Copy the KDE 3.5 branch to branches/trinity for new KDE 3.5 features.

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+<sect1 id="ai-magnitude">
+<sect1info>
+<author>
+<firstname>Girish</firstname> <surname>V</surname>
+</author>
+</sect1info>
+<title>Magnitude Scale</title>
+<indexterm><primary>Magnitude Scale</primary>
+<seealso>Flux</seealso>
+<seealso>Star Colors and Temperatures</seealso>
+</indexterm>
+<para>
+2500 years ago, the ancient Greek astronomer Hipparchus classified the
+brightnesses of visible stars in the sky on a scale from 1 to 6.  He
+called the very brightest stars in the sky <quote>first magnitude</quote>, and the
+very faintest stars he could see <quote>sixth magnitude</quote>.  Amazingly, two
+and a half millenia later, Hipparchus's classification scheme is still
+widely used by astronomers, although it has since been modernized and
+quantified.</para>
+<note><para>The magnitude scale runs backwards to what you
+might expect: brighter stars have <emphasis>smaller</emphasis>
+magnitudes than fainter stars.
+</para>
+</note>
+<para>
+The modern magnitude scale is a quantitative measurement of the
+<firstterm>flux</firstterm> of light coming from a star, with a
+logarithmic scaling:
+</para><para>
+m = m_0 - 2.5 log (F / F_0)
+</para><para>
+If you do not understand the math, this just says that the magnitude
+of a given star (m) is different from that of some standard star (m_0)
+by 2.5 times the logarithm of their flux ratio.  The 2.5 *log factor
+means that if the flux ratio is 100, the difference in magnitudes is 5
+mag.  So, a 6th magnitude star is 100 times fainter than a 1st magnitude
+star.  The reason Hipparchus's simple classification translates to a
+relatively complex function is that the human eye responds
+logarithmically to light.
+</para><para>
+There are several different magnitude scales in use, each of which serves
+a different purpose.  The most common is the apparent magnitude scale;
+this is just the measure of how bright stars (and other objects) look
+to the human eye.  The apparent magnitude scale defines the star Vega
+to have magnitude 0.0, and assigns magnitudes to all other objects using
+the above equation, and a measure of the flux ratio of each object to
+Vega.
+</para><para>
+It is difficult to understand stars using just the apparent magnitudes.
+Imagine two stars in the sky with the same apparent magnitude, so they
+appear to be equally bright.  You cannot know just by looking if the
+two have the same <emphasis>intrinsic</emphasis> brightness; it is
+possible that one star is intrinsically brighter, but further away.
+If we knew the distances to the stars (see the <link
+linkend="ai-parallax">parallax</link> article), we could account for
+their distances and assign <firstterm>Absolute magnitudes</firstterm>
+which would reflect their true, intrinsic brightness.  The absolute
+magnitude is defined as the apparent magnitude the star would have if
+observed from a distance of 10 parsecs (1 parsec is 3.26 light-years,
+or 3.1 x 10^18 cm).  The absolute magnitude (M) can be determined
+from the apparent magnitude (m) and the distance in parsecs (d)
+using the formula:
+</para><para>
+M = m + 5 - 5 * log(d)   (note that M=m when d=10).
+</para><para>
+The modern magnitude scale is no longer based on the
+human eye; it is based on photographic plates and photoelectric
+photometers.  With telescopes, we can see objects much fainter than
+Hipparchus could see with his unaided eyes, so the magnitude scale has
+been extended beyond 6th magnitude.  In fact, the Hubble Space Telescope
+can image stars nearly as faint as 30th magnitude, which is one
+<emphasis>trillion</emphasis> times fainter than Vega.
+</para><para>
+A final note: the magnitude is usually measured through a color filter
+of some kind, and these magnitudes are denoted by a subscript
+describing the filter (&ie;, m_V is the magnitude through a <quote>visual</quote>
+filter, which is greenish; m_B is the magnitude through a blue filter;
+m_pg is the photographic plate magnitude, &etc;).
+</para>
+</sect1>