The birthplace of planetary radio astronomy: The Seneca ...
The Mills Cross Array
The array was based on the design of Mills
and Little and used phase-switch receivers
based on the design of Ryle. The array
consisted of two linear arrays of 66 dipoles
spanning 2047 feet arranged to form a
slightly flattened X. The array operated at 22
MHz. Half-power beam width was 1.6 x 2.4
degrees. The addition of extra cables allowed
the beam to be moved in declination.
A view along one end of the array. The receivers were
housed in an army surplus truck visible in the distance.
Undated photo about 1954. Courtesy of the Archives of
the Carnegie Institution of Washington.
6 di p
The birthplace of planetary radio astronomy:
The Seneca, Maryland observatory 50 years
after Burke and Franklin's Jupiter radio emission discovery.
Leonard N. Garcia1, James R. Thieman2, Chuck A. Higgins3
QSS Group Inc., NASA/GSFC Code 630, Greenbelt, MD 20771, 2NASA/GSFC,
Middle Tennessee State University, Murfreesboro, TN 37132.
The Department of Terrestrial Magnetism of the Carnegie
Institution of Washington (DTM/CIW) initiated a 22 MHz sky
survey using the Mills Cross array. Acting as a transit instrument,
the array swept through a relatively narrow range of declinations
over several weeks and was then shifted by about 1 degree in
declination by the addition of phasing cables. In the first 3
months of 1955, DTM scientists Bernard Burke and Kenneth
Franklin were testing the array and mapping the northern sky,
progressively moving the beam to more southerly declinations.
During their survey they detected intermittent bursts of
interference obscuring an unknown extended source in 9 out of
31 records. In Franklins account of the discovery, they originally
attributed this interference to a farmhand driving home from a
date late at night [Franklin, 1959]. The extended radio source at
RA=7h 30m and Dec.=+22 degrees was being studied as part of
a presentation for the 92nd AAS meeting in April 1955. Initially,
data with the obscuring interference was not included in their
study. When these data were included they recognized that the
interference occurred at almost the same sidereal time. Further
analysis showed that over several months this interference
drifted slightly in sidereal time. In March 1955, they traced the
RA of the antenna beam when the interference was received
and compared it against the RA of several celestial objects. Only
Jupiter matched both the RA of the beam position and its drift.
Had the data containing the interference not been included
during the search for the unknown extended source they might
never have made this discovery. Had they been mapping
towards more northerly declinations, Jupiter would have quickly
drifted out of the beam. As it was they were inadvertently
tracking Jupiter. A search of earlier data using other antennas at
the Seneca site found that Jupiter had been detected but not
recognized in 1954 during observations of an occultation of the
Crab Nebula by the solar corona. Confirmation of their discovery
came from Australian C.A. Shain of the CSIRO who went back
through 18 MHz observations made in 1950 and 1951 and found
several instances of Jupiter data not recognized at the time.
An aerial view facing West of the
Mills Cross and surroundings. The
location of the array is indicated.
The Y-shaped line of trees below
center and the trail running rightleft in the foreground assisted us
later in identifying the former
location of the array.
Courtesy of the Archives of the
Carnegie Institution of
Where was the Seneca Observatory?
This map with handwritten notes marks the
location of antennas on the farm leased from
Mr. Brockett Muir.
The notes were made by Merle Tuve, director of
the Department of Terrestrial Magnetism.
(Dated 1956 and courtesy of the Archives of the Carnegie
Institution of Washington.)
Comparison with current maps show
that this property is now part of the
McKee-Beshers Wildlife Management
Area, about 20 miles northwest of
Washington D.C., south of River Road
along the northern shore of the
Potomac River and north of Tenfoot
The discovery by Burke and Franklin (1955) of radio
emissions from Jupiter has opened a new field of
- John D. Kraus, AJ, 61, 182, 1956.
The Washington Post and Times Herald April 7, 1955
In recognition of the
50th anniversary of the
discovery of Jupiters
radio emissions, the
Trust will erect a roadside
historic marker along River
Road near the former
Seneca Observatory. The
marker will read:
PLANETARY RADIO EMISSIONS
Our estimate for the Mills Cross
array location is indicated.
(Lat 39.078064,Long 77.393771)
Courtesy Terraserver web site.
IN 1955 SCIENTISTS BERNARD BURKE
AND KENNETH FRANKLIN FROM THE
CARNEGIE INSTITUTION OF
RADIO WAVES FROM JUPITER USING A
96-ACRE ANTENNA ARRAY. THIS
DISCOVERY LED TO GREATER
UNDERSTANDING OF PLANETARY
MAGNETIC FIELDS AND PLASMAS AND
OPENED A NEW WINDOW IN OUR
EXPLORATION OF THE SOLAR SYSTEM.
We gratefully acknowledge
the assistance of Shaun
Hardy, archivist at the
Carnegie Institution of
Washington. We also
acknowledge Jim Gass, Jay
Friedlander and Steve
Kortenkamp for their help in
the archival research.
looking for artifacts
at the McKeeBeshers Wildlife
Burke, B. F. and Franklin, K.L.,
Observations of a variable radio
source associated with the
planet Jupiter, JGR, 60, 213217, 1955.
Franklin, K.L. An account of the
discovery of Jupiter as a radio
source, AJ , 64, 37-39, 1959.
Franklin, K.L., The discovery of
Jupiter bursts, in Serendipitous
discoveries in radio astronomy,
editors Kellermann and Sheets,
pp. 252-257, 1983.
Burke and Franklin's discovery of radio
emissions from Jupiter in 1955 effectively
marked the birth of the field of planetary
radio astronomy. The discovery was made
near Seneca, Maryland using the
Department of Terrestrial
Magnetism/Carnegie Institution of
Washington's (DTM/CIW) Mills Cross
Array. Fifty years later little evidence of
this 96-acre X-shaped array of dipoles
remains. The site, now known as the
McKee-Beshers Wildlife Management
Area, is owned by the State of Maryland
Department of Natural Resources. Radio
a NASA/GSFC education and public
outreach project, will recognize the 50th
anniversary of this discovery through an
historic reenactment using the Radio Jove
receiver and dual-dipole array system. We
describe some results of our search
through the DTM/CIW archives, our visit to
the site to look for evidence of this array,
and other efforts at commemorating this
The Site Visit August, 2004
The New York Times April 10, 1955
The Historic Marker
Comparison of the map with
current and past aerial photos
(note the trail and Y-shaped line of
trees ) provided clues on where
the Mills Cross array had been 50
The Discovery and Announcement
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