  The solar diameter has been monitored at the Royal Observatory of the Spanish
Navy (today the Real Instituto y Observatorio de la Armada: ROA) almost
continuously since its creation in 1753 (i.e. during the last quarter of a
millennium). After a painstaking effort to collect data in the historical
archive of this institution, we present here the data of the solar semidiameter
from 1773 to 2006, making up an extensive new database for solar-radius
measurements can be considered. We have calculated the solar semidiameter from
the transit times registered by the observers (except values of the solar
radius from the modern Danjon astrolabe, which were published by ROA). These
data were analysed to reveal any significant long-term trends, but no such
trends were found. Therefore, the data sample confirms the constancy of the
solar diameter during the last quarter of a millennium (approximately) within
instrumental and methodological limits. Moreover, no relationship between solar
radius and the new sunspot-number index has been found from measurements of the
ROA. Finally, the mean value for solar semidiameter (with one standard
deviation) calculated from the observations made in the ROA (1773-2006), after
applying corrections by refraction and diffraction, is equal to 958.87" \pm
1.77"
