May 29, 1991 OBSEARTH.TXT OBSERVATION OF EARTH'S TV STATIONS FROM ALPHA CENTAURI SUMMARY Observers in the Alpha Centauri system with technology similar to ours could detect video carriers (but not program material) broadcast from Earth's television stations south of 31 degrees north latitude. Over the course of a year they could determine the Earth's orbit. With careful observations and a few reasonable assumptions, they could determine Earth's size, length of sidereal and solar days, and axial tilt with respect to Alpha Centauri. They could map individual stations onto a globe, determine probable antenna sizes, and might even detect windy seasons for certain stations. From Earth's orbit they could estimate Earth's temperature. By assigning a reasonable density, they could estimate Earth's mass and surface gravitation from its radius. INTRODUCTION Sullivan, Brown, and Wetherill (1978) (Science, vol. 199, no. 4327, pp. 377-388) discuss the appearance of radio emissions from Earth as seen by observers at interstellar distances. I have summarized parts of their article and examined its implications for observation of Earth from the Alpha Centauri system, a vantage point much farther south than they considered. DETECTION Sullivan, Brown, and Wetherill conclude that an Arecibo-size detector could detect a strong UHF station's carrier at 1.8 light years, and Project Cyclops arrays of 1000 100-m dishes could detect video carriers out to about 25 light years. Program detection requires about 20,000 times the sensitivity required for carrier detection (an antenna with about 100 times the diameter, or at about a hundredth the distance). Even Project Cyclops at Alpha Centauri could not get a picture from Earth's TV broadcasts. Sullivan, Brown, and Wetherill note that BMEWS-type radar could be detected as much as 10 times farther than UHF television carriers, but I believe BMEWS is confined to northern latitudes not visible from Alpha Centauri. EARTH'S ORBIT Alpha Centauri has declination -60 (60 degrees south of Earth's equator). It is visible some or all of the time south of 30 degrees north latitude, and it can only 'see' the parts of the Earth from which it is visible. This excludes about 85% of the television stations in the model used by Sullivan, Brown, and Wetherill. However, they conclude that orbital information is best determined by observing individual strong UHF stations, rather than combined signals, so this is not a problem. Observation of various strong UHF stations over the course of a year are sufficient to determine Earth's orbital period, orbital eccentricity, longitude of perihelion, time of perihelion, projected semimajor orbital axis, and mass function of the Sol- Earth system. If the aliens assume the emissions are from a planet orbiting Sol and know enough astrophysics to estimate Sol's mass, they can assume Earth's mass negligible and find the true value of the semimajor axis of Earth's orbit. Knowing the orbit, they can then estimate the approximate temperature of Earth. EARTH ITSELF The aliens can determine the length of Earth's sidereal and solar days. The sidereal day can be determined from rise and set times of individual stations if the abrupt appearance and disappearance of a station is attributed to occultation by the rotating planet. The Doppler shift is close to its daily extremes at times of rise and set; it can be used to confirm rise and set. Rise and set also differ from the station going on and off the air because the television broadcast is focused at the horizon, and signal strength is therefore greatest near rise and set. The solar day is more difficult to determine than the sidereal day. Sullivan, Brown, and Wetherill mention two methods, both relying on the fact that the daily schedule of a station is tied to the solar, not sidereal, day. The first method looks for annual modulations in the number of stations contributing to characteristic peaks of flux that are produced when concentrations of many stations are near the horizon. This method is not well suited to the large southern declination of Alpha Centauri, which prevents it from 'seeing' Europe, Japan, and much of North America. The other method of determining the solar day looks at individual stations and is therefore more suitable for use at Alpha Centauri. Sullivan, Brown, and Wetherill mention a method which notes that the time in hours between a station's two daily appearances (strongest signals at times of rise and set) is equal to the time in half-months between its two annual shutdowns (when it is off the air at rise or set). From Alpha Centauri this method only works for stations between 30 degrees north latitude and 30 degrees south latitude. Stations farther south never rise or set; they make only one broad appearance per day (when Alpha Centauri dips closest to the station's horizon), and they deliver little power because Alpha Centauri is always far above their horizontal antenna patterns. For the same transmitter power, VHF stations are more visible than UHF stations in this situation because VHF stations tend to be less focused toward the horizon. The aliens could determine Earth's size, equatorial rotation velocity, and axial tilt with respect to Alpha Centauri by careful observation and correct modeling. They will notice that some stations never set, which implies some axial tilt with respect to Alpha Centauri. (Only if the observer was in the plane of Earth's equator, would all stations rise and set, and all rises and sets would be 12 hours apart.) For each of many stations that set, the aliens would have to measure the daily maximum Doppler shift and the time between the appearance and disappearance of the station as it rises and sets. Fitting the data for enough stations to the model of a tilted, rotating sphere would give Earth's tilt with respect to Alpha Centauri and the velocity of Earth's rotation at the equator. The equatorial radius of the Earth can be determined from the equatorial velocity and the sidereal-daily period to be about 6000 km. Thus, the aliens will have determined that the source of the broadcasts is either a planet, white dwarf star, or burned-out black-dwarf star. They may choose to rule out either type of star because of the stars' very high surface gravity. Whether or not they decide that Earth is a planet, they can map the measured stations (none north of 30 degrees north latitude) onto a sphere of approximately 6000 km radius to an accuracy of a few kilometers if their timing accuracies are a few seconds and frequency resolution is no larger than 1 hertz. By following the trace of an individual station through a day, they can measure the radiation pattern of the primary lobe and may be able to measure some side lobes. If they assume a diffraction- limited aperture, they can determine the antenna size, which is typically about 15 to 20 meters. Years-long, very careful observations of individual stations might distinguish seasonal effects at those stations. UHF television broadcasts from towers are tightly focused to the horizon, and strong storm winds can tilt a tall tower enough to cause a detectable phase jitter of about 1 minute from the time the peak signal is expected for a given station. If the aliens noticed the annual cycle in this phase jitter, they might possibly attribute the phase jitter to winds during windy seasons. Since the best observations are possible for UHF stations within about 30 degrees of the equator, the aliens would be more likely to determine hurricane, typhoon, and monsoon seasons, rather than southern hemisphere temperate seasons. Luna's orbit will never pass between Earth and Alpha Centauri, so the aliens will never observe lunar occultations of the Earth. By careful observations over decades they might detect the Earth's precessional wobble. If they made all the right assumptions, they could then estimate Luna's mass and distance from Earth. LIMITATIONS As noted above, Alpha Centauri happens to be located so far south that it cannot detect BMEWS-type radar or about 85% of the television broadcast from Earth. From Alpha Centauri the aliens cannot detect or map the great concentrations of stations in Europe, Japan, and most of North America. Because the unfavorable geometry keeps Alpha Centauri more than a few degrees above the local horizon, the aliens will receive little power from UHF stations in Buenos Aires, Capetown, Sydney, or points farther south. However, they will receive VHF signals from those cities and points south to about 38 degrees south latitude, or even farther south with sufficiently sensitive detectors. The aliens' best observations and mapping will be limited to latitudes between 30 degrees north latitude and approximately 32 degrees south latitude. They will have some information farther south, but they will be unaware of much of Earth's broadcast output and geopolitical structure.