Project Blue Book Special Report No. 14 (Analysis of Reports of Unidentified Aerial Objects)

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The following points should be carefully noted about these histograms:
( 1) The negligible number of sightings when the sun is within
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10° of the zenith and nadir (angle of elevation of the sun =
± 90 °) of the observer is due to the fact that the southern-
most latitude of the U. S. is greater than the declination
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of the sun at the summer solstice, so that it would be im-
possible for the sun to reach the zenith or nadir of any
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observer in the U. S. (where most of the sightings were
made).
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(2) The time of day at which a particular angle of elevation
of the sun occurs does not remain fixed but varies from
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day to day. Consider, for example, the variation in
sunrise and sunset times over the course of a year.
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Thus, there are only two inferences to be made from this histogram:
( 1) the high peak of sightings soon after sunset, and (2) the lack of increase
in the UNKNOWNS relative to the KNOWNS near either sunset or sunrise.
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This would seem to discount the possibility that atmospheric phenomena
such as mock suns were the primary cause of the unknown reports, since
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such phenomena usually occur when the sun is near the horizon.
The Local Sun Time was computed as a step in the calculation of the
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angle of elevation of the sun. It is related to the hour angle by the equation:
Local Sun Time (L. S. T.) =HA/15 + 12. 00, where L. S. T. is in hours and
HA in degrees.
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The cards were grouped on the basis of L. S. T. in intervals of one
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hour, and the number of cards in each interval was counted. Again the
UNKNOWNS were sorted out and similarly treated. Histograms were con­
structed with the results of these tabulations of OBJECT SIGHTINGS
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(Figure 41). Here, again, there is a peak in the early evening hours.
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The cards were then broken up into seven groups on the basis of the
angle of elevation of the sun, as follows:
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Group 1 - Daylight sightings for which the sun was more than
10° above the horizon.
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Group 2 - Sunset sightings for which the sun was between 0°
and 10° above the horizon.
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Group 3 - Sunset sightings for which the sun was between 0°
and 10° below the horizon.
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Group 4 - Evening sightings for which the sun was between
10° and 40° below the horizon.
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