One of SIM's key goals is to measure accurate distances of stars throughout the Milky Way Galaxy. The technique is very different from measuring distances on Earth where we can use a measuring tape to see how long a driveway is. But even on Earth we often need to measure distances between landmarks that we can see but cannot reach. Take the width of a river as an example. Land surveyors have developed a simple geometric method for this situation, called parallax. Surveyors start by measuring the angle to a landmark from two different vantage points. They can then calculate the distance to the landmark from the difference of these two angles and the distance between the two vantage points. The distance between the two vantage points is called the length of the baseline. The longer the baseline the more accurate is the measurement of the distance.
To measure the immense distances between stars to any accuracy we need to measure tiny angular changes over very long baselines. The longest baseline astronomers can use from the ground is the diameter of Earth's orbit around the Sun. As Earth revolves every year around the Sun, we find ourselves now on one side of the solar system and six months later on the opposite side. This orbit takes us 300 million kilometers or 187 million miles away from where we start out. As we change our vantage point in this journey, six months later some stars appear to be in a different position. Nearby stars appear to move more than distant stars. Stars very far from Earth do not seem to change position at all.
To detect these subtle changes we need to measure the positions of stars with great precision. The Space Interferometry Mission will measure the position of stars with an accuracy of 4 microarcseconds. Take a moment to consider this. A full circle is divided into 360 degrees. Each degree is divided into 60 arcminutes and every arcminute into 60 arcseconds. Therefore a full circle holds 1,296,000 arcseconds. Finally, a microarcsecond is one-millionth of an arcsecond. This accuracy will enable SIM to determine stellar distances (also called parallaxes) to 10 percent accuracy out to a distance of 482,000 million million miles (or, in astronomical units, 25,000 parsecs). With this accuracy SIM would be able to measure the distance to any sufficiently bright object anywhere in our Milky Way Galaxy. This is an improvement of several hundred times over what is possible today.
