What is Gaia?
Gaia is a satellite that the European Space Agency will launch about 2011. It will measure distances, positions, and velocities of stars in our Galaxy, the Milky Way, to create the most accurate 3-D picture of our Galaxy that we've ever had.
Why Bother Measuring Distances and Velocities?
Because knowing the distance to a star allows us to determine many of the essential properties (age, mass, true luminosity, etc) of the star. Velocities give us information about where the star was millions of years ago and where it will be in the future. By measuring these quantities Gaia will determine the nature, formation history and evolution of the Milky Way.
How Do We Measure the Distance to a Star?
Astronomers use a quantity called the stellar parallax. It is the apparent angular displacement of a star in the sky when viewed from opposite points of the Earth's orbit around the Sun. Stellar parallax can be converted into distance by using simple geometry.