Refraction near the horizon

Abstract

We have studied the variation of astronomical refraction near the horizon. We have collected 144 measurements of refraction from seven sites by three techniques and have found that the variation of refraction on the horizon is substantially larger than has previously been realized. The rms deviation of our observations is 0?16, while the individual measurements range from 0?234 to 1?678. At the 95% confidence level the total refraction should vary over a range of 0?64. This surprising result has five applications: First, the time of sunrise can only be predicted with an accuracy of 4 min, despite all the extreme accuracy of modern positional astronomy. Second, during a low-altitude solar eclipse (such as is the 1990 eclipse viewed from Finland), the size and shape of the edge of the umbra will vary in an unpredictable manner by perhaps several kilometers, so that attempts to measure the diameter of the Sun may have an unexpected accuracy limit. Contact times may vary by typically 0.1 sec. Third, refraction variation will set a fundamental limit on the accuracy of any claimed archaeoastronomical alignment. Fourth, methods for aligning the Great Pyramid of Cheops to an accuracy of 2' cannot involve near-horizon observations. Fifth, the historically important claim by A. Thom that British megalithic sites were used as accurate lunar observatories is shown to be wrong because the needed accuracy is much greater than can be obtained for long averaging intervals.

Geolocation by light: accuracy and precision affected by environmental factors

“…Factors that may influence the light regime are extremely diverse, but all cause some sort of shading, resulting in spuriously shorter days. The most fundamental confounding factor is refraction near the horizon that influences the perceived sunrise/sunset by up to ±0·32° of sun elevation angle depending on the composition of the atmosphere and weather conditions (Schaefer & Liller 1990). Weather conditions in general may change perceived light conditions, for example shading by cloud cover.…”

Archaeoastronomy and Refraction Near the Earth’s Surface

“…44 On the other hand, the timed observations of sunrise and sunset over a level horizon, while addressing archaeoastronomical refraction, do so in circumstances which do not represent the common method of observing an astronomical object beyond an elevated horizon marker. 45 Future empirical studies should measure simultaneously the terrestrial refraction α and the astronomical refraction (α + ε) at the observation site and, if possible, the refraction β from the horizon marker back to the observer. The need for repeated observations of an astronomical object from a fixed site to a fixed horizon marker suggests abandoning the conventional use of sunset or sunrise and observing instead the setting or rising of a fixed star.…”

Horizon Line Stability Observations over the Sea

“…The variation of these times represents variations of atmospheric refraction. The measurements show that the range of refraction scatter is near (Schaefer and Liller, 1990) and (Sampson et al, 2003). At the same time, the horizon line position varies in the range from to (Young, 2004).…”

“…Generally speaking, the variation of refraction can be much greater than the values that we have obtained. High-precision estimations of atmospheric refraction were carried out in Schaefer and Liller (1990), Young (2004) and Sampson et al (2003). In these works, the times of sunset and sunrise were measured.…”