Measuring distances between stars is very different from measuring distances on Earth where we can lay out yardsticks or pull out a measuring tape to see, for example, how far apart two trees are. However,even on Earth we often measure distances between inacessible points that we can see but that we cannot reach. The distance between two mountain tops or the width of a river are good examples. Land surveyors have developed a simple geometric method for such situations, called parallax. They measure the angle to a distant object from two different vantage points. A surveyor can then calculate the distance to the object from the difference of these two angles and the separation between the two vantage points, or baseline. The longer the baseline is, the more accurate the measurement will be.

To measure the huge distances to stars we need a very long baseline. Actually, we are already travelling along an enormous baseline. Every year, Earth completes its journey around the Sun. In the spring we find ourselves on one side of the solar system and six months later in the fall we are on the opposite side of the Sun 300 million kilometers or 187 million miles away from where we started out in the spring. Because we have now changed our vantage point, some stars appear to be in a different position in the fall than they were in the spring. Nearby stars appear to have moved more than more distant stars. The most distant stars do not seem to change positions at all.

Because these changes are small, and get smaller the farther a star lies away from the Sun, we need to measure the positions of distant stars very precisely to determine their distance. The Space Interferometry Mission will measure the position of stars with an accuracy of 4 microarcseconds and will see parallaxes at a 10 percent accuracy level out to a distance of 482,000 million million miles (or in astronomical units 25,000 parsec). In other words, SIM would be able to see the grass in your yard grow every second, from as far away as 10 kilometers, or more than 6 miles. With this accuracy SIM will be able to measure the distance to any object in our Milky Way Galaxy, as long as it's bright enough. This is an improvement of several hundred times over what is possible today.

Once we have measured a reliable distance, we can also determine the brightness of a star. Think of a very dark night. As long as you do not know how far away from you a light is, a small flashlight close to you will appear as bright or brighter than the glaring headlights of a distant truck. In the same way, astronomers need to know the distance to a star. Only if the distance is known, they will know whether they are looking at a dull nearby star or a bright distant star.

We have seen here that precise position measurement of stars lead to reliable distance determinations. From these distances we determine the true brightness of a star. And those determinations will teach us all about the chemical composition and the evolution of the stars. The key to all these investigations is precise position measurements.