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The original report of the Eddington experiment, based on observations at the total solar eclipse of 29 May 1919.

Official publication: doi.org/10.1098/rsta.1920.0009

Original paper scan: w.astro.berkeley.edu/~kalas/labs/documents/dyson1920.pdf

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]

An eclipse occurs when an alignment of astronomical bodies obscures a source of light or places an illuminated body in another body's shadow. During a solar eclipse, the Moon interrupts the view of the Sun; during a lunar eclipse, the Moon enters Earth's shadow.

The monthly sequence of Moon phases is insufficient to produce monthly eclipses. The Moon's orbital inclination usually carries it above or below the required alignment. Eclipses become possible when the relevant phase occurs sufficiently near an orbital node.^[ref]

The finite apparent size of the Sun makes shadow geometry richer than a single dark cone: the umbra excludes the entire solar disk, whereas the penumbra excludes only part. An extended lamp and an opaque ball reproduce this distinction on a screen, allowing the shadow's sharp central region and softer surroundings to be compared without astronomical distances.

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]}

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.

A penumbra is a region behind an opaque object from which only part of an extended source of light is visible. It contrasts with the umbra, where the entire source is blocked. During a solar eclipse, an observer in the lunar penumbra sees a partial solar eclipse.^[ref]

An extended lamp, an opaque ball and a screen make the mechanism visible: different points of the lamp cast displaced shadows. Where only some of those shadows overlap, the screen receives reduced irradiance. Enlarging the lamp generally broadens the soft transition, whereas an ideal point source gives a sharp geometric boundary. The penumbra is a consequence of source extent even in geometrical optics; it does not require diffraction.

A solar eclipse occurs when the Moon passes in front of the Sun for an observer on Earth. The same event appears different from different locations: an observer in the umbra sees totality, while an observer in the penumbra sees only part of the solar disk covered.

If the Moon's apparent angular diameter is too small to cover the Sun, the central alignment produces an annular eclipse instead. The comparison is between apparent sizes, which depend on both physical size and Euclidean distance. During totality the normally overwhelming solar disk is hidden, exposing the much fainter solar corona and allowing observations of nearby background stars.^[ref]

An umbra is the region from which an opaque intervening object blocks the whole visible disk of an extended source of light. In the lunar umbra, an observer on Earth can see a total solar eclipse; the surrounding penumbra gives only partial coverage.^[ref]

For a spherical source of radius $R_s$ larger than an opaque body's radius $R_o$, separated by centre distance $d$, the common tangent rays meet on the axis behind the body atThis finite cone explains why a smaller occulting body need not produce totality at every distance. Beyond its tip the source can surround the occulting disk, producing an annular alignment. The umbra excludes direct source rays; it need not be perfectly black when scattered or refracted light enters the region.