If you hold your thumb out at arm's length and then alternately open and close each eye while looking at it, you will see that your thumb apparently moves or shifts against the background. This shift is called a parallax shift. As you move your thumb in closer to your nose and repeat the process, you should notice that the shift gets bigger. Astronomers can use this same technique to measure distances to the stars. As the Earth orbits the sun, a given star's position changes against the background of other stars. By comparing photographs of the star at six-month intervals, astronomers can measure the degree of the shift and obtain the angle of parallax (half the parallax shift = theta or Θ). By knowing the angle of parallax and the radius of the Earth’s orbit (R), astronomers can calculate the distance to the star (D) using trigonometry: D = R x cotangent (theta) or D = RCotΘ. Parallax measurements are reliable for stars with distances less than or equal to 50 parsecs. For distances greater than this, astronomers must find variable star markers and use the luminosity-distance relationships (see next page).
Early Milky Way Theories
As we mentioned, Galileo discovered that the Milky Way is made of dim stars, but what about its shape? How can you tell the shape of something if you're inside it? In the late 1700s, astronomer Sir William Herschel addressed this question. Herschel reasoned that if the Milky Way was a sphere, we should see numerous stars in all directions. So, he and his sister Caroline counted the stars in more than 600 areas of the sky. They found that there were more stars in the directions of the band of the Milky Way than above and below. Herschel concluded that the Milky Way was a disk-shaped structure. And because he found about the same numbers of stars in all directions along the disk, he concluded that the sun was near the center of the disk.
Around 1920, a Dutch astronomer named Jacobus Kapetyn measured the apparent distances to nearby and remote stars using the technique of parallax. Because parallax involved measuring the motions of stars, he compared the motions of distant stars with nearby ones. He concluded that the Milky Way was a disc approximately 20 kiloparsecs, or 65,000 light years, in diameter (one kiloparsec = 3,260 light years). Kapetyn also concluded that the sun was at or near the center of the Milky Way.
But future astronomers would question these ideas, and advanced technology would help them dispute the theories and come up with more accurate measurements.