Parallax: If we view a star now, and then view it again 6 months later, our position will have changed by the diameter of the Earth’s orbit around the sun. For nearby stars (within 100 light-years or...

Parallax: If we view a star now, and then view it again 6 months later, our position will have changed by the diameter of the Earth’s orbit around the sun. For nearby stars (within 100 light-years or so), the change in viewing location is sufficient to make the star appear to be in a slightly different location in the sky. Half of the angle from one location to the next is known as the parallax angle (see Figure 1.5). Parallax can be used to measure the distance to the star. An approximate relationship is given by

where d is the distance in light-years, and p is the parallax measured in seconds of arc.