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Several group in the World followed the solar diameter with dedicatedinstruments, namely solar astrolabes, since 1974. Their data have been gatheredin several observing stations connected in the R2S3 (Reseau de Suivi au Sol duRayon Solaire) network and through reciprocal visits and exchanges: Nice/CalernObservatory, Rio de Janeiro Observatorio Nacional/Brazil, IAG/Universidade deSao Paulo/Brazil, Antalya Observatory/Turkey, San Fernando/Spain. The traditionof these observational efforts is here briefly sketched with the aim toevidence the possibility to analyze against the solar activity all these 42years data at once by overcoming the problem of the shift between the differentseries. Each instrument has its own density filter with a prismatic effectresponsible of that shift. The overall change of the solar radius during thelast century is evident by comparing the Auwers' radius of 959.63" (1891,present IAU standard) with 959.94" (2015, from eclipses and Venus transit data,the latter either observed from space) and the role of ground-based dailymeasurements has been and it is crucial nowadays with Picard-sol and theReflecting Heliometer of Rio de Janeiro, to ensure an homogeneus and continuousmonitor of the solar diameter without the lifetime limitation of thesatellites.

42 Years of Continuous Observations of the Solar Diameter from 1974 to 2015