Measurement in Space
How do scientists determine distance to stars from the solar system Earth is part of? One way is to measure its parallax--the apparent shift in the position of an object when viewed from two different positions. Extend your arm and look at your thumb first with your left eye closed and then with your right eye closed, as the girl in Figure 3A is doing. Your thumb appears to change position with respect to the background. Now do the same experiment with your thumb closer to your face, as shown in Figure 3B. What do you observe? The nearer an object is to the observer, the greater its parallax is.
Astronomers can measure the parallax of relativity close stars to determine their distances from Earth. Figure 4 shows how a close star's position appears to change. Knowing the angle that a star's position changes and the size of Earth's orbit, astronomers, astronomers can calculate the distance of the star from Earth.
Because space is so vast, a special unit of measure is needed to record distances. Distances between stars and galaxies are measured in light-years. A light year is the distance that light travels in one year. Light travels at 300,000 km/s, or about 9.5 trillion km in one year. The nearest star to Earth, other than the Sun, is Proxima Centauri. Proxima Centauri is a mere 4.3 light years way, or about 40 trillion km.
Figure 4
Parallax can be seen if you observe the same star when Earth is at 2 different points during its orbit around the Sun. The star's positon relative to more distant background stars will appear to change. Is star A or B farther from Earth?
