Editing Yrjö Väisälä (section)

== Geodesy ==
[[File:Vaisala diary.JPG|thumb|left|A laboratory diary of Yrjö Väisälä. The text is written in 1929. On the pages seen here Väisälä describes the principle of 'a new telescope for photography'. Väisälä never published this concept and few years later Estonian [[Bernhard Schmidt]] invented the same construction which is now known as the [[Schmidt camera]].]]

In the 1920s and 1930s [[Finland]] was doing its first precision triangulation chain measurements, and to create long-distance vertices Prof. Väisälä proposed usage of flash-lights on {{convert|5|to|10|km|ft}} altitude balloons, or on some big fireworks rockets. The idea was to measure the exact position of the flash against background stars, and by precisely knowing one camera location, to derive an accurate location for another camera. This required better wide-field cameras than were available, and was discarded.

Later, Prof. Väisälä developed a method to multiply an optical length reference using [[white light interferometry]] to precisely determine lengths of baselines used in triangulation chains. Several such baselines were created in Finland for second high-precision triangulation campaign in 1950s and 1960s.

Later GPS made these methods largely obsolete. The [[Nummela Standard Baseline]] established by Väisälä is still maintained by the [[Finnish Geodetic Institute]] in [[Nummela (Vihti)|Nummela]] for the calibration of other distance measurement instruments.

Prof. Väisälä also developed excellent tools to measure earth rotational axis position by building so called [[zenith telescope]]s, and in the 1960s Tuorla Observatory was in the top rank of North Pole position tracking measurements.

In the 1980s radioastronomy was able to replace Earth rotation tracking by referring things against "non-moving background" of [[quasar]]s.

For these Zenith Telescopes, Prof. Väisälä made also one of the first experiments at doing mirrors of liquid mercury. (Such mirror needs extremely smooth rotational speeds which were achieved in the late 1990s.)