Astrometry is a part of astronomy and deals with the positions of
stars and other celestial bodies, their distances and movements. Part
of astrometry involves creating the cosmic distance ladder.
It is one of the oldest subfields of the science, the
successor to the more qualitative study of positional
astronomy. Astrometry dates back at least to Hipparchus,
who compiled the first catalogue of stars visible to
him and in doing so invented the brightness scale basically
still in use today. Modern astrometry was founded by
Friedrich Bessel with his Fundamenta astronomiae, which
gave the mean position of 3222 stars observed between
1750 and 1762 by James Bradley.
Apart from the fundamental function of providing Astronomers
with a reference frame to report their observations in,
astrometry is also fundamental for fields like celestial
mechanics, stellar dynamics and galactic astronomy. In
observational astronomy, astrometric techniques help
identify stellar objects by their unique motions. It
is also instrumental for keeping time, in that UTC is
basically the atomic time synchronized to Earth's rotation
by means of exact observations.
Astronomers use astrometric techniques for the tracking
of near-Earth objects. It has been also been used to
detect extrasolar planets by measuring the displacement
they cause in their parent star's apparent position on
the sky, due to their mutual orbit around the center
of mass of the system. NASA's planned Space Interferometry
Mission will utilize astrometric techniques to detect
gas giants around other stars, and perhaps even terrestrial
Astrometric measurements are used by astrophysicists
to constrain certain models in celestial mechanics. By
measuring the velocities of pulsars, it is possible to
put a limit on the asymmetry of supernova explosions.
Also, astrometric results are used to determine the distribution
of dark matter in the galaxy.
Astrometrics is the science of stellar measurements
and motion. Astrometrics was used, during the 1990s,
to detect extrasolar gas giants orbiting various solar
systems. This was done by observing the "stellar
wobble" of a star and calculating what kinds of
gravitational forces would cause such motion; it was
then determined that planetary forces must be affecting
the stars in question.