From ce599e4f9f94b4eb00c1b5edb85bce5431ab3df2 Mon Sep 17 00:00:00 2001 From: toma Date: Wed, 25 Nov 2009 17:56:58 +0000 Subject: Copy the KDE 3.5 branch to branches/trinity for new KDE 3.5 features. BUG:215923 git-svn-id: svn://anonsvn.kde.org/home/kde/branches/trinity/kdeedu@1054174 283d02a7-25f6-0310-bc7c-ecb5cbfe19da --- doc/kstars/skycoords.docbook | 152 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 152 insertions(+) create mode 100644 doc/kstars/skycoords.docbook (limited to 'doc/kstars/skycoords.docbook') diff --git a/doc/kstars/skycoords.docbook b/doc/kstars/skycoords.docbook new file mode 100644 index 00000000..1017be06 --- /dev/null +++ b/doc/kstars/skycoords.docbook @@ -0,0 +1,152 @@ + + + +Jason +Harris + + +Celestial Coordinate Systems + +Celestial Coordinate Systems +Overview +A basic requirement for studying the heavens is determining where in the +sky things are. To specify sky positions, astronomers have developed +several coordinate systems. Each uses a coordinate grid +projected on the Celestial Sphere, in +analogy to the Geographic coordinate +system used on the surface of the Earth. The coordinate systems +differ only in their choice of the fundamental plane, +which divides the sky into two equal hemispheres along a great circle. (the fundamental plane of the +geographic system is the Earth's equator). Each coordinate system is named for +its choice of fundamental plane. + + + +The Equatorial Coordinate System +Celestial Coordinate Systems +Equatorial Coordinates +Celestial Equator +Celestial Poles +Geographic Coordinate System + +Right AscensionEquatorial Coordinates +DeclinationEquatorial Coordinates + + +The Equatorial coordinate system is probably the most +widely used celestial coordinate system. It is also the most closely related +to the Geographic coordinate system, 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 +Celestial Equator. +Similarly, projecting the geographic Poles onto the celestial sphere defines the +North and South Celestial Poles. + +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 starsactually, the equatorial +coordinates are not quite fixed to the stars. See precession. Also, if Hour Angle is used in place of Right +Ascension, then the Equatorial system is fixed to the Earth, not to the +stars., so it appears to rotate across the sky with the stars, +but of course it is really the Earth rotating under the fixed sky. + +The latitudinal (latitude-like) angle of the Equatorial +system is called Declination (Dec for short). It +measures the angle of an object above or below the Celestial Equator. The +longitudinal angle is called the Right +Ascension (RA for short). It measures the angle of an object East +of the Vernal Equinox. 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 +Sidereal Time and Hour Angle. 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. + + + + +The Horizontal Coordinate System + +Celestial Coordinate Systems +Horizontal Coordinates +Horizon +Zenith + +AzimuthHorizontal Coordinates +AltitudeHorizontal Coordinates + +The Horizontal coordinate system uses the observer's local horizon 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 Zenith. The +pole of the lower hemisphere is called the nadir. The +angle of an object above or below the horizon is called the +Altitude (Alt for short). The angle of an object around +the horizon (measured from the North point, toward the East) is called the +Azimuth. The Horizontal Coordinate System is sometimes +also called the Alt/Az Coordinate System. + +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. + +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). + + + + +The Ecliptic Coordinate System + +Celestial Coordinate Systems +Ecliptic Coordinates +Ecliptic + + +The Ecliptic coordinate system uses the Ecliptic 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 Ecliptic Latitude, and the longitudinal angle +is called the Ecliptic Longitude. Like Right Ascension +in the Equatorial system, the zeropoint of the Ecliptic Longitude is the Vernal Equinox. + +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). + + + + +The Galactic Coordinate System + +Celestial Coordinate Systems +Galactic Coordinates + + +Milky Way +The Galactic coordinate system uses the Milky Way as its +Fundamental Plane. The latitudinal angle is called the Galactic +Latitude, and the longitudinal angle is called the +Galactic Longitude. 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. + + + -- cgit v1.2.1