The Dollond transit instrument Bought: 1807 Company: Dollond, London

The main task of practical astronomy was to determine exact astronomical coordinates. This work demands both accurate clock and telescope. The location of the star in relation to the observer on the surface of the Earth is determined by two parameters: the star’s coordinates and the time of observation. As the Earth rotates, the horizontal coordinates of the celestial bodies also change: the Sun rises in the East, both its height and the azimuth increase. Reaching the South, the celestial body reaches its maximum height (culminates); then the height decreases until setting in the West. Supposing that the Earth rotates evenly (which is right to a very good accuracy), we get a comfortable method for determining the coordinates of stars: as one coordinate we take the time and the second coordinate – the height of culminations.

In the 17th century two important inventions were made allowing to raise the accuracy of these coordinates significantly. In 1657 Christian Huygens patented the pendulum clock, in 1704 Ole Römer constructed the telescope which could be rotated in the meridian and it was called the transit instrument. The transit instrument was the most important instrument in the observatories in the 18th and the 19th centuries. Also, the Tartu Observatory started its activities with obtaining the transit instrument. The builder of this oldest transit instrument was the student K. Williams who used J. Dollond’s 2.5 inch achromatic objective. The work was completed in 1805. The apparatus was quite successful and it was sold in 1823 after buying the Reichenbach-Ertel meridian circle.

With the transit instrument the moment of time when the celestial body was observed in the meridian (exactly in the Southern or the Northern direction) is fixed. The accuracy of measurement is guaranteed by the exact position of the axis of the apparatus (in the horizontal level, with the direction from the East to the West which is quite a complicated problem technically)

Transit instrument

The first task of Struve in 1813 was to install the Dollond transit instrument for use in the new observatory on the Dome Hill where it is until today. The transit instrument was used until World War 1. In the archives of the observatory there are 33 observation registers from the period of 1814-1909, all in all 3,284 pages. The transit instrument is now in its original place in the Eastern hall of the observatory, the optical parts (the objective and the ocular) are missing. The system for opening the wall and the roof hatches have been preserved in the original shape.