On the occasion ofthe second edition ofthe book, it appeared necessary
to up- date information that was already seven years old. Astrometry has
recorded tremendous advances during these last years, so that, in
addition to cor- recting identified errors and misprints, there are many
major modifications. Among the events that forced me to modify
significantly the contents of the book, the most important are the
release of the Hipparcos and Tycho cata- logues, the introduction of CCD
astrometry, the decision of the International Astronomical Union to
adopt a new celestial reference frame, the dramatic improvement of
accurate time and frequency standards, the decision taken by space
agencies to prepare several new space astrometry satellites and the
development of optical interferometry. The description and the
consequences of these events have been included in this edition. One of
them is that a mi- crosecond of arc or microarcsecond (uas) has become a
widely used unit. On the contrary, the result was also that the
importance of some in- struments such as astrolabes or transit circles
has decreased. However, I left but because their description unchanged,
not only for their historical interest, newer techniques often use
similar data reduction methods so that one can refer to them.
Conversely, some methods or instruments have evolved and new information
is included. Finally, many new references were added to the original
list.
