  The balloon-borne Solar Disk Sextant (SDS) experiment has measured the
angular size of the Sun on seven occasions spanning the years 1992 to 2011. The
solar half-diameter -- observed in a 100-nm wide passband centred at 615 nm --
is found to vary over that period by up to 200 mas, while the typical estimated
uncertainty of each measure is 20 mas. The diameter variation is not in phase
with the solar activity cycle; thus, the measured diameter variation cannot be
explained as an observational artefact of surface activity. Other possible
instrument-related explanations for the observed variation are considered but
found unlikely, leading us to conclude that the variation is real. The SDS is
described here in detail, as is the complete analysis procedure necessary to
calibrate the instrument and allow comparison of diameter measures across
decades.
