  Out of the three quantities that characterize the state of an isolated
gaseous body: pressure, temperature and volume the radius is the only one
directly measurable for the Sun, what is specially true in the optical window
and for ground base measurements. The Heliometer of Observatorio Nacional, in
Rio de Janeiro, measures the distance between two opposite limbs of the Sun in
the same field of view, through the reflection on a 10 cm parabolic mirror
split on its half and forming an appropriate angle. This configuration is free
from optical aberrations and focal variations along the measurement direction.
The mirrors are made on CCZ vitro-ceramic and the telescope structure is of
carbon steel, resulting that there is no flexion or temperature deformation.
The instrument is compact, and can perform hundreds of measurements per duty
day, around all heliolatitudes. It attains an accuracy on the solar radius of
0.01 arcsec, becoming the ideal instrument to monitor from the ground the solar
diameter, and to bridge satellites and astrolabes historical series of data. We
discuss the first years of regular observation, with emphasis on the
instrumental calibrations and on the statistic study of the derived time
series, attitude series, and solar geometry series. On basis of these series we
obtain how well the Heliometer and Solar Astrolabe results are matched.
