The near-Earth asteroid 433 Eros was discovered in 1898. Its highly elliptical orbit which brings it close to Earth makes it the ideal object to perform parallax measurements. During the opposition of 1900-1901, a worldwide programme was launched to make parallax measurements of Eros to determine the solar parallax (or distance to the Sun). During the next (and closer) pass of Eros in 1930-1931, a similar parallax programme was launched by the RGO under Howard Spencer Jones, allowing astronomers to refine the true scale of the solar system.

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight. When a celestial body such as Eros comes close to Earth and is viewed simultaneously from two widely separated locations, its observed positions with respect to the background stars are different. If this ‘parallactic displacement’ is measured and the distance between the two observing locations is known, the distance to the body can be computed through trigonometry. Once the distance between Eros and earth is established, the distance to the sun is also known from Kepler’s Third Law.

To be able to accurately measure the parallax angle, the position of all reference stars up to the 9th magnitude along the path of Eros had first to be determined with high precision. The preliminary work for the 1931 opposition started in the 1920s, when a list of 419 reference stars was carefully prepared by the Rechen-Institut in Berlin. To account for atmospheric dispersion, the brightness, spectrum, and colour index of the stars was also to be determined.

During January 1931, numerous observatories measured the position of Eros with respect to these reference stars. This was done either visually, using micrometer measurements, or photographically, where the exposed plates were measured afterwards. The path of Eros was photographed at the Bergedorf, Greenwich, Leipzig, Lick and Cape Observatories with further cooperating observatories providing lists of positions of secondary comparison stars to aid the work of measuring the path plates.

At Greenwich, photographic observations of Eros preponderated over visual observations. As the observations were intended for the dual purpose of deriving the solar parallax and also the mass of the Moon, photographic plates were measured both in right ascension and declination. Two Greenwich photographic telescopes were used: the 26-inch Refractor and the 13-inch Astrographic, and reductions were made based on these observations, which were then compared with results from telescopes worldwide.

The solar parallax value established was the most accurately known value for the solar distance up to that time and remained a standard until 1968, when interplanetary radar began making direct distance measurements.