You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
153 lines
7.3 KiB
153 lines
7.3 KiB
<sect1 id="ai-skycoords">
|
|
<sect1info>
|
|
<author>
|
|
<firstname>Jason</firstname>
|
|
<surname>Harris</surname>
|
|
</author>
|
|
</sect1info>
|
|
<title>Celestial Coordinate Systems</title>
|
|
<para>
|
|
<indexterm><primary>Celestial Coordinate Systems</primary>
|
|
<secondary>Overview</secondary></indexterm>
|
|
A basic requirement for studying the heavens is determining where in the
|
|
sky things are. To specify sky positions, astronomers have developed
|
|
several <firstterm>coordinate systems</firstterm>. Each uses a coordinate grid
|
|
projected on the <link linkend="ai-csphere">Celestial Sphere</link>, in
|
|
analogy to the <link linkend="ai-geocoords">Geographic coordinate
|
|
system</link> used on the surface of the Earth. The coordinate systems
|
|
differ only in their choice of the <firstterm>fundamental plane</firstterm>,
|
|
which divides the sky into two equal hemispheres along a <link
|
|
linkend="ai-greatcircle">great circle</link>. (the fundamental plane of the
|
|
geographic system is the Earth's equator). Each coordinate system is named for
|
|
its choice of fundamental plane.
|
|
</para>
|
|
|
|
<sect2 id="equatorial">
|
|
<title>The Equatorial Coordinate System</title>
|
|
<indexterm><primary>Celestial Coordinate Systems</primary>
|
|
<secondary>Equatorial Coordinates</secondary>
|
|
<seealso>Celestial Equator</seealso>
|
|
<seealso>Celestial Poles</seealso>
|
|
<seealso>Geographic Coordinate System</seealso>
|
|
</indexterm>
|
|
<indexterm><primary>Right Ascension</primary><see>Equatorial Coordinates</see></indexterm>
|
|
<indexterm><primary>Declination</primary><see>Equatorial Coordinates</see></indexterm>
|
|
|
|
<para>
|
|
The <firstterm>Equatorial coordinate system</firstterm> is probably the most
|
|
widely used celestial coordinate system. It is also the most closely related
|
|
to the <link linkend="ai-geocoords">Geographic coordinate system</link>, because
|
|
they use the same fundamental plane, and the same poles. The projection of the
|
|
Earth's equator onto the celestial sphere is called the
|
|
<link linkend="ai-cequator">Celestial Equator</link>.
|
|
Similarly, projecting the geographic Poles onto the celestial sphere defines the
|
|
North and South <link linkend="ai-cpoles">Celestial Poles</link>.
|
|
</para><para>
|
|
However, there is an important difference between the equatorial and
|
|
geographic coordinate systems: the geographic system is fixed to the
|
|
Earth; it rotates as the Earth does. The Equatorial system is
|
|
fixed to the stars<footnote id="fn-precess"><para>actually, the equatorial
|
|
coordinates are not quite fixed to the stars. See <link
|
|
linkend="ai-precession">precession</link>. Also, if <link
|
|
linkend="ai-hourangle">Hour Angle</link> is used in place of Right
|
|
Ascension, then the Equatorial system is fixed to the Earth, not to the
|
|
stars.</para></footnote>, so it appears to rotate across the sky with the stars,
|
|
but of course it is really the Earth rotating under the fixed sky.
|
|
</para><para>
|
|
The <firstterm>latitudinal</firstterm> (latitude-like) angle of the Equatorial
|
|
system is called <firstterm>Declination</firstterm> (Dec for short). It
|
|
measures the angle of an object above or below the Celestial Equator. The
|
|
<firstterm>longitudinal</firstterm> angle is called the <firstterm>Right
|
|
Ascension</firstterm> (<acronym>RA</acronym> for short). It measures the angle of an object East
|
|
of the <link linkend="ai-equinox">Vernal Equinox</link>. Unlike longitude,
|
|
Right Ascension is usually measured in hours instead of degrees, because the
|
|
apparent rotation of the Equatorial coordinate system is closely related to
|
|
<link linkend="ai-sidereal">Sidereal Time</link> and <link
|
|
linkend="ai-hourangle">Hour Angle</link>. Since a full rotation of the sky
|
|
takes 24 hours to complete, there are (360 degrees / 24 hours) = 15 degrees in
|
|
one Hour of Right Ascension.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="horizontal">
|
|
<title>The Horizontal Coordinate System</title>
|
|
|
|
<indexterm><primary>Celestial Coordinate Systems</primary>
|
|
<secondary>Horizontal Coordinates</secondary>
|
|
<seealso>Horizon</seealso>
|
|
<seealso>Zenith</seealso>
|
|
</indexterm>
|
|
<indexterm><primary>Azimuth</primary><see>Horizontal Coordinates</see></indexterm>
|
|
<indexterm><primary>Altitude</primary><see>Horizontal Coordinates</see></indexterm>
|
|
<para>
|
|
The Horizontal coordinate system uses the observer's local <link
|
|
linkend="ai-horizon">horizon</link> as the Fundamental Plane. This conveniently
|
|
divides the sky into the upper hemisphere that you can see, and the lower
|
|
hemisphere that you can't (because the Earth is in the way). The pole of the
|
|
upper hemisphere is called the <link linkend="ai-zenith">Zenith</link>. The
|
|
pole of the lower hemisphere is called the <firstterm>nadir</firstterm>. The
|
|
angle of an object above or below the horizon is called the
|
|
<firstterm>Altitude</firstterm> (Alt for short). The angle of an object around
|
|
the horizon (measured from the North point, toward the East) is called the
|
|
<firstterm>Azimuth</firstterm>. The Horizontal Coordinate System is sometimes
|
|
also called the Alt/Az Coordinate System.
|
|
</para><para>
|
|
The Horizontal Coordinate System is fixed to the Earth, not the Stars.
|
|
Therefore, the Altitude and Azimuth of an object changes with time, as the
|
|
object appears to drift across the sky. In addition, because the Horizontal
|
|
system is defined by your local horizon, the same object viewed from different
|
|
locations on Earth at the same time will have different values of Altitude and
|
|
Azimuth.
|
|
</para><para>
|
|
Horizontal coordinates are very useful for determining the Rise and Set times of
|
|
an object in the sky. When an object has Altitude=0 degrees, it is either
|
|
Rising (if its Azimuth is < 180 degrees) or Setting (if its Azimuth is >
|
|
180 degrees).
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="ecliptic">
|
|
<title>The Ecliptic Coordinate System</title>
|
|
|
|
<indexterm><primary>Celestial Coordinate Systems</primary>
|
|
<secondary>Ecliptic Coordinates</secondary>
|
|
<seealso>Ecliptic</seealso>
|
|
</indexterm>
|
|
<para>
|
|
The Ecliptic coordinate system uses the <link
|
|
linkend="ai-ecliptic">Ecliptic</link> for its Fundamental Plane. The
|
|
Ecliptic is the path that the Sun appears to follow across the sky over
|
|
the course of a year. It is also the projection of the Earth's
|
|
orbital plane onto the Celestial Sphere. The latitudinal angle is
|
|
called the <firstterm>Ecliptic Latitude</firstterm>, and the longitudinal angle
|
|
is called the <firstterm>Ecliptic Longitude</firstterm>. Like Right Ascension
|
|
in the Equatorial system, the zeropoint of the Ecliptic Longitude is the <link
|
|
linkend="ai-equinox">Vernal Equinox</link>.
|
|
</para><para>
|
|
What do you think such a coordinate system would be useful for? If you
|
|
guessed charting solar system objects, you are right! Each of the
|
|
planets (except Pluto) orbits the Sun in roughly the same plane, so they always
|
|
appear to be somewhere near the Ecliptic (&ie;, they always have small ecliptic
|
|
latitudes).
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="galactic">
|
|
<title>The Galactic Coordinate System</title>
|
|
|
|
<indexterm><primary>Celestial Coordinate Systems</primary>
|
|
<secondary>Galactic Coordinates</secondary>
|
|
</indexterm>
|
|
<para>
|
|
<indexterm><primary>Milky Way</primary></indexterm>
|
|
The Galactic coordinate system uses the <firstterm>Milky Way</firstterm> as its
|
|
Fundamental Plane. The latitudinal angle is called the <firstterm>Galactic
|
|
Latitude</firstterm>, and the longitudinal angle is called the
|
|
<firstterm>Galactic Longitude</firstterm>. This coordinate system is useful for
|
|
studying the Galaxy itself. For example, you might want to know how the density
|
|
of stars changes as a function of Galactic Latitude, to how much the disk of the
|
|
Milky Way is flattened.
|
|
</para>
|
|
</sect2>
|
|
</sect1>
|