January 1960 L E T T E R S T O T H E E D I T O R 85

Star Visibility in Daylight at High Altitudes A. H . MIKESELL

U. S. Naval Observatory, Washington, D. C. (Received July 31, 1959)

ON May 6-7, 1958, an open gondola was manned for a day-night balloon ascension which reached a 40 000-ft height

over Mille Lacs Lake, Minnesota. An earlier radiosonde flight indicated that the first tropopause was located at about 36 000 ft. Very near this level the two-man crew perceived an inversion layer which was sharply defined by haze. Above the tropopause the sunlit sky first appeared deep blue or purple.

Several minutes after passing the tropopause, and nearly thirty minutes before sunset, rotation of the gondola facihtated a view to the southeast. Jupiter was seen by the naked eye immediately, without searching for it. I t appeared silvery white and so disk-like that the observer first thought of a weather balloon which was launched several hours earher. Its illumination was such that it appeared to show no irradiation against the adjacent, dark-colored sky. The navigational star Spica, less than four degrees from Jupiter, was more than three magnitudes fainter, as well as somewhat bluer, than the latter. Spica was not seen, despite a careful search. Neither was Arcturus located, although it is one magnitude brighter and considerably more orange than Spica. Possibly the observer's eyes did not retain infinity focus while sweeping 35° across the sky from Jupiter to the Arcturus region. The angular altitude of Jupiter and Spica was 20°, and of the sun, 0?5 (about 5° above the apparent gondola horizon). The azimuths of these objects were 126°, 130°, and 294°, respectively. Jupiter's diameter and stellar magnitude were 41 sec and —2.0.

An interesting observation revealed the duration of nautical twilight, sometimes defined as the time interval between sunset and the first appearance of 3rd magnitude stars. The casual statement of aviators that it does not exist, was supported. Stars as faint as the third magnitude, and within 30° of the sunset point, could be seen with the naked eye five minutes after sunset. Astronomical twilight, as shown by photoelectric photometer readings of the southern sky, lasted only as long as any twilight glow could be discerned. This amounted to about 70 min, by which time computations (kindly furnished by Mr. R. F. Haupt) place the center of the sun 10° below the gondola horizon. The corresponding interval for a ground-based observer, when the sun would be 18° below his horizon, is estimated at 126 min.