# Yrjö Väisälä

Yrjö Väisälä (Finnish: ; 6 September 1891 – 21 July 1971) was a Finnish astronomer and physicist.
His main contributions were in the field of optics. He was also active in geodetics, astronomy and optical metrology. He had an affectionate nickname of Wizard of Tuorla (Observatory/Optics laboratory), and a book with the same title in Finnish describes his works. His discoveries include 128 asteroids and 3 comets.
His brothers were mathematician Kalle Väisälä (1893–1968) and meteorologist Vilho Väisälä (1889–1969). His daughter Marja Väisälä (1916–2011) was an astronomer and discoverer of minor planets.
Väisälä was a fervent supporter of Esperanto, presiding over the Internacia Scienca Asocio Esperantista ("International Association of Esperanto Scientists") in 1968.

## Optician

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 Contemporary to Bernhard Schmidt's design, but unpublished was also Prof.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 (also see photographic 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 Multiple Mirror Telescope.

## Geodesy

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 5 to 10 kilometres (16,000 to 33,000 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 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 telescopes, 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 quasars.
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.)

## Astronomer

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.)
Yrjö Väisälä is credited by the Minor Planet Center with the discovery of 128 asteroids (see below) during 1935–1944. He used to name them with the names of his personal friends that had birthdays. One of them was the professor Matti Herman 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.
Besides minor planets, he has also discovered 3 comets. The parabolic comet C/1944 H1 observed in 1944 and 1945, as well as the two short period comets, 40P/Väisälä, a Jupiter-family comet, and C/1942 EA, a Halley-type and near-Earth comet. Together with Liisi Oterma he co-discovered the Jupiter-family comet 139P/Väisälä–Oterma, which was first classified as asteroid and received the provisional designation "1939 TN".

## Honors and awards

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 is named after him, and so are the minor planets 1573 Väisälä and 2804 Yrjö.

## List of discovered minor planets

| 1391 Carelia | 16 February 1936 | list |
| 1398 Donnera | 26 August 1936 | list |
| 1405 Sibelius | 12 September 1936 | list |
| 1406 Komppa | 13 September 1936 | list |
| 1407 Lindelöf | 21 November 1936 | list |
| 1421 Esperanto | 18 March 1936 | list |
| 1424 Sundmania | 9 January 1937 | list |
| 1446 Sillanpää | 26 January 1938 | list |
| 1447 Utra | 26 January 1938 | list |
| 1448 Lindbladia | 16 February 1938 | list |
| 1449 Virtanen | 20 February 1938 | list |
| 1450 Raimonda | 20 February 1938 | list |
| 1451 Granö | 22 February 1938 | list |
| 1453 Fennia | 8 March 1938 | list |
| 1454 Kalevala | 16 February 1936 | list |
| 1460 Haltia | 24 November 1937 | list |
| 1462 Zamenhof | 6 February 1938 | list |
| 1463 Nordenmarkia | 6 February 1938 | list |
| 1471 Tornio | 16 September 1938 | list |
| 1472 Muonio | 18 October 1938 | list |
| 1473 Ounas | 22 October 1938 | list |
| 1477 Bonsdorffia | 6 February 1938 | list |
| 1478 Vihuri | 6 February 1938 | list |
| 1479 Inkeri | 16 February 1938 | list |
| 1480 Aunus | 18 February 1938 | list |
| 1483 Hakoila | 24 February 1938 | list |
| 1488 Aura | 15 December 1938 | list |
| 1492 Oppolzer | 23 March 1938 | list |
| 1494 Savo | 16 September 1938 | list |
| 1495 Helsinki | 21 September 1938 | list |
| 1496 Turku | 22 September 1938 | list |
| 1497 Tampere | 22 September 1938 | list |
| 1498 Lahti | 16 September 1938 | list |
| 1499 Pori | 16 October 1938 | list |
| 1500 Jyväskylä | 16 October 1938 | list |
| 1503 Kuopio | 15 December 1938 | list |
| 1518 Rovaniemi | 15 October 1938 | list |
| 1519 Kajaani | 15 October 1938 | list |
| 1520 Imatra | 22 October 1938 | list |
| 1521 Seinäjoki | 22 October 1938 | list |
| 1523 Pieksämäki | 18 January 1939 | list |
| 1524 Joensuu | 18 September 1939 | list |
| 1525 Savonlinna | 18 September 1939 | list |
| 1526 Mikkeli | 7 October 1939 | list |
| 1527 Malmquista | 18 October 1939 | list |
| 1529 Oterma | 26 January 1938 | list |
| 1530 Rantaseppä | 16 September 1938 | list |
| 1532 Inari | 16 September 1938 | list |
| 1533 Saimaa | 19 January 1939 | list |
| 1534 Näsi | 20 January 1939 | list |
| 1535 Päijänne | 9 September 1939 | list |
| 1536 Pielinen | 18 September 1939 | list |
| 1541 Estonia | 12 February 1939 | list |
| 1542 Schalén | 26 August 1941 | list |
| 1548 Palomaa | 26 March 1935 | list |
| 1549 Mikko | 2 April 1937 | list |
| 1551 Argelander | 24 February 1938 | list |
| 1552 Bessel | 24 February 1938 | list |
| 1567 Alikoski | 22 April 1941 | list |
| 1631 Kopff | 11 October 1936 | list |
| 1646 Rosseland | 19 January 1939 | list |
| 1656 Suomi | 11 March 1942 | list |
| 1659 Punkaharju | 28 December 1940 | list |
| 1677 Tycho Brahe | 6 September 1940 | list |
| 1678 Hveen | 28 December 1940 | list |
| 1696 Nurmela | 18 March 1939 | list |
| 1699 Honkasalo | 26 August 1941 | list |
| 1723 Klemola | 18 March 1936 | list |
| 1740 Paavo Nurmi | 18 October 1939 | list |
| 1757 Porvoo | 17 March 1939 | list |
| 1883 Rimito | 4 December 1942 | list |
| 1928 Summa | 21 September 1938 | list |
| 1929 Kollaa | 20 January 1939 | list |
| 1947 Iso-Heikkilä | 4 March 1935 | list |
| 2020 Ukko | 18 March 1936 | list |
| 2067 Aksnes | 23 February 1936 | list |
| 2091 Sampo | 26 April 1941 | list |
| 2096 Väinö | 18 October 1939 | list |
| 2194 Arpola | 3 April 1940 | list |
| 2204 Lyyli | 3 March 1943 | list |
| 2243 Lönnrot | 25 September 1941 | list |
| 2258 Viipuri | 7 October 1939 | list |
| 2292 Seili | 7 September 1942 | list |
| 2299 Hanko | 25 September 1941 | list |
| 2333 Porthan | 3 March 1943 | list |
| 2379 Heiskanen | 21 September 1941 | list |
| 2397 Lappajärvi | 22 February 1938 | list |
| 2454 Olaus Magnus | 21 September 1941 | list |
| 2464 Nordenskiöld | 19 January 1939 | list |
| 2479 Sodankylä | 6 February 1942 | list |
| 2486 Metsähovi | 22 March 1939 | list |
| 2502 Nummela | 3 March 1943 | list |
| 2512 Tavastia | 3 April 1940 | list |
| 2535 Hämeenlinna | 17 February 1939 | list |
| 2638 Gadolin | 19 September 1939 | list |
| 2639 Planman | 9 April 1940 | list |
| 2678 Aavasaksa | 24 February 1938 | list |
| 2679 Kittisvaara | 7 October 1939 | list |
| 2690 Ristiina | 24 February 1938 | list |
| 2715 Mielikki | 22 October 1938 | list |
| 2716 Tuulikki | 7 October 1939 | list |
| 2733 Hamina | 22 February 1938 | list |
| 2737 Kotka | 22 February 1938 | list |
| 2750 Loviisa | 30 December 1940 | list |
| 2802 Weisell | 19 January 1939 | list |
| 2820 Iisalmi | 8 September 1942 | list |
| 2826 Ahti | 18 October 1939 | list |
| 2885 Palva | 7 October 1939 | list |
| 2898 Neuvo | 20 February 1938 | list |
| 2962 Otto | 28 December 1940 | list |
| 2972 Niilo | 7 October 1939 | list |
| 3037 Alku | 17 January 1944 | list |
| 3099 Hergenrother | 3 April 1940 | list |
| 3166 Klondike | 30 March 1940 | list |
| 3212 Agricola | 19 February 1938 | list |
| 3223 Forsius | 7 September 1942 | list |
| 3272 Tillandz | 24 February 1938 | list |
| 3281 Maupertuis | 24 February 1938 | list |
| 3522 Becker | 21 September 1941 | list |
| 3606 Pohjola | 19 September 1939 | list |
| 3897 Louhi | 8 September 1942 | list |
| 4181 Kivi | 24 February 1938 | list |
| 4266 Waltari | 28 December 1940 | list |
| 4512 Sinuhe | 20 January 1939 | list |
| 5073 Junttura | 3 March 1943 | list |
| 5153 Gierasch | 9 April 1940 | list |
| (6073) 1939 UB | 18 October 1939 | list |
| 6572 Carson | 22 September 1938 | list |

## Gallery

* An early 20th century wooden house in Joensuu at the corner of Sepänkatu and Papinkatu streets. The building was Väisälä brothers' home in 1904–19.
* A commemorative plaque attached to the building in 1976. The text reads: "At this place was Väisälä scientist brothers, Vilho, Yrjö and Kalle's, home in 1904–1919. Joensuu Lyceum ex-students".