Struve and the Tartu meridian

(Read at a leaflet from the Tartu tourist office, in Estonia)

F. G.Wilhelm von Struve, astronomer and initiator of the Struve Geodetic Arc, now a UNESCO World Heritage Site. 

On the floor of the entrance hall of Tartu Observatory there is a marker commemorating "Struve's astro-geodetical point". A few meters above this brass plaque, in 1825, F.G.W Struve had installed the world's largest telescope at the zero point of what was to become the Tartu Meridian - a breakthrough measurement of the longitude of planet Earth.

At the time, Tartu was known as Dorpat, and the Baltic-German astronomer Struve was using the old French "toise" (about 2 meters) as the unit of measurement for length. Around this time in France the "meter" was being introduced as a much more logical unit defined as 1/10.000.000th of the Earth's meridian along a quadrant. But, the question remained: how long was 1/4 of the polar circumference of the Earth? After decades of surveying work, Struve provided an amazingly accurate result for the shape and size of the globe. The size and shape of the earth was calculated by F.W. Bessel in 1840 using Struve's data. This work remained as a major baseline for all international mapping until the advent of the Global Positioning System pioneered in the 1960s.

The significance of Struve's achievement is only recently being publicly understood - in 2005 the Struve Arc was accepted onto the UNESCO World Heritage List and in 2006 a commemorative marker was unveiled at Tartu Observatory.

From 1816 to 1855 Struve directed the measurement of a meridian arc of over 2,820 km, stretching from the vicinity of Hammarfest in northern Norway to near Izmail on the Black Sea. Struve's work is amazing on many levels: accuracy, scale and historical significance.

The need to maximise accuracy was supreme and the most modern equipment and telescopes were used for triangulation. The basic method was to physically mark out precise baselines - for example, frozen lake surfaces in Finland provided excellent horizontal surfaces on which to measure very long and precise baselines. The baselines were calibrated with a special tool constructed by Struve. The tool was calibrated with a sample of one "toise" measure brought from Paris. Next, by a combination of astronomical measurements and triangulation to surface points, a long north-south chain of measurements could be made. Over the years this chain was spread so long that it was possible to accurately calculate the length and curvature of the earth.

In its time, the Struve Arc was the longest measurement of the form and size of our globe. It comprised 258 main arcs with 265 triangulation points in all, with 65 ancillary points. The Arc measurement traverses many countries: nowadays Norway. Sweden, Finland, Russia, Estonia, Latvia, Lithuania, Belarus, Moldova and Ukraine.

Struve's work is remarkable as an early example of international scientific cooperation. Struve built on existing surveying work and linked up with work which was managed by C. Tenner in Latvia, Lithuania, Belarus, Ukraine and Moldova for the southern part of the meridian. Over the years he managed to extend the length of the arc, time and again being given funding by tsars and kings and the support of military teams to survey parts of the arc. These were times when good maps were becoming of increasing strategic military advantage, and when clearly defined national borders were part of political alliances as Europe began to reorganize itself into nation-states during the 19th century.

The early 1800s was a time when the acceleration of scientific knowledge was becoming evident. Advances in hardware technology and applied mathematics allowed conceptual theories to be tested. However, there was also a background of great uncertainty for Struve's work - there was debate about the shape of the earth, debate about the location of the prime meridian (Paris or Greenwich) and even confusion about basic units of measure. Imperial measures had basic failings - it was not possible to define the exact length of an English "foot" or a French "toise" and all over Europe there was a patchwork of different measurement systems. Struve's work helped to prove that Newton was correct when he theorized that the shape of the Earth was an oblate spheroid.

As a scientist, Struve is as highly regarded as Isaac Newton in many countries. Similarly, the Struve Arc, or the Tartu Meridian, is of as much scientific, historical and practical importance as the Greenwich Meridian (which was adopted as an international standard some 50 years after Struve's work in 1884).

Having made the final significant measurement with the French "toise" system, Struve contributed to the international acceptance of the metric system.

Only two other physical markers of Struve’s triangulation points survive in Estonia. These are both endpoints of the Simuna-Voivere baseline in Lääne-Viru County. At Simuna the endpoint is marked by a 1.9 m high granite column with the number "1849". The second endpoint in Voivere was thought to have been destroyed but it was found by GPS measurements carried out in 2001. A granite marker with a hole drilled in its centre was found buried underground. Struve's baseline measured in 1827 differed from modem measurements by only 14 mm over the 4.5 km length.

Today much has changed. With an internet connection and laptop computer, one could even track their own journey in real-time using Google Earth, a virtual globe of satellite images. This would be entertaining on a day- trip, but trivial compared to the highly significant achievement and 40 years of hard work by Struve.