passed
through different thicknesses of Earth's atmosphere. The second
instrument was an improved version of Langley's bolometer,
which was essentially a wire whose electrical resistance
would change as radiation of a particular wavelength was
directed at it by a prism. By passing the whole solar spectrum
across the wire, he could create a radiation curve of the Sun's
output (a bolograph) by plotting intensity versus wavelength.
Abbot collected data on solar radiation and atmospheric
transmission at 44 different wavelengths over multiple zenith
angles, then mathematically extrapolated this information to
determine what the radiation values should be outside Earth's
atmosphere. Analyzing his data in 1908, Abbot found a mean
value for the solar constant of 2.01 calories, equivalent to
about 1,403 watts per square meter. By 1915, with accumulating
observations and improved methodology, Abbot refined
this number to 1.93 calories - close to the current value of
1.95, or 1,361 watts per square meter.
Sadly, Abbot's great mentor, Samuel Langley, died in 1906
and wasn't able to enjoy the results of his partner's efforts.
Although Langley's early accomplishments in solar physics
were pivotal, he's better remembered for his great contributions
to early aviation. Several important facilities bear his
name, including Langley Air Force Base and NASA's Langley
Research Center, in Virginia.
A Voyage to the Sun
What's the Sun's surface temperature? To solve this second
question Abbot would need to determine the solar constant at
uFLYING TO THE SUN Samuel
Pierpont Langley was a visionary
solar scientist and passionate
enthusiast of early aviation who
wrote that " the observation of the
amount of heat the Sun sends the
earth is among the most important
and difficult in astronomical
physics. "
uuSPLENDID SPOT While
serving as the first director of the
Allegheny Observatory in Pennsylvania,
Langley used the facility's
13-inch Fitz-Clark refractor to
make this detailed drawing of a
" typical " sunspot that crossed the
Sun's face in December 1873.
sk yand tele scope .o r g * JULY 2022 31
SUMMIT SHELTER This stone-and-steel structure 14,502 feet above
sea level atop Mount Whitney in California was one of the locations where
Abbot collected data in search of a solar constant. Built in 1909, the
building still exists today and is frequently used by hikers seeking shelter.
the Sun's surface. But how? Math helps. As with gravitational
force, radiation from the Sun varies inversely with the square
of the distance. To illustrate, imagine two enormous spheres
each having the Sun at their center. The first has a radius of
almost 700,000 kilometers (435,000 miles) - big enough to
encompass the photosphere, the visible surface of the Sun. The
other, vastly larger sphere extends about 150 million km to
Earth's orbit. A space probe traveling from the outer sphere
to the inner one would record a steady increase in the solar
constant. At the halfway point, the energy recorded will have
increased fourfold. When it reaches the photosphere, the
solar constant will be 46,000 times greater. Multiplying this
by the solar constant measured at the distance of Earth, we
can calculate the solar radiation emitted from each square
meter at the Sun's surface (the solar flux).
Using the modern value of 1,361 watts per square meter
(equivalent to a solar constant of 1.95) for the solar radiation
received outside of Earth's atmosphere, we find a solar flux
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Sky & Telescope - July 2022

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