138 lines
6.2 KiB
HTML
138 lines
6.2 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
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<HTML>
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<HEAD>
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<title>Xephem LG #25</title>
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</HEAD>
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<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#A000A0"
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ALINK="#FF0000">
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<H4>
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"Linux Gazette...<I>making Linux just a little more fun!</I>"
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</H4>
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<P> <HR> <P>
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<center>
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<H2>Xephem</H2>
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<h4>By <a href="mailto: layers@marktwain.net">Larry Ayers</a></h4>
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</center>
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<P> <HR> <P>
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<center><h3>Introduction</h3></center>
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<p><blockquote><b>ephemeris</b> <i>n.,</i> <i>pl.</i> <b>ephemerides</b>
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<b>1.</b> A table giving the coordinates of one or a number of celestial
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bodies at a number of specific times during a given period. <b>2.</b> A
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publication that presents a collection of such tables; an astronomical
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almanac.
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</blockquote>
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<p>The above definition came to mind when, some time ago, I happened upon a
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Debian package called <em>xephem</em> while browsing the contents of a
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distribution CD. At the time I dismissed any thought of installing it; I
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could visualize (falsely, as I later learned) a simple X application
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displaying scrollable lists of sun, moon, and planet rising and setting times
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for various latitudes. This sort of information is easily available from
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printed ephemerides and hardly justified installing a probably old package.
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<p>A salient aspect of free software is that it's not advertised, and
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word-of-mouth has its limitations. News of an application with wide appeal,
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such as an editor or file-manager, will eventually be spread via the internet,
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but a program which occupies a specialized niche might not receive the
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attention it deserves.
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<p>Some time later I saw a brief description of xephem in a usenet posting
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which was enough to spark my curiosity. After trying it out, I was impressed,
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and thought the word should be spread.
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<center><h3>Description and Features</h3></center>
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<p>Xephem is a Motif-based X application which goes far beyond the name's
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implication. It's a multi-purpose astronomical program which can present
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detailed, zoom-able star-charts as well as views of the earth, moon, planets
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and the entire solar-system. These views can be from any location on Earth,
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at any time in the past or future.
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<p>This application can be effective on several levels. The casual star-gazer
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can consult xephem just to see what planets and constellations are visible on
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a certain night, and perhaps print out a star-chart. As a teaching aid
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xephem's graphical and animated displays could spark a student's interest.
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The serious amateur astronomer can set up a link between a telescope and the
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program, so that the sky-view displays whichever spot the telescope is also
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seeing.
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<p>This review will be more comprehensible if a screenshot is presented first.
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The first window which appears when xephem is started is the control
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window:<br>
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<p><img alt="xephem controls" src="./gx/ayers/xeph_1.gif">
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<p>In this window various parameters, such as location, date, and time, can be
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set. From the menubar the view windows can be summoned, as well as which of
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the various astronomical databases (included in the distribution) should be
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loaded into memory. These databases are quite a useful resource to have
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available. They include the Messier and NGC databases of deep-sky objects,
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along with databases of asteroids, comets, and satellites. Updated versions of
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the latter two are available on the xephem web-pages.
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<p>Here is a screenshot of a skyview window:<br>
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<p><img alt="Skyview window" src="./gx/ayers/skyview.gif">
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<p>This window is much more than a simple star-chart of a certain date, time,
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and location. Right-mouse-button clicking on a star or other astronomical
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object summons a small window showing various facts about the object. Zooming
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in can also be done with the mouse, and a zoomed view can be panned using the
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scroll-bars. A variety of viewing options can be set from the menubar. The
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constellation names and outlines can be shown, and if any of the xephem
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databases are loaded the objects in them will be visible, if desired.
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<p>One view window which I find particulary interesting is the earth view. A
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representation of the earth from an orbital viewpoint is shown, with the sun's
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illumination and current zenith-point highlighted. This is updated in
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real-time, and equivalent views displaying the zenith location and area
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illumination of either the moon or the other planets are menu options.<br>
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<p><img alt="Earth View" src="./gx/ayers/earth.gif">
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<p>Another view-window displays the solar-system in schematic form. This and
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the earth-view windows can be animated, a sort of cartoon-movie which shows
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the relative movements of the various celestial objects.
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<center><h3>Availability</h3></center>
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The xephem <a href="http://iraf.noao.edu/~ecdowney/xephem.html">web-site</a>
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is the place to visit if you'd like to investigate this application. Source
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for current development versions is available there; I've had good luck
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compiling and running these. Users lacking the Motif libraries can obtain
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statically-linked binary releases from this site, and updated databases are
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available as well. Elwood Downey is the author of xephem. If you install
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it, I'm sure he would be glad to hear any comments you might have.
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<!-- hhmts start -->
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Last modified: Sat 31 Jan 1998
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< !-- hhmts end -->
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<!--===================================================================-->
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<P> <hr> <P>
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<center><H5>Copyright © 1998, Larry Ayers <BR>
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Published in Issue 25 of <i>Linux Gazette</i>, February 1998</H5></center>
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<!--===================================================================-->
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<P> <hr> <P>
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<A HREF="./index.html"><IMG ALIGN=BOTTOM SRC="../gx/indexnew.gif"
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ALT="[ TABLE OF CONTENTS ]"></A>
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<A HREF="../index.html"><IMG ALIGN=BOTTOM SRC="../gx/homenew.gif"
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ALT="[ FRONT PAGE ]"></A>
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<A HREF="./ayers2.html"><IMG SRC="../gx/back2.gif"
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ALT=" Back "></A>
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