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Preliminary Analysis of the Mount Wilson Sunset Observations

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Technical Report

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US Naval Observatory Washington United States

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A total of 250 observations of the times of sunset were made from Mount Wilson, California from 1987 Mar. 16 through 1991 Nov. 10. Along with the time of observation, the current atmospheric temperature, dew point, and pressure at the observer were recorded. An initial reduction to a subset of that data was performed by J. Bangert in 2007. The weather data are used to determine the horizontal refraction, computed using the Hohenkerk and Sinclair 1985 algorithm and compared to the timing data. The observed sunsets all occurred over the Pacic Ocean giving a uniform, level, depressed horizon. This note examines the resulting O - C values for the time of sunset. They show a signicant 41 plus or minus 58 s delay between the observed and computed time of sunset. These values also show a signicant positive skew to the sunset times and evidence of extreme refraction events. The systematic dierences found here are attributed not to a problem with the Hohenkerk and Sinclair algorithm, but to possible boundary layer lapse rates diering from the standard rate used in the reduction. The boundary layer lapse rate difference is thought to arise from the contact of the relatively cold water of the Pacific Ocean with the overlying atmosphere. Boundary layer lapse rates differing from that of the rest of the troposphere are not included in the Hohenkerk and Sinclair algorithm. Finally, a set of suggested future actions to improve the refraction algorithm is made.

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  • Meteorology

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