XEphem
| XEphem
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|---|---|
 XEphem 3.7.3: epicyclic orbit of Mars in the constellation Cancer in late 2009 / early 2010 |
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| Basic data
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| developer | Elwood C. Downey / Clear Sky Institute |
| Publishing year | 1990 |
| Current version | 3.7.7 (August 9, 2015) |
| operating system | Unix-like operating systems |
| category | Astronomy program |
| License | Proprietary , source code available free of charge for private use |
| German speaking | No |
| http://www.clearskyinstitute.com/xephem/ | |
XEphem is an astronomy program under X11 / Motif for Linux , Mac OS X and other Unix-like operating systems . It is proprietary software, the source code of which is available free of charge for private use.
By using VSOP87 , XEphem achieves a very high level of accuracy when calculating the ephemeris , the current planetary positions. This explains the name XEphemer : Ephemeris under the X Window System.
Functionality
The program, which has been continuously developed since 1993, has a very large range of functions that are clearly geared towards the needs of the observing star friend. Essential elements of this are outlined below.
Star map
XEphem can first of all create a current, i.e. time and location-related, star map (XEphem: sky view ) that can contain the respective positions of planets, asteroids , comets and satellites . The section or zoom factor and the objects to be displayed can be selected according to type and brightness. Detailed maps with a high or maximum zoom factor can also be underlaid with images from the Digitized Sky Survey via the Internet .
Ephemeris
The program ephem , from which the X11 program XEphem emerged, was able to calculate ephemeris (XEphem: data table ), i.e. the current positions of the planets. XEphem's ephemeris table can be expanded to include numerous quantities that can be derived from the coordinates as well as other objects of the solar system defined via catalogs or path elements .
Solar system
A model view of the ecliptic , the common plane in which the planets revolve around the sun, is also possible. Here, too, further objects of the solar system can be integrated; To do this, select an object under data index and save it as a favorite. The resulting three-dimensional model of our solar system can be viewed from different perspectives.
Lunar atlas
The side of the moon facing the earth is available as an interactive lunar atlas. On request, fixed stars and planets close to the moon can also be displayed so that lunar coverages can also be simulated.
Jupiter
Jupiter , its four Galilean moons and their shadows can also be displayed in their current position. The Great Red Spot, a very stable hurricane system in the Juptier atmosphere, is also recorded. Since the geographic longitude of the Great Red Spot is subject to certain fluctuations, the length of the spot can be set manually.
Similar functionality is also offered for the planets Saturn and Uranus .
earth
The earth can also be represented both as a sphere and in a cylinder projection. In an animation, the diurnal and seasonal shifts of the hemisphere illuminated by the sun can be observed. Phenomena such as the midnight sun near the pole and questions about the shifting of sunrise and sunset times are easy to understand here. The umbra zone during a solar eclipse are also calculated and displayed here.
The current global weather situation can also be displayed via the Internet.
Animations
Moving representations can be generated as a sequence of individual images with a constant time interval. In some viewing festivals (e.g. in the earth view and in the solar system view ) there is a separate menu item Animation demo for this . The adjacent animation of the day-night boundary was created with XEphem. In reality, the transition from day to night is of course not geometrically as sharp-edged as the graphic suggests. Rather, it is a strip a few hundred kilometers wide. At sunset, the earth's atmosphere acts like a lens and apparently delays it. This effect, which also affects stars near the horizon, is taken into account by XEphem when creating a star map in the horizon system. The information on the altitude above sea level and the current temperature in the main window are used for this.
The time interval between two images in the adjacent animation is 14 days plus or minus the time until the local sunset. The individual images were created in the earth view and added to a XEphem movie . By saving, a sequence of png files was created , which was converted into GIF format with netpbm tools and combined to a GIF animation with gifslice.
Trails
In addition, orbits of planets and other portable objects can be generated as so-called trails , i.e. as a kind of light trail that these objects seem to pull behind them. This is how the Mars loop was created in the screenshot above on the right.
Such trails usually only make sense when using the equatorial coordinate system . When using the azimuthal coordinate system, XEphem warns the user of appropriately confusing star maps.
Time series and graphs
The possibility of having XEphem generate a sequence of snapshots at the same time interval can be used both to create time series with output in a text file ( list values under tools ) and to graphically display astronomical quantities and their dependencies ( plot values ) .
Catalogs
The XEphem program itself knows only a few permanently programmed objects: the planets and their most important moons. All other celestial bodies are defined in so-called catalogs (see star catalog ), which can be loaded and unloaded at runtime. XEphem uses its own text format here , which usually uses the file extension .edb . The most important of these catalogs are included with the XEphem source code - the CD version that can be ordered from the XEphem website has a significantly expanded range of such catalogs. Astronomical institutes often offer catalogs for XEphem for download on the Internet.
Track elements
The orbital elements of asteroids, comets and satellites, which can only be used for short periods of time, can even be conveniently updated via the Internet directly in XEphem (menu item Data Download ) by the relevant institutes ( Minor Planet Center of Harvard University , International Astronomical Union , Lowell Observatory ).
Field stars
The fixed star catalog SKY2k65.edb used by XEphem by default contains a good eight thousand fixed stars up to about 7 mag . This corresponds roughly to those stars that can be seen with the naked eye under favorable visibility conditions (about 6.5 mag). Even with a powerful binoculars, however, significantly smaller stars, up to about 10 mag, can be observed.
XEphem has a sophisticated mechanism for displaying the already large number of faint stars: the so-called field stars . There is no need to load all of these stars into memory. Because it makes sense to display these stars only with a correspondingly high zoom factor and thus a small image section. This is exactly what happens: With a large zoom factor, exactly the faint stars required for the respective section of the sky are loaded from a special catalog (in a binary format, as a so-called quadtree is used).
Individual evidence
- ↑ See. Gifslice site (English).
- ↑ See also section 6.8 Movie loop in the XEphem manual
- ↑ See for example Xephem or Telescope
- ↑ Details on the setup of the field stars can be found in Uoregon.edu
Web links
- XEphem on the Clear Sky Institute website.
- the detailed XEphem Manual (Engl.)