Case 40

South Mountain

Fall 1967

Investigator: Ayer

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A light witnessed and photographed from a moutain slope was analyzed by rough photometry and reference to a map of the area. It was attributed almost certainly to headlights of a surface vehicle in the valley.


Two young college men decided to watch for UFOs over a valley from the flank of a mountain peak. In the evening, they drove off a highway east of city A, north on a road about 0.75 mi. past a ranch access road, then turned east on a dirt road about 0.5 mi. up the slope of a mountain. There they set up their camera on a tripod. It was a Yashica-D with 80-mm lens, 2.25 by 2.25-in. frame, loaded with Eastman Tri-X film. The moon was high and the sky clear.

About 1:20 a.m., a white light appeared in the valley to the west, apparently above the valley floor but below the line of lights that marked a well travelled highway on the valley floor. About 1:30 a.m., while the light was still stationary, two photographs were taken with exposures of 40 and 80 sec. Later the light moved northward at both low and high speeds, then returned to its starting point. Its apparent path is shown in Fig. 8.


The latest, unpublished Geological Survey map indicates that the altitude of the camera site was about 7,800 ft. From this and other known altitudes, it was deduced that the line of sight to the UFO intersected the valley floor about seven miles from the camera. The camera position was almost due east of city B, which lies in a valley between a mountain to the south and other mountains to the


north. These features can be approximately identified on the photographs. They indicate that the bearing of the UFO from the camera was 290°.

The positions and lengths of the star tracks, corrected for the camera motion apparent on the longest exposure, indicate that the first exposure was roughly three times as long as the second, and that the reported exposure times were approximately correct. A vertical microdensitometer tracing of the region to the right of the edge of the disc of the UFO spot on the 80 sec. exposure indicated substantial illumination of the valley floor, suggesting that the light was on a vehicle on the ground.

The eye usually can distinguish two objects having an angular separation less than one minute of arc, or about ten feet at seven miles. This limitation would explain why the boys saw only one light, even though the source may have been a pair of headlights. Application of Rayleigh's criterion for resolving power to the camera lens indicates that if of excellent quality it could have resolved headlights at any top opening greater than f/12; presumably it was used wide open.

However, the two headlight images would have been only 8.6 µ apart on the camera film. Tri-X film is rather coarse-grained; the manufacturer's specifications indicate that it cannot register separate image details, even with poor efficiency, unless they are at least 15 µ apart. Contrast effects between bright headlights and the dark background would further reduce the resolution on the film. It seems clear that a pair of headlights could not have been distinguished from a single light in the photographs. A horizontal densitometer trace showed three shallow peaks of unequal height, but the separation of the two greater ones was roughly ten times the expected value for headlights. The shallowness of the peaks suggested they might be artifacts.

The intensity of the unknown source was determined approximately from the geometry of the situation and the density of the


image of the source on the film. If we call the intensity of the source I, the light flux from the source into the camera lens F, the area of the lens opening A, and its distance from the source R, then F = IA/R2. Absorption and other losses in the lens reduce this flux by a factor T, estimated as 0.8. The remaining light flux falls on an image spot of area at the film. Therefore, if J is the illumination at the image, Ja = TIA/R2.

The lens opening is assumed to have been f/3.5, or 2.28 cm. diameter. The diameter of the image spot on the 40-sec. negative was determined from a densitometer trace as 0.4 mm. The density of the image spot, corrected for background, was 3.2. The H-D curve published by Eastman for Tri-X film with antihalation base, developed seven minutes in D-76 at 86 F., shows only the toe and straight section. If the exposure is determined by a linear extrapolation of the straight section, a minimum value of the illumination results, namely 4.0 meter-candles.

If the preceding equation for the intensity I of the unknown source is solved with these data, I = JaR2/TA = 197,000 candlepower. However, this equation has assumed implicitly that the unknown source was radiating uniformly in all directions. Since headlight beams are concentrated in the forward direction, the result above must be reduced by the ratio of the solid angle effectively filled by the headlight beam to that of the full sphere. Since the distribution of light in the beam is not uniform and depends on the individual headlight design and condition, no accurate correction of this result is possible. It can only be noted that the solid angle effectively filled by a headlight is roughly .05 to 0.1 of the full sphere, reducing the computed source intensity to an estimated 10,000 to 20,000 candlepower. Further uncertainties occur as to whether the assumed headlights were pointing directly toward the camera, and in estimating the source distance, lens stop used, and illumination of the film.

Maximum intensities of the high beams of automobile headlights lie in the range 15,000 to 50,000 candlepower. The results



Figure 8: King Ranch

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of the photometric computation of the source intensity therefore are compatible with automobile headlights, though subject to broad uncertainties.

The following hypothesis can nbw be advanced: a vehicle, probably 4-wheel driven, moved in the valley along a path similar to that shown in Fig. 8. No wheeled vehicle can move cross-country in the valley because of the ubiquitous stiff vegetation: but a map of the area shows crude roads or sand tracks that approximate the path described by the boys. These roads are blocked by barbed-wire fences along the section lines. Stopping to open take-down gates in these fences accounts for the interrupted progress of the UFO. The fading of the original light is explained by the change in direction of the vehicle, and the appearance of a red color by the coming in view of a tail-light.

The UFO was reported to have moved toward the boys at high speed. The segment AB of the path marked on Fig. 8 is a straight black-topped road, in the valley with a sufficient "toward" component to correspond to the analogous part of the track in Fig. 8.

Finally, the statement that the UFO returned to its starting point is made plausible by the circuitous pattern of roads and tracks on maps of the area.

Many questions remain, not the least of which is: how is it that such a bright light suddenly appeared in the middle of a vast expanse of scrub, and what were the occupants of the vehicle doing at that hour? Perhaps they were trying to jack-light deer (out of season) or rabbits. Since such a pursuit was illegal, hunters would have chosen a late hour to avoid being seen.

Thanks are due Dr. Elmo Bruner of Laboratory Atmospheric and Space Physics for making the densitometric measurements.