# On a supposed alteration in the amount of astronomical aberration of light **George Biddell Airy, C.B., Astronomer Royal** — Royal Observatory, Greenwich *Proc. R. Soc. Lond.* **20**, 35–39 (1871). Received November 17, 1871. --- ## Summary This is **the paper**. Airy reports his water-filled telescope experiment testing Klinkerfues's prediction that the aberration constant should increase when light passes through a refracting medium. The result: **no change detected**. The hypothesis of Klinkerfues is "untenable." Airy observed γ Draconis (the same star Bradley used to discover aberration) with a zenith telescope whose tube was filled with water. He compared spring observations (when aberration displaces the star in one direction) with autumn observations (opposite direction). The geographical latitude derived from both seasons agreed to within a fraction of an arcsecond, proving the aberration constant was unchanged by the water. --- ## Page-by-Page Breakdown # Page 1 ![[page-1.png]] This page contains a preceding paper on sound velocity measurements, then begins Airy's paper at section IV. Key content: **Context**: A discussion on the Continent, partly in *Astronomische Nachrichten* and partly in independent pamphlets, about whether the undulatory theory predicts a change in aberration when light passes through a refracting medium. The most important papers are by **Professor Klinkerfues** (Leipzig, August 1867; *Astronomische Nachrichten* No. 1669, October 1867; Netherlands Royal Academy, 1869). **Klinkerfues's prediction**: The 8-inch column of water and a prism in the telescope interior would increase the aberration constant by **eight seconds of arc**. **Klinkerfues's own observation**: The aberration appeared to increase by **7".1**, but Airy notes "it does not appear that this observation was repeated." **Airy's decision**: "A result of physical character so important, and resting on the respectable authority of Professor Klinkerfues, merited and indeed required further examination." Airy chose a **vertical telescope** observing γ Draconis, the same star that established aberration's existence and laws. **The instrument**: A plane glass closing the lower surface of the object-glass, with the whole tube filled with water. Water column: **35.3 inches** (89.7 cm). The lenses were adapted to correct for spherical and chromatic aberration in conjunction with the water. # Page 2 ![[page-2.png]] **Telescope construction**: Built on a plan arranged by Airy himself for double observation in reversed positions. The vertical reference is given by **two spirit levels**, read at every single observation. Construction by **Mr. James Simms**. Distilled water supplied by H. W. Chisholm, Esq., Warden of Standards. **Key statement**: "Had the result of the observations been confined to the determination of an astronomical constant, or the variation of its value for different telescopes, I should not have thought it worthy of communication to the Royal Society. But it is really a result of great physical importance, not only affecting the computation of the velocity of light, but also influencing the whole treatment of the Undulatory Theory of Light." **Observation strategy**: The instrument was mounted at a small Occasional Observatory first constructed for Mr. Struve. The seasons at which the meridional zenith distance of γ Draconis is most affected by aberration in opposite directions are the Equinoxes. **Critical methodology for understanding the data table**: An apparent value of geographical latitude is formed from every observation by subtracting the observed instrumental zenith distance north of the star from the tabular declination in the *Nautical Almanac*. The observed zenith distance is affected with the **True Aberration** as seen in the instrument. The tabular declination is affected with the **Received Aberration** used in computing the Almanac. Therefore the apparent geographical latitude is affected by the **difference between the True Aberration and the Received Aberration**. If adding water changed the aberration, the apparent latitude would be different in spring vs autumn. # Page 3 ![[page-3.png]] **The data table**: Observations from February 28 to October 6, 1871. Spring observations (Feb-March): Mean latitude of instrument = **51° 28' 34.4"** Autumn observations (Aug-Oct): Mean latitude of instrument = **51° 28' 33.6"** The spring and autumn means agree within **0.8 arcseconds**. > [!danger] The Key Result > "Remarking that the mean results for Geographical Latitude of the Instrument (determined from observations made when the Aberration of the star had respectively its largest + value and its largest − value) agree within a fraction of a second, **I think myself justified in concluding that the hypothesis of Professor Klinkerfues is untenable.** Had it been retained, the Aberrations to be employed in the corrections would have been increased by +15" and −15" respectively, and the two mean results would have disagreed by 30"." This is the kill shot. If Klinkerfues were right, the spring and autumn latitudes would disagree by **30 arcseconds**. They agreed to within **0.8 arcseconds**. The prediction failed by a factor of ~37. # Page 4 ![[page-4.png]] **Latitude agreement**: The latitude from the instrument (51° 28' 34.4" spring, 51° 28' 33.6" autumn) agrees well with the geodetic latitude of the transit circle (51° 28' 38".4) and the geodetic latitude of the instrument (51° 28' 35".05). **Micrometer scale verification**: The micrometer plate contains 26 wires and the fixed part contains 25 crosses, each interval being nearly 256". Every wire interval is measured with great ease. Verification by placing the instrument in a proper position: 25 intervals of wires ought to be 0.8693 inch, measured with compasses as 0.871 or 0.875. Agreement is as close as can be expected from the rudeness of the operation, showing **no error of principle in the method of evaluating the micrometer scale**. # Page 5 ![[page-5.png]] This page begins a new paper (V. "Magnetic Survey of the East of France in 1869") and is not part of Airy's paper. --- ## See Also - [[1728_Bradley_New_Motion_Fixed_Stars|Bradley 1728]] — Airy observed the same star (γ Draconis) with the same instrument type (zenith sector) - [[1867_Klinkerfues_Aberration_Wave_Theory|Klinkerfues 1867]] — The +8" prediction that Airy tested and found "untenable" - [[1887_MichelsonMorley_Relative_Motion|Michelson-Morley 1887]] — Opens with Airy's null result as the foundational motivation - [[1895_Lorentz_Versuch_Airy_Sections|Lorentz 1895]] — §61 explains Airy's result via "corresponding states"; cites Airy in footnote 3 - [[1921_Pauli_Theory_of_Relativity|Pauli 1921]] — §36(γ) calls Airy's result "self-evident" and "trivial" from the relativistic viewpoint - [[1911_Watson_Velocity_of_Light|Watson 1911]] — States "No such effect is, however, observed" without invoking any rescue - [[2025_Ligabue_Airy_Relativity|Ligabue 2025]] — Modern wavevector treatment; the subject of our rebuttal - [[Franco|Response to Franco]] — 7-part rebuttal with spreadsheet summary - [[Aberration_in_Air_vs_Water|Null Hypothesis]] — Formal $H_0$/$H_1$ framework for Airy's experiment ## Critical Notes for the Airy Argument > [!important] What Airy Actually Measured > Airy did NOT compare "water telescope" vs "air telescope" simultaneously. He used the water telescope to observe γ Draconis at two seasons 6 months apart, when aberration acts in opposite directions. If water changed the aberration, the two seasons would give different latitudes. They didn't. > [!important] The 30" Prediction > Klinkerfues predicted an increase of ~8" in the aberration constant. Over a full year (spring to autumn), this would produce a discrepancy of 2 × 15" = 30" between the two seasonal latitude determinations. Airy found 0.8". The prediction is excluded by a factor of 37. > [!note] Airy's Instrument Design > The zenith telescope is spirit-leveled vertical. It does not tilt to track stars. The water surface at the top is horizontal. This is critical for the Sun's rest frame argument: in that frame, light enters the horizontal water surface at normal incidence. > [!note] Airy References Klinkerfues Directly > Airy explicitly names Klinkerfues, cites his prediction of +8", notes his observation of +7".1, and then demolishes the hypothesis with his own data. This is a direct experimental refutation of the wave theory prediction for a moving refracting medium.