  Several group in the World followed the solar diameter with dedicated
instruments, namely solar astrolabes, since 1974. Their data have been gathered
in several observing stations connected in the R2S3 (Reseau de Suivi au Sol du
Rayon Solaire) network and through reciprocal visits and exchanges: Nice/Calern
Observatory, Rio de Janeiro Observatorio Nacional/Brazil, IAG/Universidade de
Sao Paulo/Brazil, Antalya Observatory/Turkey, San Fernando/Spain. The tradition
of these observational efforts is here briefly sketched with the aim to
evidence the possibility to analyze against the solar activity all these 42
years data at once by overcoming the problem of the shift between the different
series. Each instrument has its own density filter with a prismatic effect
responsible of that shift. The overall change of the solar radius during the
last 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 daily
measurements has been and it is crucial nowadays with Picard-sol and the
Reflecting Heliometer of Rio de Janeiro, to ensure an homogeneus and continuous
monitor of the solar diameter without the lifetime limitation of the
satellites.
