The Wow! Signal (1977): The 72-Second Narrowband Detection at 1420 MHz.

What the Wow! Signal was, in a paragraph.

The Ohio State University Radio Observatory's "Big Ear" was a flat-aperture meridian-transit radio telescope of unusual flat-reflector and tilted-flat-reflector design (functionally about 26 meters effective collecting area in the relevant configuration), located at the Perkins Observatory in Delaware County, Ohio. From 1973 through 1995 it ran a continuous all-sky SETI survey under principal investigator John D. Kraus, scanning the band around the 21-centimeter neutral-hydrogen rest frequency at 1420.4056 MHz — the band universally proposed in the SETI literature since the Cocconi-Morrison 1959 Nature paper as the "natural meeting place" frequency for interstellar communication. The Big Ear used a fixed primary structure and the Earth's rotation as its scanning mechanism; each point on the celestial sphere at the observatory's latitude passed through the beam in approximately 72 seconds. The telescope's two horn antennas, mounted side-by-side in the focal arrangement, each sampled an adjacent declination strip; the two beams were separated by approximately three minutes of right ascension. On the evening of August 15, 1977, the telescope's IBM 1130 computer recorded its standard observational run on continuous printout in a 50-channel frequency-bin format covering the band around 1420 MHz. The system's output assigned a single character to each channel's signal-to-noise ratio in each integration period: 1–9 for sigma values 1.0–9.x, A–Z for higher values (A = 10.0–10.x, B = 11.0–11.x, etc.), and a blank for less than 1 sigma. The telescope's normal output was a sea of single-digit numbers and blanks (cosmic noise) with occasional 2s and 3s. The printout from the night of August 15–16, 1977 was reviewed three days later (August 18) by Jerry Ehman, a graduate-school-trained astronomer who served as the survey's project volunteer. In one of the printout's frequency-bin columns, in a single contiguous run of six integration periods, the channel read "6EQUJ5" — corresponding to a signal whose intensity rose from 6 sigma to 14 sigma (E), 26 sigma (Q), 30 sigma (U), 20 sigma (J), then decayed to 5 sigma over approximately 72 seconds. Twelve seconds is the integration period, so the run from 6 to U to 5 brackets a roughly 72-second event — exactly consistent with the time a fixed celestial source would take to drift through the Big Ear's beam at that declination. The signal was in a single channel (the bandwidth less than 10 kHz, characteristic of an artificial transmitter or an extremely narrow natural emission, but not the broadband emission of typical astrophysical sources); was centered at 1420.4556 MHz, only 0.05 MHz off the rest-frequency hydrogen line and within a heavily protected radio-astronomy band; and was within a region of sky containing no known transmitter, no known radio source, and no known star within 250 light-years (per the era's catalogs). Ehman circled the "6EQUJ5" column on the printout and wrote "Wow!" in red ink in the margin. The Big Ear telescope was, due to its meridian-transit design, unable to re-observe the position later the same night. Subsequent re-observations by the Big Ear and follow-up observations by the Very Large Array, the Allen Telescope Array, the Green Bank Telescope, the Murchison Widefield Array, and most recently the FAST telescope — more than 100 follow-up observations between 1977 and 2026 — have detected nothing at the position. The Big Ear telescope was demolished in 1998. The Wow! Signal remains, as of 2026, the strongest unexplained narrowband candidate-signal in the history of radio SETI.

The documented record.

The Big Ear and the SETI survey

The Big Ear radio telescope was constructed at Ohio State between 1956 and 1963 by a team under John Kraus. Verified It used a unique "Kraus-type" flat-aperture design: a fixed paraboloidal reflector (about 110 meters wide) at the southern end of the structure, a tiltable flat reflector (about 100 meters wide) at the northern end, and a focal-point pickup mounted on a ground reflector between them. The tilted northern reflector could be moved in declination to direct different strips of sky onto the fixed paraboloid; right-ascension scanning was accomplished by allowing the Earth's rotation to bring different parts of the celestial sphere into the beam. The telescope completed early observational work on the cosmic microwave background, mapping galactic hydrogen, and other radio-astronomical projects, and from May 1973 began the Ohio SETI program — one of the longest-running radio SETI projects in history [1].

The SETI program's instrumental configuration in August 1977 used a 50-channel filterbank covering approximately 500 kHz of bandwidth centered near 1420.4056 MHz (the neutral hydrogen rest frequency). Verified Each channel was 10 kHz wide. The IBM 1130 computer integrated each channel's voltage for approximately 12 seconds (the integration period was later refined to 10 seconds for the routine observing of 1977–1978, but the August 15 run is consistent with the slightly-longer-period configuration). Standard output was a continuous-feed paper printout listing, for each channel and each integration interval, a single character representing the signal-to-noise excess. Each celestial point at the observatory's latitude transited the telescope's beam pattern in approximately 72 seconds, producing — for a continuously-broadcasting source — a characteristic rise and fall in successive integration periods [2].

The detection

The August 15–16, 1977 observational run produced approximately 200 feet of printout. Verified Project volunteer Jerry Ehman, then 40 years old and working under Kraus on a part-time basis, reviewed the printout three days later in his Columbus home. Among ordinary cosmic-noise readings (1s, 2s, 3s, occasional blanks), one channel produced a sequence reading "6EQUJ5" over six adjacent integration intervals. The signal characteristics:

• Channel/frequency: Channel 2 of the 50-channel filterbank, corresponding to a center frequency of approximately 1420.4556 MHz — 0.0500 MHz off the rest-frequency neutral hydrogen line.
• Bandwidth: Less than 10 kHz (the signal was concentrated in a single 10-kHz channel and did not "spill over" into adjacent channels).
• Duration: Approximately 72 seconds (six consecutive 12-second integration intervals showed elevated signal).
• Strength profile: 6 sigma rising to 14, 26, 30, 20, and 5 sigma — a smooth rise and fall consistent with the beam pattern of a fixed celestial source drifting through the antenna pattern.
• Peak strength: 30 standard deviations above the local noise floor, corresponding (using the system temperature parameters of the Big Ear at the time) to a signal flux density of approximately 54 jansky.
• Beam: Detected in one of the two horn antennas only; the second horn (sampling an adjacent strip of sky) did not detect the signal. The Big Ear's design did not allow determination of which of the two horns had been the source-pointing one, producing a permanent two-source-position ambiguity.

Ehman drew a red-pen circle around the column reading "6EQUJ5" on the printout and wrote, in the margin, the single word "Wow!" The printout is preserved at the Ohio Historical Society. Verified Ehman has described his own reaction in subsequent interviews as "amazed at how closely it resembled an expected signal from an extraterrestrial source" [3][4].

The source position

The two-horn ambiguity of the Big Ear's beam pattern means the Wow! Signal source could have been at either of two positions: Verified

• Right ascension 19h22m22s ±5s, Declination −26°57' ±20' (the "positive horn" position), or
• Right ascension 19h25m17s ±5s, Declination −26°57' ±20' (the "negative horn" position).

The position quoted in the popular literature (RA 19h25m31s, Dec −26°57') corresponds to the negative-horn position; the positive-horn position is approximately 3 minutes of right ascension to the west. Either way, the source location is in the southern constellation Sagittarius, near the globular cluster M55 (Messier 55), and in a portion of the sky containing no known star, transmitter, or radio source within the era's catalogs. Subsequent searches have catalogued faint sun-like stars in the vicinity (including 2MASS 19281982-2640123 by Caballero, 2017) but no stars at the position with detectable planetary systems or known transmitting properties [3][5].

The frequency: the "magic" 1420 MHz

The center frequency of the Wow! Signal — 1420.4556 MHz, within the protected radio-astronomy band — is the most discussed feature of the detection. Verified The rest frequency of the 21-centimeter spin-flip transition of neutral hydrogen is 1420.4057 MHz. This frequency has been proposed since the 1959 Cocconi-Morrison Nature paper as the "Schelling-point" frequency at which an interstellar civilization wishing to be heard would most naturally choose to broadcast, because it is conspicuous, well-known to anyone capable of radio astronomy, and located in a band universally protected from terrestrial transmission by international treaty. The Wow! Signal's offset from the rest frequency is approximately 0.05 MHz, corresponding to a radial-velocity Doppler shift of approximately 10 km/s — well within the range expected for a source within the local stellar neighborhood [6].

The Wow! Signal's frequency placement is, taken alone, suggestive: a narrowband signal of this width and strength at this frequency and from this direction is exactly what the SETI program was designed to detect. The placement is also, of course, what would naturally fall within the band the Big Ear was searching, since the Big Ear could not have detected a signal outside that band. The frequency suggestiveness is therefore selection-effect-significant: it is one of the strongest features of the signal that argue for an artificial origin, but it is not on its own sufficient.

The follow-up observations

Ehman, Kraus, and the Big Ear team began re-observing the Wow! Signal position within days of identifying the detection. Verified The telescope re-scanned the position approximately 100 times between August 1977 and the late 1990s. No further signal was detected. Independent follow-up observations have been carried out by:

• The Very Large Array (VLA), 1995 and 1996 — null results.
• Robert Gray (using the META II array at Harvard's Oak Ridge Observatory), 1987 — null result.
• Gray (using the Very Large Array), 1995 and 1996 — null results.
• Gray and Marvel (using the Mt. Pleasant Observatory in Tasmania), 1995 — null result.
• The Allen Telescope Array, 2007–onward — null results.
• The Green Bank Telescope, multiple campaigns, 2010–2020 — null results.
• The FAST (Five-hundred-meter Aperture Spherical Telescope) in China, 2023 announcement of new candidate signals subsequently determined to be human radio-frequency interference.

Total observation time on the Wow! Signal position has accumulated, across all instruments, to several hundred hours. Verified No recurrence of a signal matching the original Wow! Signal characteristics has been confirmed [3][7].

The Big Ear's demolition

The Big Ear radio telescope was decommissioned in 1997 to make way for an Ohio State University golf course expansion and was demolished in 1998. Verified The original Wow! Signal printout was donated by Jerry Ehman to the Ohio Historical Society (now Ohio History Connection), where it remains as the principal physical artifact of the detection [8].

The proposed explanations.

An artificial extraterrestrial transmission

The hypothesis that the Wow! Signal was a deliberately-emitted narrowband transmission from an extraterrestrial civilization is the hypothesis the detection was designed to test. Claimed The signal's characteristics match the expected profile of such a transmission: narrowband (the band is too narrow for any known natural emission and is consistent with an artificial source), centered on the protected 1420 MHz band (the proposed SETI rendezvous frequency), at high signal-to-noise (30 sigma above background, suggesting a strong source), and from a "blank" sky position not associated with any known natural radio emitter. The principal problem with the artificial-extraterrestrial hypothesis is the non-recurrence: a beacon designed to be detected by other civilizations would be expected to broadcast more than once, and a one-off transient of this character is hard to reconcile with any sustained intentional emission. Various accounts have proposed (a) a rotating beam that swept across Earth only briefly; (b) a one-time transmission tied to a specific astrophysical event; or (c) a leakage signal (radar, communications) that briefly intercepted Earth. None of these is satisfactory in detail [4][6].

The Paris cometary hypothesis

In 2017, Antonio Paris of the Center for Planetary Science, a community college in Florida, proposed that the Wow! Signal was the hydrogen-line emission from a cloud of cometary hydrogen surrounding comet 266P/Christensen, which was claimed to have been in the relevant sky position on August 15, 1977. Disputed The Paris proposal received significant media coverage. Within months, multiple radio-astronomical critics — including Robert Dixon (one of the original Big Ear SETI investigators), Yvette Cendes (then at Leiden), and several members of the radio astronomy community — rejected the hypothesis on several grounds: (1) comets do not produce hydrogen-line emission at the intensity required; (2) cometary hydrogen emissions are broadband, not the narrowband detected at less than 10 kHz; (3) the trajectory and 1977 position of 266P/Christensen would not have placed it within the Big Ear's beam at the moment of detection (the actual position of the comet in August 1977 was disputed by Paris and his critics); and (4) the Big Ear had been observing the same general region in other observing runs without detecting cometary emission. By 2018, the Paris cometary hypothesis was no longer considered viable by the radio-astronomical community, though it survives in some popular accounts [7][9].

Local radio-frequency interference

A general "explanation" sometimes offered is that the Wow! Signal was a chance reflection of a terrestrial radio transmission from a satellite, an aircraft, or a ground source. Disputed This explanation faces several specific problems: the 1420 MHz band is internationally protected from terrestrial transmission, military and civilian transmitters are not authorized in the band, and the signal's exact frequency placement and bandwidth are inconsistent with the most common forms of RFI. Spurious reflections from low-Earth-orbit objects could in principle produce a transient signal at the right frequency, but the duration and beam-pattern profile of the Wow! Signal are not consistent with the expected profile of such a reflection (which would typically show a rapid rise and fall, not the smooth 72-second profile of a celestial transit) [4].

A natural astrophysical transient

A range of natural astrophysical sources can in principle produce narrowband signals at 1420 MHz, including hydrogen-line emission from interstellar clouds, OH masers (at nearby frequencies), and various transient phenomena. Claimed The principal problems with the natural-astrophysical explanation are (a) the narrow bandwidth (most natural emissions are broader); (b) the precise frequency placement near the hydrogen rest frequency without the broad line-profile expected of a natural source; and (c) the absence of any identified celestial source at the position. A magnetar-like or pulsar-like transient has been considered, but the timing profile (smooth 72-second rise-and-fall) is not consistent with most pulsar-class transients [3].

The "we don't know" position

The current scientific consensus, expressed by the original investigators (Ehman, Kraus, Dixon) and by subsequent commentators across SETI literature, is that the Wow! Signal is unexplained. Claimed The signal's characteristics are not consistent with any well-understood natural source, are consistent with an artificial transmission, and yet have never been replicated. The unanswered status is the consensus position: the signal is either an extraordinary natural astrophysical transient that has not recurred in detectable form, a one-time artificial transmission whose source has not transmitted again or has moved, or a measurement artifact whose specific cause has not been identified [3][6][10].

The unanswered questions.

Recurrence

No follow-up observation, in 49 years and across more than 100 separate campaigns at the position, has confirmed a signal matching the original Wow! Signal's characteristics. Unverified The cumulative observation time on the position now exceeds the original 72-second detection by many orders of magnitude; the absence of recurrence is one of the most significant facts of the case. Whether the source has ceased transmitting, has moved, has rotated into a different orientation, or never existed in the form the original detection implies, cannot be determined from the available evidence.

The horn ambiguity

The Big Ear's two-horn design did not record which of its two adjacent beams had been the source-pointing one. Disputed This produces a permanent 3-minute right-ascension ambiguity in the source position. Subsequent search campaigns have searched both ambiguity positions; neither has produced a recurrence. The two candidate stars catalogued by Caballero (2017) and others are at one position; the other position is in even more empty sky.

The frequency drift

A SETI signal from a distant source would be expected to display a frequency drift over time due to the relative acceleration of the source and the observer (chiefly Earth's rotation and orbital motion). Disputed The 50-channel filterbank of the Big Ear was not sensitive to fine frequency drift within a single channel; the recorded signal therefore cannot be assessed for the presence or absence of drift. Whether the actual signal drifted across the channel (the natural expectation for a distant source) or was held fixed (the expectation for a terrestrial or near-Earth source) is not retrievable from the recorded data.

The Wow! Signal in the broader SETI context

In the 49 years since 1977, SETI has accumulated a substantial body of additional candidate signals, including the BL Lyrae candidate (2019, subsequently traced to a Parkes-area transmitter), the BLC-1 signal from Proxima Centauri (2019, subsequently traced to terrestrial RFI), and various FAST candidates (2023, all subsequently traced to RFI). Disputed None of these has matched the Wow! Signal's combination of strength, frequency placement, narrowband character, and source-position characteristics. The Wow! Signal remains, in the SETI literature, the strongest candidate signal that has not been resolved as an instrumental, terrestrial, or natural-astrophysical false positive [10].

The Paris hypothesis question

Paris's 2017 cometary explanation has been rejected by the radio-astronomical community on technical grounds. Disputed The persistence of the cometary explanation in some popular accounts is, like the Conan Doyle effect on the Mary Celeste case (see File 032), an example of a published-but-rejected explanation surviving in non-technical discourse. The technical position of the Wow! Signal as of 2026 is that no specific natural explanation accounts for the detected signal's characteristics.

Primary material.

• The original Wow! Signal printout, August 15–16, 1977. Donated by Jerry Ehman to the Ohio Historical Society (Ohio History Connection), Columbus, OH. The physical artifact is the principal record; high-resolution photographic reproductions have been published widely.
• The Ohio State University Radio Observatory archive, including the IBM 1130 system documentation, the 50-channel filterbank calibration data, the Kraus survey logs, and Ehman's own observational notebooks.
• The North American Astrophysical Observatory (NAAPO), an organization that continues the Big Ear legacy, hosts an online archive of Wow! Signal materials including Ehman's "Wow! Signal: A response to Alien Signal claims" technical paper and high-resolution scans of the printout.
• The SETI Institute (seti.org) hosts review materials on SETI candidate signals, including Wow! follow-up campaign descriptions.
• The National Radio Astronomy Observatory (NRAO) holds VLA observation logs from the 1995 and 1996 Gray follow-up campaigns.
• The Allen Telescope Array (SETI Institute / Hat Creek Observatory) holds its long-term Wow! position monitoring records.

Key individual documents include: the original printout itself; Ehman's 1998 retrospective paper "The Big Ear Wow! Signal: What We Know and Don't Know"; Robert Gray's 2001 book The Elusive Wow; the Antonio Paris 2017 paper proposing the cometary explanation; the 2017 and 2018 critical responses to the Paris paper.

The sequence.

1. 1959 Cocconi and Morrison publish "Searching for Interstellar Communications" in Nature, proposing the 1420 MHz hydrogen line as the natural SETI frequency.
2. 1956–1963 Big Ear radio telescope constructed at Ohio State by John Kraus.
3. May 1973 Ohio SETI program begins.
4. 22:16 EDT, August 15, 1977 Wow! Signal detected.
5. August 18, 1977 Jerry Ehman, reviewing the printout, identifies the "6EQUJ5" sequence and circles it with "Wow!" in red ink.
6. August–December 1977 Big Ear re-observes the source position multiple times; no recurrence.
7. 1987 Robert Gray's first follow-up campaign using META II at Oak Ridge Observatory; null result.
8. 1995–1996 Gray uses the VLA and Mt. Pleasant for further follow-up campaigns; null results.
9. 1997—1998 Big Ear decommissioned and demolished. Printout donated to Ohio Historical Society.
10. 2007 onward Allen Telescope Array begins long-term Wow! position monitoring; null results.
11. 2001 Robert Gray, The Elusive Wow, the principal book-length synthesis.
12. 2010–2020 Green Bank Telescope multiple campaigns; null results.
13. January 2017 Antonio Paris and Evan Davies publish cometary-emission hypothesis (266P/Christensen) in the Journal of the Washington Academy of Sciences.
14. February—June 2017 Multiple radio-astronomical critics reject the cometary hypothesis on technical grounds.
15. 2017 Alberto Caballero catalogues faint sun-like stars in the Wow! Signal position; identifies 2MASS 19281982-2640123 as a candidate.
16. 2020 Caballero conducts further follow-up observations of his candidate stars; no signal detected.
17. 2023—2024 FAST telescope (China) reports candidate signals from various positions subsequently traced to RFI; Wow! position included in monitoring program.

Cases on this archive that connect.

The Nimitz Tic Tac (2004) — another sustained-attention case in which sensor data of an unidentified character has been recorded, documented, and not resolved across multiple decades. The Wow! Signal and the Nimitz incidents share the structural feature of a single instrumented observation that has not recurred in resolvable form, despite continued monitoring.

The Roswell Incident (1947) — the founding event of American UFO discourse, treated here for its archival rather than its physical interest.

Planned: the BL-1 candidate signal from Proxima Centauri (2019); the Lorimer Burst (2007) and the broader history of fast radio bursts; the SETI Breakthrough Listen program (2016–present).

Full bibliography.

1. Kraus, John D., Big Ear, Cygnus-Quasar Books, 1976. The history of the telescope by its principal investigator.
2. Dixon, Robert S., "A SETI System Using the 1500 Channel Filterbank at the Ohio State Radio Observatory," published in the IEEE Annual Symposium on Information Theory, 1977 proceedings.
3. Ehman, Jerry R., "The Big Ear Wow! Signal: What We Know and Don't Know About It After 20 Years," NAAPO Wow! Signal archive, 1997. The principal investigator's retrospective.
4. Gray, Robert H., The Elusive Wow: Searching for Extraterrestrial Intelligence, Palmer Square Press, 2001. The principal book-length synthesis.
5. Caballero, Alberto, "An approximation to determine the source of the WOW! Signal," International Journal of Astrobiology, 2022. The Sun-like stars at the position.
6. Cocconi, Giuseppe and Morrison, Philip, "Searching for Interstellar Communications," Nature 184(4690):844–846, 1959.
7. Gray, Robert H. and Marvel, Kevin B., "A VLA Search for the Ohio State 'Wow,'" The Astrophysical Journal 546(2):1171–1177, 2001.
8. Ohio Historical Society / Ohio History Connection, Big Ear archive, including the original Wow! Signal printout.
9. Paris, Antonio and Davies, Evan, "Hydrogen Clouds from Comets 266P/Christensen and P/2008 Y2 (Gibbs) are Candidates for the Source of the 1977 'WOW' Signal," Journal of the Washington Academy of Sciences 103(2):25–46, 2017.
10. SETI Institute, candidate-signal review documents, 1995–present. seti.org.
11. North American Astrophysical Observatory (NAAPO), Wow! Signal documentation archive. bigear.org.
12. Cendes, Yvette, response to the Paris cometary hypothesis, Astronomy Stack Exchange and personal blog, June 2017.
13. Shostak, Seth, Confessions of an Alien Hunter, National Geographic, 2009. Coverage of the Wow! Signal in broader SETI context.
14. Tarter, Jill C., "The Search for Extraterrestrial Intelligence," Annual Review of Astronomy and Astrophysics 39:511–548, 2001.
15. Croft, Steve, "The Wow! Signal at 40," presentation to the SETI Institute Colloquium Series, August 2017.