Sky & Telescope - August 2020 - 14

The Long View

infrared astronomy, but challenges posed by Earth's atmosphere limited observations. The sounding rockets that
would give us our ﬁrst glimpses of the gamma-ray, X-ray, and
ultraviolet universe were still more than two decades away.
And the longest wavelengths we know of, radio waves, were
about to become a surprising and mesmerizing window onto
the cosmos - one that has gone from revealing our Sun's
magnetic nature to tracing super-fast streams of matter gushing out of distant galaxies and the buzz that echoes back to us
from the Big Bang.

The First Signals

S REBER'S BACKYARD SCULPTURE Grote Reber stands at Green
Bank in the 1960s after reassembling his radio telescope there. Reber
originally built the instrument in 1937.

1933 paper, coincided with the center of the Milky Way. We
now know the radio signals that Jansky captured originate
from a 4-million-solar-mass black hole called Sagittarius A*.
But since astronomers at the time didn't know black holes
existed - or, for that matter, that celestial objects emit radio
waves - scientists did not grasp the full signiﬁcance of Jansky's ﬁnding.
Enter Grote Reber, American pioneer of radio astronomy.
Reber, an electrical engineer by training, was an amateur
radio operator. When he heard of Jansky's discovery, he
designed and built an instrument in his own backyard - a

T EMISSION FROM THE MILKY WAY Left: Reber gathered these radio maps' data in 1943, observing the sky at 160 MHz. The Milky Way's disk
lies along a line intersecting 0 on the y-axis, and the galactic center is in the contour marked 11.2 on the far right. (The maps are plotted in an older
version of galactic coordinates, so you may notice the x-axis doesn't quite match today's coordinates.) Contour lines are in terms of 10-22 watts per
square centimeter, circular degree, megahertz. Right: This 2018 image, made with the MeerKAT radio telescope in South Africa, is one of our clearest
images of the galactic center. The image spans about 2° by 1° - only a small segment of Reber's map.

AUGUST 2 02 0 * SK Y & TELESCOPE

REBER WITH HIS SCOPE: NR AO / AUI / NSF;
CON TOURS: G. REBER / S&T 1949; M ODER N VIE W OF
G A L ACTIC CEN TER: SA R AO / MEERK AT

It all began thanks to the work of a young physicist and engineer by the name of Karl Jansky. Employed by Bell Telephone
Laboratories in the late 1920s and early 1930s to work in the
nascent ﬁeld of radio communications, Jansky's job was to
investigate sporadic static that could interfere with transmissions. In his quest to discover the origin of this nuisance,
he built a telescope - basically a gloriﬁed antenna of brass
piping and timber, some 30 meters across and 6 meters high
(about 100 feet by 20 feet). He mounted this contraption on
the wheels of a Model T Ford, which allowed it to rotate in
any direction and inevitably earned it the nickname "Jansky's
merry-go-round."
Jansky distinguished three kinds of interference, readily
dismissing two that originated in thunderstorms. But the
third kind was a steady hiss that periodically reached maximum intensity every 23 hours and 56 minutes (you might
recognize this as Earth's sidereal period). Jansky initially
attributed this hiss to the Sun, but when its intensity failed to
decrease during the solar eclipse of August 31, 1932, he had
to rustle up another explanation. He found that the signal
was strongest in the direction of the galactic plane, speciﬁcally at a right ascension of 18h, somewhere in Sagittarius.
This location, Jansky noted with cautious excitement in a

Sky & Telescope - August 2020

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