1 <sect1 id="ai-parallax">
4 <firstname>James</firstname> <surname>Lindenschmidt</surname>
7 <title>Parallax</title>
8 <indexterm><primary>Parallax</primary></indexterm>
9 <indexterm><primary>Astronomical Unit</primary><see>Parallax</see></indexterm>
10 <indexterm><primary>Parsec</primary><see>Parallax</see></indexterm>
12 <firstterm>Parallax</firstterm> is the apparent change of an observed
13 object's position caused by a shift in the observer's position. As an
14 example, hold your hand in front of you at arm's length, and observe
15 an object on the other side of the room behind your hand. Now tilt
16 your head to your right shoulder, and your hand will appear on the
17 left side of the distant object. Tilt your head to your left
18 shoulder, and your hand will appear to shift to the right side of the
22 Because the Earth is in orbit around the Sun, we observe the sky from
23 a constantly moving position in space. Therefore, we should expect
24 to see an <firstterm>annual parallax</firstterm> effect, in which the
25 positions of nearby objects appear to <quote>wobble</quote> back and forth in
26 response to our motion around the Sun. This does in fact happen, but
27 the distances to even the nearest stars are so great that you need to
28 make careful observations with a telescope to detect
29 it<footnote><para>The ancient Greek astronomers knew about parallax;
30 because they could not observe an annual parallax in the positions of
31 stars, they concluded that the Earth could not be in motion around
32 the Sun. What they did not realize was that the stars are millions of
33 times further away than the Sun, so the parallax effect is impossible
34 to see with the unaided eye.</para></footnote>.
37 Modern telescopes allow astronomers to use the annual parallax to
38 measure the distance to nearby stars, using triangulation. The
39 astronomer carefully measures the position of the star on two dates,
40 spaced six months apart. The nearer the star is to the Sun, the
42 the apparent shift in its position will be between the two dates.
45 Over the six-month period, the Earth has moved through half its orbit
46 around the Sun; in this time its position has changed by 2
47 <firstterm>Astronomical Units</firstterm> (abbreviated AU; 1 AU is
48 the distance from the Earth to the Sun, or about 150 million
49 kilometers). This sounds like a really long distance, but even the
50 nearest star to the Sun (alpha Centauri) is about 40
51 <emphasis>trillion</emphasis> kilometers away. Therefore, the annual
52 parallax is very small, typically smaller than one
53 <firstterm>arcsecond</firstterm>, which is only 1/3600 of one degree.
54 A convenient distance unit for nearby stars is the
55 <firstterm>parsec</firstterm>, which is short for "parallax
56 arcsecond". One parsec is the distance a star would have if its
57 observed parallax angle was one arcsecond. It is equal to 3.26
58 light-years, or 31 trillion kilometers<footnote><para>Astronomers
59 like this unit so much that they now use <quote>kiloparsecs</quote> to measure
60 galaxy-scale distances, and <quote>Megaparsecs</quote> to measure intergalactic
61 distances, even though these distances are much too large to have an
62 actual, observable parallax. Other methods are required to determine
63 these distances</para></footnote>.