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I live in a big suburban area where pollution is quite high, and I have been testing a sextant by measuring the angle between pairs of stars. I compared the measured angle with the one obtained from ephemerides, where the latter takes account of the alteration of the body altitude due to refraction in the atmosphere.

For instance, I measured the angle between Castor and Pollux, both at apparent altitudes of ~ 39 degrees.

The comparison works pretty well, but the sextant appears to systematically overestimate the angle by ~ 1'. Can the air pollution in my area (not light pollution) alter the refraction index of the surrounding air mass and thus be responsible for this discrepancy?

Thanks!

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    $\begingroup$ Will do, thanks! $\endgroup$
    – Louise
    Commented Apr 23, 2021 at 18:25

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The air pollution has a negligible effect on the refraction, because pollution barely affects the air density.

However, the refraction is proportional to the air pressure and inversely proportional to the air temperature in Kelvin. So different weather conditions or ground elevations will result in different refraction values.

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  • $\begingroup$ I know, but the altitude correction tables in the nautical almanac show that the corrections due to non standard temperature and pressure values are negligible. $\endgroup$
    – Louise
    Commented May 3, 2021 at 18:10

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