34 ASTRONOMY • MAY 2020was slow in coming. Some thought they
were within the Milky Way, embryonic
planetary systems in their early stages of
formation.
In 1909, Slipher began recording spec-
tra of spiral nebulae, urged on by Lowell,
who thought they might show spectral
similarities to our solar system. This task
was difficult, however, because these
objects were faint. Nonetheless, Slipherconsulted with astronomers at other
observatories and experimented with
equipment, including faster lenses and
observing techniques that might mini-
mize the difficulty.
In the fall of 1912, Slipher recorded a
plate of the “Andromeda Nebula” that he
felt was sufficiently good to obtain its
radial velocity. No radial velocities of
nebulae were known at that time. He
recorded better plates in November and
December 1912, and still a better resulton the nights of December 29, 30, and 31,
and into the predawn hours of New
Year’s Day 1913. He measured the plates
over the first half of January, finding that
the nebula was moving three times faster
than any previously known object in the
universe.
Confusion ensued, and Slipher spent
more time measuring the plates. On
February 3, 1913, he wrote to Lowell that
the Andromeda Nebula was approaching
Earth at the unheard-of velocity of
186 miles per second (300 kilometers persecond), still an accurate value. “It looks
as if you had made a great discovery,”
wrote Lowell. “Try some other spiral
nebulae for confirmation.”
Slipher next went after what we now
call the Sombrero Galaxy (M104) in
Virgo. He found its spectral lines shifted
far toward the red, indicating that it is
receding from Earth at 620 miles per sec-
ond (1,000 km per second). By the 1914
American Astronomical Society meeting
in Evanston, Illinois, Slipher was able toannounce results for 15 spirals. Nearly all
were receding at high velocities. Three
years later, Dutch astronomer Willem de
Sitter theorized that the universe is
expanding. It was Slipher’s observations
of the so-called spiral nebulae that estab-
lished this fact.Slipher’s varied work
In addition to discovering that the spiral
nebulae were receding at great velocities,
Slipher found that radial motions existed
within the spiral nebulae themselves.
That is, they were rotating. These first
discoveries again included what we now
know as the Andromeda and Sombrero
galaxies. This finding contradicted what
astronomer Adriaan van Maanen of
Mount Wilson Observatory had reported
earlier, that the spiral arms of these
objects were unwinding. This would
suggest they were close and not at great
distances, or such a detection would be
impossible.
As with many aspects of astronomy,
debates ensued, and most astronomers
took the side of van Maanen rather than
Slipher. Another decade passed before
the realization came that Slipher was
right and van Maanen wrong. By the end
of World War I, other astronomers rein-
forced Slipher’s work on spiral nebulae
with their own observations, and the tide
of belief began to turn. And then in 1923
came Hubble’s discovery of the nature of
galaxies. By 1929, Hubble derived his
crucial velocity-distance relationship forgalaxies, using, as Hubble wrote Slipher,
“your velocities and my distances.”
Slipher and Hubble had together uncov-
ered the expanding universe, the nature
of galaxies, and a way to measure extra-
galactic distances.
Slipher also conducted critical work
on aurorae and on the phenomenon
known as sky glow, the brightness of the
night sky. He turned to these areas in
part because he had mostly exhausted
the research capability of the 24-inchV.M. Slipher (bottom) accompanies Carl Lampland
at the business end of the 24-inch Clark refractor, in
an undated image.Although Slipher was a cautious thinker, he adopted some of the more aggressive and
controversial ideas of his employer, Percival Lowell, throughout the early part of his career.