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66 lines
3.3 KiB
66 lines
3.3 KiB
15 years ago
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<sect1 id="ai-parallax">
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<sect1info>
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<author>
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<firstname>James</firstname> <surname>Lindenschmidt</surname>
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</author>
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</sect1info>
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<title>Parallax</title>
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<indexterm><primary>Parallax</primary></indexterm>
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<indexterm><primary>Astronomical Unit</primary><see>Parallax</see></indexterm>
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<indexterm><primary>Parsec</primary><see>Parallax</see></indexterm>
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<para>
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<firstterm>Parallax</firstterm> is the apparent change of an observed
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object's position caused by a shift in the observer's position. As an
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example, hold your hand in front of you at arm's length, and observe
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an object on the other side of the room behind your hand. Now tilt
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your head to your right shoulder, and your hand will appear on the
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left side of the distant object. Tilt your head to your left
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shoulder, and your hand will appear to shift to the right side of the
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distant object.
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</para>
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<para>
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Because the Earth is in orbit around the Sun, we observe the sky from
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a constantly moving position in space. Therefore, we should expect
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to see an <firstterm>annual parallax</firstterm> effect, in which the
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positions of nearby objects appear to <quote>wobble</quote> back and forth in
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response to our motion around the Sun. This does in fact happen, but
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the distances to even the nearest stars are so great that you need to
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make careful observations with a telescope to detect
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it<footnote><para>The ancient Greek astronomers knew about parallax;
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because they could not observe an annual parallax in the positions of
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stars, they concluded that the Earth could not be in motion around
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the Sun. What they did not realize was that the stars are millions of
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times further away than the Sun, so the parallax effect is impossible
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to see with the unaided eye.</para></footnote>.
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</para>
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<para>
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Modern telescopes allow astronomers to use the annual parallax to
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measure the distance to nearby stars, using triangulation. The
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astronomer carefully measures the position of the star on two dates,
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spaced six months apart. The nearer the star is to the Sun, the
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larger
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the apparent shift in its position will be between the two dates.
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</para>
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<para>
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Over the six-month period, the Earth has moved through half its orbit
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around the Sun; in this time its position has changed by 2
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<firstterm>Astronomical Units</firstterm> (abbreviated AU; 1 AU is
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the distance from the Earth to the Sun, or about 150 million
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kilometers). This sounds like a really long distance, but even the
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nearest star to the Sun (alpha Centauri) is about 40
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<emphasis>trillion</emphasis> kilometers away. Therefore, the annual
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parallax is very small, typically smaller than one
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<firstterm>arcsecond</firstterm>, which is only 1/3600 of one degree.
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A convenient distance unit for nearby stars is the
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<firstterm>parsec</firstterm>, which is short for "parallax
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arcsecond". One parsec is the distance a star would have if its
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observed parallax angle was one arcsecond. It is equal to 3.26
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light-years, or 31 trillion kilometers<footnote><para>Astronomers
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like this unit so much that they now use <quote>kiloparsecs</quote> to measure
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galaxy-scale distances, and <quote>Megaparsecs</quote> to measure intergalactic
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distances, even though these distances are much too large to have an
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actual, observable parallax. Other methods are required to determine
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these distances</para></footnote>.
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</para>
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</sect1>
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