Peering through the fog
Joseph R
Introduction Hershel discovered the planet Uranus from his backyard in Bath, 1 but today our biggest and most-cited telescopes are in far more exotic locations. The middle of the Chilean Desert, a shield volcano 13,803 ft 2 above sea level, in low earth orbit, the Sun-Earth L2 Lagrange point. 3 Why? Our atmosphere changes the light that reaches the earth in ways which can make optical measurements very difficult or even impossible. In this essay I will explore the difficulties that the Earth’s atmosphere presents to observational astronomy, and the methods which scientists and engineers use to minimize or even escape its effect. Refraction Think back to GCSE physics and Snell’s law: the ratio of the sines of the angle of incidence and the angle of refraction is equal to ratio of the refractive indices of the two media. The refractive index of a vacuum is (by definition) 1, and the refractive index of air is 1.000273 (at 1atm, 0 Celsius and 589.20nm). 4 Therefore, the atmosphere acts like as a lens, changing the direction of light, although it is important to note that since the atmosphere changes in density and temperature with height so light rays appear to ‘bend’ rather than sharply change direction as would be observed with a glass prism. This naturally makes modelling this effect more complex and the use of numerical integration is called for beyond small zenith angles. However, it is not excessively difficult – especially with modern computing power – to find the true zenith angle of a celestial body. 5
Figure 1-Refraction deviation angles for an observer
Figure 2- Zenith Angle vs Dispersion
at sea level, for inclination
1 Stuart, C. ‘The Discovery of Uranus’, https://www.rmg.co.uk/stories/space-astronomy/astronomy/discovery- uranus. Consulted: 26/08/2025. 2 Maunakei Observatories. SCIENCE. https://www.maunakeaobservatories.org/science. Consulted: 26/08/2025. 3 McCoy, M. ‘James Webb Fact Sheet’, https://science.nasa.gov/mission/webb/fact-sheet/. Consulted 26/08/2025. 4 Polyanskiy, M. ‘Refractiveindex.info database of optical constants’, https://refractiveindex.info/?shelf=other&book=air&page=Ciddor. Consulted: 26/08/2025. 5 Auer, L. and E. Standish (2000) ‘Astronomical refraction: computational method for all zenith angles’, The Astronomical Journal 119: 2472.
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