The Last Meridian Circles -- an Epilogue

TL;DR

This paper documents the history and contributions of meridian circles in astronomy over the last 35 years, highlighting their role before space-based astrometry took over, and includes accounts from colleagues involved.

Contribution

It provides a detailed historical account of the last 18 meridian instruments and their significance in astronomical measurements before space astrometry became dominant.

Findings

Meridian circles provided fundamental star positions for centuries.

Transition to space astrometry and VLBI has replaced meridian circles.

The paper includes personal accounts from colleagues involved in the last 35 years.

Abstract

The aim is to document in some detail the last 35 years of meridian circles, a type of instrument with a fundamental role in astronomy for a very long time, and to do so while witnesses are still alive and can contribute. This is about finding facts. Meridian circles provided fundamental star positions for centuries. These positions were tied to a well-defined celestial coordinate system of right ascension and declination, and accurate proper motions ensured a transformation of the positions over long periods of time. This function of the meridian circles has been taken over by space astrometry and VLBI. The Hipparcos astrometric satellite was approved by ESA in 1980 and launched to a successful 3-year mission in 1989. Its successor, Gaia, completed a mission of 10.5 years in January 2025. An account is given of the last 18 meridian instruments, which were active for some part of the 35 years between 1980 and 2015. This account is based on input kindly supplied in correspondence with many colleagues.