![]() Section 7 of this chapter describes how astronomers measure distances to more distant objects. However, most stars even in our own galaxy are much further away than 1000 parsecs, since the Milky Way is about 30,000 parsecs across. Space based telescopes can get accuracy to 0.001, which has increased the number of stars whose distance could be measured with this method. This limits Earth based telescopes to measuring the distances to stars about 1/0.01 or 100 parsecs away. Parallax angles of less than 0.01 arcsec are very difficult to measure from Earth because of the effects of the Earth's atmosphere. Limitations of Distance Measurement Using Stellar Parallax This simple relationship is why many astronomers prefer to measure distances in parsecs. The distance d is measured in parsecs and the parallax angle p is measured in arcseconds. There is a simple relationship between a star's distance and its parallax angle: d = 1/ p Stellar parallax diagram, showing how the 'nearby' star appears to move against the distant 'fixed' stars when Earth is at different positions in its orbit around the Sun. The star's apparent motion is called stellar parallax. Astronomers can measure a star's position once, and then again 6 months later and calculate the apparent change in position. This definition is the same as the apparent motion that would be observed if the two observation points were the sun and Earth. As the Earth orbits the Sun, a nearby star will appear to move against the more distant background stars. This effect can be used to measure the distances to nearby stars. Your hand will appear to move against the background. ![]() ![]() Another way to see how this effect works is to hold your hand out in front of you and look at it with your left eye closed, then your right eye closed. ![]()
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