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0001 <sect1 id="ai-parallax"> 0002 <sect1info> 0003 <author> 0004 <firstname>James</firstname> <surname>Lindenschmidt</surname> 0005 </author> 0006 </sect1info> 0007 <title>Parallax</title> 0008 <indexterm><primary>Parallax</primary></indexterm> 0009 <indexterm><primary>Astronomical Unit</primary><see>Parallax</see></indexterm> 0010 <indexterm><primary>Parsec</primary><see>Parallax</see></indexterm> 0011 <para> 0012 <firstterm>Parallax</firstterm> is the apparent change of an observed 0013 object's position caused by a shift in the observer's position. As an 0014 example, hold your hand in front of you at arm's length, and observe 0015 an object on the other side of the room behind your hand. Now tilt 0016 your head to your right shoulder, and your hand will appear on the 0017 left side of the distant object. Tilt your head to your left 0018 shoulder, and your hand will appear to shift to the right side of the 0019 distant object. 0020 </para> 0021 <para> 0022 Because the Earth is in orbit around the Sun, we observe the sky from 0023 a constantly moving position in space. Therefore, we should expect 0024 to see an <firstterm>annual parallax</firstterm> effect, in which the 0025 positions of nearby objects appear to <quote>wobble</quote> back and forth in 0026 response to our motion around the Sun. This does in fact happen, but 0027 the distances to even the nearest stars are so great that you need to 0028 make careful observations with a telescope to detect 0029 it<footnote><para>The ancient Greek astronomers knew about parallax; 0030 because they could not observe an annual parallax in the positions of 0031 stars, they concluded that the Earth could not be in motion around 0032 the Sun. What they did not realize was that the stars are millions of 0033 times further away than the Sun, so the parallax effect is impossible 0034 to see with the unaided eye.</para></footnote>. 0035 </para> 0036 <para> 0037 Modern telescopes allow astronomers to use the annual parallax to 0038 measure the distance to nearby stars, using triangulation. The 0039 astronomer carefully measures the position of the star on two dates, 0040 spaced six months apart. The nearer the star is to the Sun, the 0041 larger 0042 the apparent shift in its position will be between the two dates. 0043 </para> 0044 <para> 0045 Over the six-month period, the Earth has moved through half its orbit 0046 around the Sun; in this time its position has changed by 2 0047 <firstterm>Astronomical Units</firstterm> (abbreviated AU; 1 AU is 0048 the distance from the Earth to the Sun, or about 150 million 0049 kilometers). This sounds like a really long distance, but even the 0050 nearest star to the Sun (alpha Centauri) is about 40 0051 <emphasis>trillion</emphasis> kilometers away. Therefore, the annual 0052 parallax is very small, typically smaller than one 0053 <firstterm>arcsecond</firstterm>, which is only 1/3600 of one degree. 0054 A convenient distance unit for nearby stars is the 0055 <firstterm>parsec</firstterm>, which is short for "parallax 0056 arcsecond". One parsec is the distance a star would have if its 0057 observed parallax angle was one arcsecond. It is equal to 3.26 0058 light-years, or 31 trillion kilometers<footnote><para>Astronomers 0059 like this unit so much that they now use <quote>kiloparsecs</quote> to measure 0060 galaxy-scale distances, and <quote>Megaparsecs</quote> to measure intergalactic 0061 distances, even though these distances are much too large to have an 0062 actual, observable parallax. Other methods are required to determine 0063 these distances</para></footnote>. 0064 </para> 0065 </sect1>