September 6: Yrjö Väisälä

Yrjö Väisälä

September 6, 1891 – July 21, 1971

Yrjö Väisälä was a Finnish astronomer and physicist.

His main contributions were in the field of optics, but he was also very active in geodetics, astronomy and optical metrology. He even had an affectionate nickname of Wizard of Tuorla (Observatory/Optics laboratory), and there is a book with the same title in Finnish describing his works.

He developed several methods for measuring the quality of optical elements, as well as a lot of practical methods of manufacturing said elements. This allowed the construction of some of the earliest high-quality Schmidt cameras, in particular a "field-flattened" version known as Schmidt-Väisälä camera.

Contemporary to Bernhard Schmidt's design, but unpublished was also Prof. Yrjö Väisälä's identical design which he had mentioned in lecture notes in 1924 with a footnote: "problematic spherical focal surface".

Once he saw Schmidt's publication, he promptly went ahead and "solved" the field flattening problem by placing a doubly-convex lens slightly in front of the film holder (back in the 1930s, astronomical films were glass plates.) The resulting system is known as the Schmidt-Väisälä camera or sometimes as the Väisälä camera. (This solution is not perfect, as images of different colour end up at slightly different places.)

Prof. Väisälä made a small test unit of 7 mirrors in a mosaic on stiff background steel frame, however it proved to be impossible to stabilize as "just adjust and forget" structure, and next time anybody tried it, was with active controls on the Multiple Mirror Telescope.

The big Schmidt-Väisälä telescope he built was used at the University of Turku for searching asteroids and comets. His research group discovered 7 comets and 807 asteroids.

For this rather massive photographic survey work, Prof. Väisälä developed also a protocol of taking two exposures on same plate some 2–3 hours apart and offsetting those images slightly. Any dot-pairs that differed from background were moving, and deserved follow-up photos. This method halved the film consumption compared to method of "blink comparing", where plates get single exposures, and are compared by rapidly showing first and second exposures to human operator. (Blink-comparing was used to find e.g. Pluto.)

The University of Turku Astronomy department is known as VISPA: Väisälä Institute for Space Physics and Astronomy in honour of its founder.

The Lunar crater Väisälä and the asteroid 1573 Väisälä are also named in his honor.

Yrjö Väisälä discovered 128 asteroids. He used to name them with the names of his personal friends that had birthdays. One of them was the professor Matti Palomaa, after whom an asteroid 1548 Palomaa was named. (For this reason the Palomar Mountain Observatory in California has never had an asteroid bearing its name – the rules for naming asteroids state that the names have to differ from each other with more than one letter.)

July 31: Friedrich Robert Helmert

Friedrich Robert Helmert

July 31, 1843 – June 15, 1917

Friedrich Robert Helmert was a German geodesist and an important writer on the theory of errors.

Helmert was born in Freiberg, Kingdom of Saxony. After schooling in Freiberg and Dresden, he entered the Polytechnische Schule, now Technische Universität, in Dresden to study engineering science in 1859. Finding him especially enthusiastic about geodesy, one of his teachers, August Nagel, hired him while still a student to work on the triangulation of the Erzgebirge and the drafting of the trigonometric network for Saxony. In 1863 Helmert became Nagel's assistant on the Central European Arc Measurement. After a year's study of mathematics and astronomy Helmert obtained his doctor's degree from the University of Leipzig in 1867 for a thesis based on his work for Nagel.

In 1870 Helmert became instructor and in 1872 professor at RWTH Aachen, the new Technical University in Aachen. At Aachen he wrote Die mathematischen und physikalischen Theorien der höheren Geodäsie (Part I was published in 1880 and Part II in 1884). This work laid the foundations of modern geodesy.

The method of least squares had been introduced into geodesy by Gauss and Helmert wrote a fine book on least squares (1872, with a second edition in 1907) in this tradition. Hald (p. 633) gives this assessment: "[It] is a pedagogical masterpiece; it became a standard text until it was superseded by expositions using matrix algebra." In 1876 Helmert published an article deriving the distribution of the sample variance for a normal population. The work was described in German textbooks, including his own, but the English statisticians 'Student' and Fisher did not know of it and re-derived the distribution.

From 1887 Helmert was professor of advanced geodesy at the University of Berlin and director of the Geodetic Institute.

Helmert received many honours. He was president of the global geodetic association of "Internationale Erdmessung", member of the Prussian Academy of Sciences in Berlin, was elected a member of the Royal Swedish Academy of Sciences in 1905, and recipient of some 25 German and foreign decorations.