World Science Scholars
1.4 Parallax

You’ve seen briefly how the method of stellar parallax uses the orbit of the Earth to observe an apparent “shift” in position of an object against the background of other distant objects. The angle at which we observe it changes as the Earth revolves around the Sun, most noticeably at periods six months apart, when the Earth is on opposite sides of the Sun. The distance between the Sun and that object can then be calculated trigonometrically, having measured the parallax angles and knowing the distance between the Sun and Earth. In this demonstration, you can control the unknown distance of a nearby celestial body. Notice how the parallax angle will change based on the object’s distance — it’s smaller for further objects and larger for closer ones. Then, using simple geometry, the distance $x$ is calculated and displayed below. Stellar parallax is a fairly accurate technique for objects closer than ~100 parsecs (326 light-years) but is not very useful beyond that, because the angle of parallax becomes so small.

Note that for the sake of demonstration, the distance from the Earth to the object in question is much smaller (and, correspondingly, the parallax angle is much larger) than it would normally be. One astronomic unit is roughly eight light minutes — nowhere close to stellar parallax’s maximum distance of ~326 light years!


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