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Fireside Chat ‘A Quadrant like Blaeu’s: the construction, use and accuracy of a full-size replica’

September 28 @ 5:00 pm - 6:00 pm BST

Taking observations with Blaeu’s wooden quadrant replica (photo by Rasjied Sloot).

Presented by Nicolàs de Hilster, Independent Scholar, the Netherlands


Recently Nicolàs made a replica of Willebrord Snellius’ wooden quadrant that was made for him by Willem Jansz Blaeu at the beginning of the seventeenth century as a successor to the iron quadrant Snellius used in his measurement of the circumference of the Earth. This original wooden quadrant is now located in the Rijksmuseum Boerhaave in Leiden, the Netherlands.

The construction of the replica (on a full scale) was initiated at the request of the television program Jekels Jacht (broadcast date 12 June 2024, 10:17pm, NPO2) and was made with the assistance of students and teachers from the furniture construction department of the Horizon College in Heerhugowaard, the Netherlands and the chairman of the Heerhugowaard public observatory Saturn, Paul Volman.

Detail of the Blaeu quadrant replica (photo by N. de Hilster).

The replica of Snellius’ quadrant was built according to a design by Tycho Brahe, similar to what he used in his observatory on the island Hven. This means that observations are done with open sights, so there is no telescope on the instrument. All measurements made with this replica were carried out in this way and with the fireside chat he hopes to provide the listener with insight into the accuracy that can be achieved with such an instrument. Attention is paid to the possible sources of error and a distinction is made between horizontal (such as in triangulation) and vertical measurements (such as in altitude measurements).



Nicolàs de Hilster is a hydrographic surveyor by education and the founding owner of a company in specialized measurements in the hydrographic and geodetic field. In his private life he collects and researches nautical and geodetic instruments. The main focus of that research lies on instruments used for latitude determination at sea from Thomas Harriot to John Hadley and their contemporaries. Part of the research is creating reconstructions and replicas of the instruments and to use them in the field for a better understanding of their capabilities. In addition to that he also has a keen interest in geodetic instruments and their development from Martin Waldseemüller to Heinrich Wild and their contemporaries.

On 9 May 2018 Nicolàs graduated on the topic of the history and development of early modern navigational instruments. Since then, the focus of his research has shifted to astronomy, dealing with both its early-modern and modern history and the modern techniques available to amateur astronomers, which include equipment calibrations, instrument making, data acquisition and processing. In 2021 he joined the AAVSO’s Solar Observing Project that aims to re-calibrate historical sunspot observations. Observations for that project are done with the Galilean Type Telescope in his private observatory.

Apart from being a researcher Nicolàs is the Dutch representative and web-editor for the Scientific Instrument Society (SIS) and the web-editor for the Scientific Instrument Commission (SIC).


September 28
5:00 pm - 6:00 pm BST
Event Category:


On-line, for SIS-members only.


Louise Devoy
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