Sky & Telescope - July 2022 - 33

ATMOSPHERIC TRANSMISSION CURVES: GREGG DINDERMAN / S&T; SOURCE: DOUGLAS
MACDOUGAL; ABBOT WITH SOLAR BOILER: HARRIS & EWING COLLECTION - LIBRARY OF
CONGRESS / PUBLIC DOMAIN; ABBOT IN 1930: SMITHSONIAN INSTITUTION ARCHIVES
Log intensity
length of a star's peak intensity, you can approximate its
temperature. However, locating the peak isn't always easy, as
a glance at Abbot's solar curve (page 32, bottom right) demonstrates.
Nonetheless, Abbot confirmed that the Sun's spectrum
is similar in shape to a blackbody at around 6,000K.
Later Life
Despite Abbot's growing trove of data, whether or not the
Sun's output was constant remained a mystery. He took thousands
of measurements to address that question. From 1903
to 1914 Abbot collected data from various locations, including
Washington, D.C., a balloon over Algeria, and Mount
Wilson and Mount Whitney in California. And he kept at it.
By 1922 he and his colleagues had amassed 1,244 observations.
Studies of the constancy of the total solar irradiance
continue even today, though now the observations are made
with instruments of incredible precision aboard satellites and
the International Space Station. A definitive answer about the
long-term constancy of TSI remains elusive.
Abbot took up directorship of the SAO after Langley's
death, and in 1928 he became Secretary of the Smithsonian
Institution, leading it during the Depression and war
years. A passionate advocate for solar energy, Abbot never
stopped inventing. He constructed a solar oven for Mount
Wilson Observatory, a solar boiler for generating power
in arid places, and a solar still for extracting freshwater
from the sea. In 1932, at age 60, he authored a volume in
the Smithsonian's Scientific Series - a collection he himself
initiated. His book was, appropriately enough, entitled Great
Inventions. It's a lucid tour de force explaining the most
important electrical and mechanical inventions of the 19th
and 20th centuries. Abbot was a vibrant, engaged man to the
very end of his life. His last patent was issued in 1972, the
year of his 100th birthday.
2.0
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1.1270 micron
0.7690 micron
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Air mass
WHY SUNSETS ARE RED Curves of atmospheric transmission at
selected wavelengths from Abbot's Mt. Wilson 1906-1907 data. Through
the course of a day, the Sun's light passes through varying thicknesses
of atmosphere, or air masses (1 air mass = the zenith), which affect
individual wavelengths differently. As shown by its relatively level line, red
light penetrates dense air more easily than blue light does, which is why
sunsets appear red. At left is the extrapolation of these curves to zero air
mass, outside Earth's atmosphere.
Meanwhile, Abbot's quest continues. NASA's Total and
Spectral Solar Irradiance Sensor - 2 is scheduled to launch
in 2024. Its sensitive instruments will begin adding to the
many decades of continuous measurements of solar irradiance
that began with the dedicated efforts of a curious and
talented farm boy.
¢ DOUGLAS MACDOUGAL is the author of Newton's Gravity:
An Introductory Guide to the Mechanics of the Universe
(Springer, 2012). You can visit his website and blog at
www.douglasmacdougal.com.
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POWERED BY THE SUN Abbot points at one of his solar-powered
boilers in this 1936 photo. This was just one of the many solar devices
he experimented with throughout his life. Steam generated by this device
could power an engine.
A LENGTHY CAREER Abbot is pictured here in the Smithsonian
Institution Building in 1930. He remained active throughout his life and
passed away in December 1973 at the age of 101. A 10-km-diameter
lunar crater south of Mare Crisium is named in his honor.
sk yand tele scope .o r g * JULY 2022 33
0.5525 micron
0.4762 micron
0.4494 micron
0.4091 micron
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Sky & Telescope - July 2022

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