The Repsold Heliometer Bought: 1873 Company: Repsold and Söhne, Hamburg

At the beginning of the 18th century one of the main aims of astronomical observations was to describe as accurately as possible the Solar System. It was necessary to carry out exact measuring of the orbits of planets, but, however, there were difficulties with measuring the main parameter – the distance of the Earth from the Sun. So the parameters of the planet system were recorded in relative units, the main unit being the average distance of the Earth from the Sun, the so-called astronomical unit. The fact how big the exact distance in earthly units was (for example, in kilometers) remained a secret.

The distances of celestials bodies are determined measuring their parallax – the shift of the star or the planet in relation to other (more distant) stars. In the case of the Sun this method cannot be used because next to the bright Sun all other celestial bodies cannot be seen. In 1716 Edmund Halley recommended that for determining the distance the passing of the inner planets the Venus and the Mercury over the disc of the Sun should be used, measuring in different geographical points the arrival of the planet on the disc of the Sun as exactly as possible. The best moment was the so-called “second contact” – the moment when the edge of the planet is on the outer edge of the disc of the Sun.

The passing of the Venus over the Sun is repeated after 130 years. The first possibility to use Halley’s method came in 1761. To make the measurement even more accurate, John Dollond designed and built a special instrument, which he called the heliometer, in 1754. The “working organ” of the heliometer is the objective sawn into halves, the halves of which can be moved along the diameter. The two parts of the objective function as two lenses which form the images of the star in the telescope. The distance between them is determined by the size of the shift between them.

F. W. Bessel was the first to understand that the heliometer can also be used for determining the angular distance between the stars. Bessel became the director of the Observatory of Königsberg in 1810 (being 26 years old!). Besides buying other instruments, he also purchased the heliometer – it was built in the workshop of Joseph Fraunhofer. Its build was identical with the refractor of Tartu. The good observer and even a better mathematician, Bessel took this instrument into the history; for the first time the star parallax was reliably measured with it and it was predicted on the basis of the star’s movement (Sirius) that an unseen satellite existed. During Bessel`s life-time the Venus did not pass over the Sun….

The Tartu heliometer. When the astronomers of the whole world prepared for observing the pass of the Venus over the Sun in 1874, the director of the Pulkovo Observatory Otto Struve (Wilhelm Struve’s son) ordered three similar heliometers, one of them was received by the Tartu observatory and it reached Tartu in 1873.

Observations. The telescope belonged to the Tartu observatory but neither passing of the Venus over the Sun in the 19th century was observable in Tartu. In the autumn of 1874 the heliometer started its trip to South-Eastern Siberia lasting for two months. The Venus passing over the Sun was watched on 8 December 1874. The conditions for observation were favorable only in 13 observation stations, including Nerchinsk on the border of China where the director Schwarz of the Tartu Observatory also went. The equipment of the Tartu expedition consisted of the Repsold heliometer, a smaller telescope, the Ertel transit instrument and two chronometers. The local employee of the meteorological observatory helped Schwarz. Observations took place at –18º C. Unfortunately the expedition did not give the expected results because the damages of the transit instrument caused errors in determining the temporal moments.

In 1876 the above mentioned unique instrument was sent as requested by Repsold to London to the exhibition of scientific apparatuses. In 1922 the instrument was temporarily borrowed by a French expedition which went to the island of Martinique in the Caribbean Sea to watch the passing of the Venus over the disc of the Sun. The expedition was led by the famous French astronomer François Tisserand (1843-1896), but the data about the observation are missing.

In 1884–1885 the astronomer-observer Ernst Hartwig of the Tartu observatory worked with the heliometer. Unfortunately the data of his observations have not survived and this is why it is difficult to establish the volume of the whole work done. He studied the errors of the instrument, established the limb diameters of the Sun, the Saturn and other planets. He dealt with the determination of the location of the supernova discovered in the Andromeda nebula, the determination of the position of the Saturn in relation to the star ξ Taur, the measurement of the satellites of the Jupiter, the determination of the location of the Moon’s crater Mösting A with respect of the edge of the Moon, etc. Hartwig did not publish scientific papers about this work.

After Hartwig’s departure the heliometer was not used at all. When in 1915 the university properties were evacuated, the heliometer was also sent to Nizhny Novgorod. In 1921 the box with the tube and the objective of the heliometer were given back but the oculars and other technical parts had been lost. Quickly developing astrophotography had ousted the visual methods of exact measurement at that time.

The remaining parts of the Tartu heliometer
The Repsold mechanism for moving the parts of the objective