skyandtelescope.com • NOVEMBER 2019 35
his troops as “Old Stars” because of his fame as an astrono-
mer — with protecting the Union army’s left fl ank and cap-
turing the railroad that served as a Confederate supply line
from the east. Mitchel undertook a rapid forced march and
captured Huntsville, Alabama, with its large depot and repair
station for the Memphis and Charleston Railroad line.
But most famous was Mitchel’s role in enabling a dar-
ing raid intended to split the Confederacy and change the
course of the war. En route to Huntsville, a spy named J. J.
Andrews approached him with the idea of disabling a major
Confederate railroad that connected Marietta, Georgia,
with Chattanooga, Tennessee. Andrews’s plan was to steal a
train, burn the bridges behind him, and, just south of Chat-
tanooga, turn the train west and join Mitchel in Huntsville.
Mitchel supplied specially chosen troops. The Andrews Raid-
ers made it to Marietta, stole a train and got away, but were
unable to burn any of the bridges because they were hotly
pursued for 87 miles in the legendary “Great Locomotive
Chase,” immortalized in Buster Keaton’s 1927 fi lm The Gen-
eral and Walt Disney’s 1956 suspense fi lm The Great Locomo-
tive Chase. The raiders were caught, tried, and some were
hung, but after the war some of the survivors were awarded
the fi rst Medals of Honor.
In September 1862, Mitchel, by then promoted to Major
General, was reassigned to the Department of the South, in
Hilton Head, South Carolina, where he founded Mitchelville,
the fi rst town deliberately designed for African-Americans
transitioning from slavery. The next month, on October 30th,
he died from yellow fever at the age of 53.Mitchel as an Astronomer
In 2003, the late historian Donald E. Osterbrock described
Mitchel as “America’s First Carl Sagan” (although more
accurately, Sagan should have been called “America’s Second
O. M. Mitchel”). Like Sagan, Mitchel
was a gifted speaker whose astronom-
ical lectures electrifi ed a generation.
Like Sagan, Mitchel believed in the
possibility of life on other worlds. But
also like Sagan, Mitchel was a serious
research astronomer.
In June 1846, Mitchel found
that fi rst-magnitude Alpha Scorpii(Antares) had a faint, 5.5-magnitude companion star. In
October 1846, he became one of the fi rst American astrono-
mers to observe the planet Neptune after its discovery had
been announced. In October 1848, Mitchel participated in a
pioneering experiment using the telegraph to determine the
longitude of Cincinnati west of Philadelphia; for the experi-
ment, Mitchel invented an electrochronograph, a mechanism
for simultaneously recording clock signals along with the
timings of observations; his design was adopted by several
observatories.
Less well known are Mitchel’s systematic micrometri-
cal measurements of double stars at declinations below the
celestial equator, performed at the direct request of the great
Russian astronomer Wilhelm Struve, founding director of
Pulkovo, who wanted to take advantage of both the Cincin-
nati telescope’s large aperture and its more southerly latitude.
Also important were Mitchel’s thousands of transit-telescope
observations of stellar positions for the U.S. Coast Survey. In
addition, The Sidereal Messenger was widely read by Ameri-
can astronomers, in part because Mitchel published his own
translations of French and German articles from foreign
journals, and reprinted correspondence surrounding the
discovery of Neptune.
Mitchel was recognized with honorary degrees and other
accolades. Among others, in 1846, he was offered the Rum-
ford Professorship at Harvard University, which he declined
because he didn’t want to relocate to Cambridge. In 1850,
he was one of the fi rst fi ve Americans elected to be a Foreign
Associate of the Royal Astronomical Society, and was elected
a member of the American Philosophical Society in 1853.
But Mitchel’s most enduring legacy is probably his descrip-
tion of our Milky Way and other galaxies as “island uni-
verses.” Although that marvelous image has been ascribed to
several 19th-century scientists, Mitchel was gifted in coin-
ing memorable turns of phrase. He
used the phrase “island universe” in
lectures and The Sidereal Messenger
numerous times as early as 1847
when describing research by Johann
Heinrich von Mädler, which Mitchel
translated from German.¢ Contributing Editor TRUDY E. BELL,
formerly an editor for Scientifi c Ameri-
can and IEEE Spectrum and senior
writer for the University of California
High-Performance AstroComputing
Center, is author of a dozen books and
over 500 articles. Her journalism prizes
include the 2006 David N. Schramm
Award of the American Astronomical
Society. In 2017, asteroid (323552)
was named Trudybell 2004 TB in
recognition of her career. She can be
reached at [email protected].uPERFECT TIMING In the revolving-disk
electrochronograph Mitchel invented in late
1848, a make-circuit clock marked every
other second with a dot on a 22-inch paper
disk. At the end of every revolution, the
paper shifted and a new concentric circle
of dots began. The moment an astronomer
observed a star to be bisected by the vertical
wire in the eyepiece of a meridian telescope,
he pressed a telegraph key to make an addi-
tional mark on the revolving disk. In this way,
the time of the meridian transit was recorded
AN in relation to the dots that marked the time.
NA
LS
OF
TH
E^ D
UD
LE
Y^ O
BS
ER
VA
TO
RY
(^1
86
6 )^
/^ P
UB
LIC
DO
MA
IN^
/^ C
OU
RT
ES
Y^ O
F^ A
UT
HO
R