Thom's terrestrial refraction observations

Thom has done some 600 observations of refraction during day and night time:
Overview of
      observatiosn done by Thom

The left part of the table provides an overview of observations evaluated in Thom's article (1958) and right part shows how many observations have been transcribed from Thom's notebooks (MS/430/27 and MS/430/64), which were found and copied by Thomas Gough (pers. comm. 2017). Thom recorded many parameters around his apparent altitude measurements for terrestrial refraction; not many people did similar recordings.

Issues around transcribing

There are a few issues that were encountered while transcribing:


Thom's evaluation can certainly not be mapped on Negative Apparent Altitude Refraction (NAAR), some can be mapped on Around Zero Apparent Altitude Refraction (AZAAR), Thom himself evaluated mostly the Positive Apparent Altitude Refraction (PAAR) observations. Thom also did several Positive Altitude Astronomical Refraction (PAAR) observations of stars, but these were not evaluated in Thom (1958) or this webpage.

As refraction is influences mostly by a high Temperature Gradients, we need to find situations where this happens. High Temperate Gradients happen near ground level, aka low heights. And both in terrestrial and astronomical environments this happens when one gazes over a vast level  surface, where the light path grazes over the far away landscape (with a possible high Temperature Gradient).
As soon as one gazes over a terrestrial object it is normally higher up and thus the Temperature Gradient is lower.

The sites which Thom removed (the lighthouse observations: Pl and HI), are included in the evaluation on this web page and thus extending the range into the AZAAR environment. No targets were removed (except Mark I, II and III: which don't have coordinates and elevations) as all had similar theodolite accuracy (as derived by Thom's direct and reverse observations).

Still need to verify the two possible uncertainties of the measurement methodology: temperature gradient and bubble readings.
If you have ideas how to solve these uncertainties, let me know.


ASTM Committee E-20 on Temperature Measurement. Subcommittee E20.04 on Thermocouples. 1981. Manual on the use of thermocouples in temperature measurement (ASTM).
Reijs, Victor M.M., 2017a, "Geniet: Thom's terrestrial refraction observations", http://www.archaeocosmology.org/eng/Thom-refraction.htm
Reijs, Victor M.M., 2017b, "Geniet: Temperature gradients at Skerries", http://www.archaeocosmology.org/eng/TempGradientSkerries.htm
Thom, Alexander. 1954. "Notebook.", MS/430/27, Edinburgh: Historic Environment Scotland.
Thom, Alexander. 1956. "Notebook.", MS/430/64, Edinburgh: Historic Environment Scotland.
Thom, Alexander. 1958. 'An empirical investigation of atmospheric refraction', Empire Survey Review, Vol 14: pp. 248-62.
Thom, Alexander. 1971. Megalithic lunar observatories (Oxford University Press).


I would like to thank people, such as Thomas Gough, Stephen McCluskey, Marcel Tschudin and others for their help and constructive feedback. Any remaining errors in methodology or results are my responsibility of course!!! If you want to provide constructive feedback, let me know.

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Major content related changes: August 10, 2017