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Arthur Eddington helped carry out the 1919 eclipse test of general relativity, observing the apparent directions of stars near the eclipsed Sun. He coauthored the resulting report with Frank Watson Dyson and Charles Rundle Davidson.

Comparing stellar positions on eclipse and reference plates tested a small radial displacement. Changes in plate scale and focus could alter the signal, so the report assessed instrumental errors alongside its gravitational interpretation.^[ref]

Charles Rundle Davidson coauthored the original report of the 1919 Eddington experiment, with Frank Watson Dyson and Arthur Eddington. His contribution belongs to an observational project that compared stellar images taken during a total solar eclipse with reference images.

Comparing stellar positions on eclipse and reference plates tested a small radial displacement. Changes in plate scale and focus could alter the signal, so the report assessed instrumental errors alongside its gravitational interpretation.^[ref]

Institutional name record: ^[ref]

The 1919 solar eclipse expeditions organized by Frank Watson Dyson and Arthur Eddington compared background-star positions near the obscured Sun with reference photographs. The optical train used a slowly rotating flat mirror to send the light into a horizontal photographic telescope. A smaller telescope at Sobral provided a separate set of plates. Focus and plate-scale changes had to be distinguished from the radial pattern predicted by Schwarzschild light deflection. The experiment therefore tested a spatial pattern and its amplitude, not merely whether a star image had moved.^{[ref] [ref]}

Frank Watson Dyson coauthored the report of the 1919 eclipse observations with Arthur Eddington and Charles Rundle Davidson. The experiment tested the predicted bending of stellar light passing near the Sun.

Comparing stellar positions on eclipse and reference plates tested a small radial displacement. Changes in plate scale and focus could alter the signal, so the report assessed instrumental errors alongside its gravitational interpretation.^[ref]

In general relativity, light follows null geodesics. A weak gravitational field outside an isolated spherical mass gives the Schwarzschild light deflectionfor impact parameter $b\gg GM/c^2$. At the solar limb this is approximately $1.75$ arcseconds. A Newtonian corpuscular calculation gives half this value; the full result depends on spatial curvature as well as the gravitational time dilation. The parametrized post-Newtonian formalism predicts $\alpha =2(1+{\gamma }_{\mathrm{PPN}})GM/(b{c}^2)$, with ${\gamma }_{\mathrm{PPN}}=1$ in general relativity.

The Eddington experiment compared photographic star positions near the eclipsed Sun with reference exposures taken without the Sun in that field. Rays bend toward the Sun, but their backward extrapolations place the apparent stellar images farther from it. Plate scale, focus changes, atmospheric refraction and comparison-field alignment must be separated from a radial displacement proportional to $1/b$. The original report discusses the instruments, plate reductions and competing predicted deflections; the test is a quantitative comparison with uncertainty, not a unique proof of a gravitational theory.^[ref]

radio interferometry permits an independent test without an eclipse. A 2005 Very Long Baseline Array experiment observed four compact sources at 43, 23 and 15 GHz. Differential astrometry suppressed common antenna errors, while a fit $\mathbf{p}(\nu )={\mathbf{p}}_0+\mathbf{I}{\nu }^{-2}$ separated refraction in the solar corona from achromatic gravitational deflection. Tropospheric errors and radio-source structure also required calibration. The reported ${\gamma }_{\mathrm{PPN}}=0.9998\pm 0.0003$ was consistent with general relativity.^[ref]

A light ray follows a null geodesic. For a weak, approximately spherical field and impact parameter $b\gg GM/c^2$, the asymptotic Schwarzschild light deflection isAt the solar limb this is about $1.75$ arcseconds, whereas a simple Newtonian fast-particle calculation gives half that value. The difference tests relativistic spacetime geometry beyond the universality of slow-body free fall. The quoted angle compares incoming and outgoing directions far from the deflector, not a locally measured turn at a sharp vertex.

For an eclipse measurement, photograph background stars near the obscured Sun and compare their apparent positions with reference plates taken when that field is away from the Sun. Back-projecting the deflected incoming light makes a star appear displaced outward from the Sun's apparent position. Fit the radial displacement against inverse angular separation from the Sun, while allowing plate translation, rotation and scale. The expected pattern must be separated from atmospheric refraction, optical distortion and changes of focus or scale, rather than inferred from one displaced star.

The 1919 expeditions obtained observations at Sobral and Príncipe, published by Frank Watson Dyson, Arthur Eddington and Charles Davidson in 1920. The original report compares eclipse and reference plates and discusses instrument scale and image-quality limitations; it should not be treated as an exact modern precision measurement.^[ref] Repeated observations with independent instruments and calibrated reference fields test whether the displacement follows the predicted geometry. vacuum gravitational bending is achromatic to this order, a useful distinction from dispersive propagation effects.

The three classic tests probe orbital motion, the propagation of light, and clock rates in a gravitational field.

Schwarzschild perihelion precession. For an orbit with semi-major axis $a$ and orbital eccentricity $e$, the leading advance per revolution isThe relevant observation for Mercury is the residual advance after the much larger ordinary perturbations have been included, approximately $43$ arcseconds per century.^[ref] An orbital fit must account for the other planets and reference-frame effects before interpreting that residual as a relativistic contribution.

Schwarzschild light deflection. A ray with large impact parameter $b$ passing a nearly spherical mass is deflected throughAt the Sun's limb this is about $1.75$ arcseconds. The Eddington experiment used the 1919 solar eclipse to photograph background stars near the obscured Sun, then compared their positions with reference photographs. The measured effect is an apparent outward displacement of the background stars, not a direct photograph of a curved light path. Telescope focus, plate scale and atmospheric effects must be controlled when extracting such small shifts.^[ref]

Gravitational redshift. For a photon emitted lower in a weak static field and received a height $H$ above it,The Pound-Rebka experiment used the narrow recoil-free nuclear resonance supplied by the Mössbauer effect. A controlled source motion supplies a small Doppler shift that compensates the gravitational shift, allowing a change in resonant absorption to reveal it across a laboratory tower.^[ref] The leading weak-field redshift follows from the equivalence principle and therefore does not, by itself, distinguish every competing metric theory of gravity.