Abstract

Low altitude astronomical refraction (LAAR) of the setting Sun was measured over a sea horizon from a coastal location in Barbados, West Indies. The altitude of the upper limb of the Sun and the apparent horizon were determined using a digital video camera (Canon XL2) and a digital SLR camera (Canon EOS 5D). A total of 14 sunsets were measured between 2005 and 2007. From these measurements LAAR variability was estimated at 14 standard altitudes of the refracted Sun between $0°.01$ and $4°.5$. The relative variability decreases with increasing altitude from $\pm 0.0195$ of mean refraction at an altitude of $0°.01$ to $\pm 0.0142$ at $4°.5$. If extrapolated to an altitude of $15°$, a linear fit to the data produces a relative variability of $\pm 0.0038$ and an absolute variability of $\pm 0^{\prime \prime }.45$. Statistical analysis of the relative variability in LAAR appears to support the decreasing trend. However, error propagation analysis further suggests that the observed values of refraction may exceed the accuracy of the measurement system at altitudes higher than $2°$.

© 2008 Optical Society of America

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