The diameter of the Sun over the past three centuries

17th century measurements of the solar diameter indicate values which are about 10” (0.5 per cent) larger than the present figure of approximately 1920”. The apparent enlargement is smaller (3”) for measurements made during the 19th century. Has the diameter of the Sun diminished significantly over the past three centuries or is the reason for the difference located in the methods used or the instruments employed during the period? In this paper, a comparison of 30 series of measurements, obtained at various epochs over the last three centuries, is made with about 900 modern measurements applying early techniques and instruments. This investigation reveals the necessity of instrumental corrections, notably the effect of diffraction. When these corrections are taken into account, a homogenised database, extending over three centuries, can be considered. This does not reveal any sensible secular variation in the solar diameter and leads to a result for the semi-diameter at 1 AU of 960”.0± 0”.1.