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Niels Bohr Institute > Who, What, When > > Surveying the sky > Astrometry

23 November 2010


Astrometry is the branch of astronomy that Tycho Brahe practised by measuring the position of stars. With precise measurements over several years you also get the movements and distances of the stars. The other branches of astronomy are known collectively as astrophysics.

The Meridian circle at Østervold in Copenhagen, which Bengt Strömgren worked with in 1925, when he was 17 years old. On the right are the slits across whichs the star glides. Bengt Strömgren also began with astrometry and he always promoted astrometry, but he soon became an eminent astrophysicist. For six years he was the director of American observatories, he took over Albert Einstein's office at the Institute for Advanced Study in Princeton, and lived in the Carlsberg Residence of Honour from 1967 until his death in 1987.

Astrometric results are used in all branches of astronomy to get a physical understanding of the structure of the entire universe - of stars, which are glowing balls of gas - of planets orbiting the Sun and other stars - and of galaxies, which are rotating solar systems, etc.

An important new development in astrometry began in 1925 when Bengt Strömgren made some experiments on the old meridian circle in Copenhagen. A meridian circle is a telescope mounted in a special way to make measurements of positions and was invented by Ole Rømer 300 years ago. The telescope sits perpendicular on an axis, which rests on a column in the east and a column in the west, so that you can only see stars when they cross the north-south direction.

The observer sees a star enter into the field of view and it glides through due to the rotation of the Earth. He measures the exact time that the star passes across the meridian and he measures the tilt of the telescope in relation to the horizontal axis by taking a reading of an accurately divided circle, attached to the axis. These two measurements give the position of the star corresponding to a place's longitude and latitude on Earth.

Meridian circle in Brorfelde. Erik Høg was the first to observe using the new meridian circle in Brorfelde, already in August 1954, when he was 22 years old. He was completely alone out there and sometimes slept in a haystack after the observations. He was a student of Peter Naur and learned a lot from him. They had a good working relationship developing the meridian circle, but Erik Høg had a completely different opinion of the work than Peter Naur, who thought that it was incredibly boring. But for Erik Høg it was just the thing. He was interested in the development of instruments and he knew that the work was important for the development of astronomy.

New method of measurement

Bengt Strömgren was a teenager, just 17 years old, when he showed that stellar positions can be measured when you let a star glide over some slits in the meridian circle. You can capture the light of the star with a photocell that converts the light to an electric current. The timing of the star's passage over each slit is used to find the position of the star, and this can in principle be measured through the changes in the electric current.

Strömgren's photoelectric technique was the best of his time, but was not yet developed enough that it was practical. But Strömgren's photoelectric technique was the start of an entirely new method of measurement that revolutionised astrometry.

Erik Høg, an astronomer at the University of Copenhagen, knew the principle from his student days and in 1960 he was able to adapt Strömgren's idea to counting photons instead of measuring an electric current and this worked well on meridian circles and also later on the Hipparcos satellite.

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