Where was the Seneca Observatory?

1. 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. The birthplace of planetary radio astronomy:The Seneca, Maryland observatory 50 yearsafter Burke and Franklin's Jupiter radio emission discovery.Leonard N. Garcia1, James R. Thieman2, Chuck A. Higgins31QSS Group Inc., NASA/GSFC Code 630, Greenbelt, MD 20771, 2NASA/GSFC,3Middle Tennessee State University, Murfreesboro, TN 37132. Abstract 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 Jove (http://radiojove.gsfc.nasa.gov), 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 anniversary. The Discovery and Announcement 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. 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 Franklin’s 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 right-left in the foreground assisted us later in identifying the former location of the array. Courtesy of the Archives of the Carnegie Institution of Washington. 2047 feet long, 66 dipoles 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 Island. “The discovery by Burke and Franklin (1955) of radio emissions from Jupiter has opened a new field of radio astronomy” - John D. Kraus, AJ, 61, 182, 1956. Leonard Garcia (Leonard.Garcia@gsfc.nasa.gov) with a copy of the map overlaid on an aerial photo of the site. Jim Thieman (left) (James.R.Thieman@nasa.gov) and Chuck Higgins (right) (Higgins@physics.mtsu.edu) looking for artifacts at the McKee- Beshers Wildlife Management Area Bill Pine, Leonard Garcia, Max Kepler,and Jay Friedlander exploring the discovery site. The Site Visit August, 2004 Press Coverage The Washington Post and Times Herald April 7, 1955 The Washington Star April 7, 1955 The New York Times April 6, 1955 The New York Times April 10, 1955 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. Burke, B. F. and Franklin, K.L., Observations of a variable radio source associated with the planet Jupiter, JGR, 60, 213-217, 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. Acknowledgements and References The Historic Marker In recognition of the 50th anniversary of the discovery of Jupiter’s radio emissions, the Maryland Historical Trust will erect a roadside historic marker along River Road near the former Seneca Observatory. The marker will read: PLANETARY RADIO EMISSIONS DISCOVERY SITE IN 1955 SCIENTISTS BERNARD BURKE AND KENNETH FRANKLIN FROM THE CARNEGIE INSTITUTION OF WASHINGTON ACCIDENTALLY DISCOVERED NATURALLY-GENERATED 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. 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 years ago Our estimate for the Mills Cross array location is indicated. (Lat 39.078064,Long 77.393771) Courtesy Terraserver web site.