  This tutorial covers the use of absolute astrometry, in particular from the
combination of the Hipparcos and Gaia missions, to identify faint companions to
nearby stars and to measure the masses and orbits of those companions. Absolute
astrometry has been used with increasing success to discover new planets and
brown dwarfs and to measure masses and orbits for systems with periods as long
as centuries. This tutorial summarizes the nature of the underlying astrometric
data, the approach typically used to fit orbits, and the assumptions about that
data implicit throughout the process. It attempts to provide intuition for the
sensitivity of astrometry as a function of stellar and companion properties and
how the available constraints depend on the character and quantity of data
available. This tutorial is written for someone with some background in
astronomy but with no more than a minimal acquaintance with astrometry or orbit
fitting.
