Basic instructions

    This program provides a realistic, complete and rigorous visualization of the sky and any astronomical phenomena. Simulation starts by default in horizontal coordinates (azimuth and elevation) and the current instant, or for the coordinates and time of a given astronomical event. The buttons at the top allow to reset the main properties of the visualization to those present when the program started, or to go to full screen mode. Visualization options, simulation time, and observer position can be modified with the options available in the pop up window that appears after clicking with the right button of the mouse on the program window. The most common projection is stereographic, and the horizontal system of coordinates, to show the apparent sky in a similar way as it is observed, without showing objects below horizon. Coordinates in this case are azimuth (the direction of observation: 0 degrees for north, 180 for south, 90 for east, and 270 for west, or any other intermediate between them), and elevation (degrees above horizon, 0 for the horizon and 90 for the cenit, which is the direction just above our head). To show the sky as it is visible in maps or celestial charts you can select the equatorial system of coordinates (right ascension and declination), and enable all objects below horizon.

   At the bottom you will see some information about the current point selected with the mouse, and the closest object to it. The right button of the mouse will show a popup menu with additional options to obtain more information about the object or to vary other parameters that affects the visualization (below some useful details are given about some of them). When you zoom in a planet you will be able to see the coordinates in its surface (moving the mouse on the planet) and the closest geological feature (Martian regions, lunar craters, and so on). Solar spots are shown up to the latest available date (the last update of the program), and comets and asteroids are correct for several years around that date. Limiting magnitude is by default 7.5 for stars and 10.5 for deep sky objects (adequate for observing with binoculars), although both can be modified with the options present in the menu. As reference, the limiting magnitud observing with naked eye from a dark place is 6.5 for stars (greater values of limiting magnitude correspond to even fainter objects), and the typical limiting magnitude is around 4 from a city). The search option allows to center a star, Solar System object, or deep sky object by its name, while the export option allows to save the screen to an image in png format. It will throw an error in case the Java configuration doesn't allow the program to access you hard disk for writting (at the end a workaround is explained).

  In case your computer has a CPU with multiple cores the program will show after a while an additional option at the bottom of the pop up window, that allows to explore the main astronomical events for the selected year.

   Astronomical phenomena are reproduced with accuracy, including solar/lunar eclipses, transits and shadows of satellites, and even mutual phenomena between satellites of Jupiter, Saturn, and Uranus. Positions are corrected for refraction if the sky below horizon is not shown or the coordinate system selected is the horizontal one. In case Internet connection is available and the program has enough priviledges, the orbital elements for comets and asteroids are automatically updated. Comet tails are shown assuming a size of 20 million km, showing both the ionic tail (in a direction opposite respect to the Sun) in blue, and the dust tail (in a direction between the ionic tail and the position of the comet on previous days) in white. In the computations the latest theories are used (precession algorithm of  Vondrák 2011, IAU 2006 precession/nutation theory, L1 Theory by Lainey 2004 for the satellites of Jupiter, TASS1.7 theory for the satellites of Saturn, GUST86 for those of Uranus, and the 2007 theory for the martian satellites by A. Vienne. For dwarf satellites elliptic elements by JPL are used). Lunar and planetary positions are computed using the fit to JPL DE404 developed by S. L. Moshier, with a precision of 0.05",  although  in recent times the accuracy is better than 0.01". Discrepancies with latest ephemerides theories are within 0.01" in recent times and a few arcseconds around year 1000 B.C., although the discrepancy is slightly greater for the Moon.

    Some additional details about program control

    Solar and lunar eclipse maps: To show visibility maps of solar or lunar eclipses just move the mouse towards the Sun or the Moon, and use the right button of the mouse to show the popup menu. One of the options at the top will be 'Next solar/lunar eclipse'. Clic on it. The program will show the next eclipse visible from the selected location after the current simulation time. Local circumstances of solar/lunar eclipses are available for the entire time span of validity of the ephemerides for the Sun and the Moon.

    Full screen mode: This mode allows a better experience in powerful computers, besides the export of images to greater resolutions. The applet can be reverted to normal mode by pressing the Esc key or selecting again the full screen button. To show the applet just select it in the task bar or using the Alt+Tab key combination.

    Control with mouse: You can move around the sky by moving the mouse while keeping the left button down. Right button is used to show the pop up menu. Double clicking with the left button will center an object, and double clicking with the middle one will show details about it. The dragging lock mode can be also disabled using the right button of the mouse. The right button will show the popup window with options.

    Planetographic positions: At the bottom-left corner you will see the object closest to the mouse. When you zoom in a planet and you see the disk, here you will see the planetographic position of the mouse within the disk of the planet/satellite. Mutual phenomena: eclipses, occultations, shadow transits (or 'annular eclipse' when the satellite is transiting the Sun on some point of the planet surface and its size is lower than the Sun) ... are also shown. Without textures the coordinates on the disk are shown using as reference the IAU recommendatios for the prime meridian of the planets. At the bottom-left corner you will see the degrees towards east or west instead, according to the planetary nomenclature. There's a flip in the east/west direction for outer planets (Mars and beyond) respect the rest of bodies.

    More information at the bottom: Besides the point currently selected with the mouse, the object selected to maintain it centered (when the date or something changes and the sky image is updated), or the planetographic position, at the bottom-right part you will also see for certain telescopes symbols like [H] or [V]. [H] means that the image is inverted horizontally (east towards right) and [V] that it is inverted vertically (north downwards). You should take this into account when simulating something visible to the naked eye, like eclipses. Image inversion can be disabled from the menu for correct visualization in wide fields of view.

    Object centered: The program maintains centered the object selected in the search option of the popup menu (or the one double-clicked with the mouse). Its name will appear in the bottom-right corner of the screen. To avoid the automatic centering in the second case you can use the search option and leave the object field empty.

    Trajectories: To occult the trajectory of a body select the option and clic on cancel (or set to show a trajectory during 0 days). The trajectories of stars are corrected by all phenomena affecting their positions. It can be an interesting option if you have knowledge of positional astronomy or want to use the program for teaching.

    Dates B.C.: The simulation of the sky in ancient times is accurate taking into account the inherent uncertainties. A date B.C. is introduced as a negative year, but the year cannot be 0. Year 10 B.C. is -10, not -9 as in other programs. You can simulate eclipses thousands of years ago without problems. Date is selectable between years -200 000 (= 200 000 B.C.) and 200 000 A.D., although planets will be shown only between -3001 and 3000

    Automatic update: It is possible to update the sky with different speeds or time steps. This allows to observe the sequence of an eclipse in case you first search the Moon (to maintain it centered when changing the time) and zoom in it (to see its surface). The static mode can be recovered with one of the options of the menu. It is recommended to reduce the kind of objects shown (to disable comets and asteroids for instance) before selecting this option or modifying the date and time by hand multiple times.

    Color squeme: The default color squeme uses a black background, but white background or the special print mode can be useful to print a sky chart. There's also an anaglyph mode (red-cyan glasses) that allows a 3d view of the sky and planets, even when using their textures.

    Comets and asteroids: The number of objects of these types shown depends on the limiting magnitud stablished for deep sky objects, which is selectable in the last option of the menu. Changing this value will force the computation of ephemerides for the new limiting magnitude (in case the date is close to the latest update of the program). In ancient times the main comets visible are also shown. The list of comets/asteroids with their coordinates is listed in the Java console, in case you want to see the list to know which objects of these types are shown and can be searched for. The program can show some objects close to Earth (NEOs) when comets are enabled.

    Nebula and Milky Way: The option to show nebula and Milky Way can be clicked several times depending on which types of contours you prefer to see: both of them, one of them, and with/without Milky Way textures. Textures can also be disabled using the draw textures option. The quality of them can be reduced for a faster visualization in old computers, or improved in powerful computers to obtain a very realistic view of the planets.

    As an additional option it is possible to change the texture for the Milky Way using the key M. The texture used will iterate between these options: use no texture at all, use the optical image by Nick Risinger, the H-alpha image by Finkbeiner (2003) that shows emission nebula, the 21 cm line of HI by Kalberla (2005) that shows the galactic plane, the CO line by Dame (2001) that shows the molecular clouds, the IRAS map at 100 microns by Schlegel (1998) that shows the emission/absorption due to dust, and the background microwave map from WMAP. In case the key is not working first you have to give focus to the program window so it can recognize any key event. To do so just clic on the full screen button, for instance.

    Export images to big sizes: Probably you will not be able to export charts when executing the program from the web page. Below a possible workaround is explained, although the easiest way is to download the program compressed in zip format, and uncompress it to execute from your computer. It is possible to export images to high resolutions by setting as file name a number with the width desired for the output image in pixels, for instance 3000. The maximum possible value depends on the available memory in your computer.

    The sky from other planets: As a very advanced option it is possible to simulate the sky as it would be visible from a given location on the surface of another planet or satellite (or from a comet or asteroid). To do that just select a given place on another body (with the planetary disk visible) and press Ctrl+left button of the mouse. The popup menu only allows to select Earth locations, so it can be used to go back to our planet. Visual accuracy in not guarranteed in some elements (like planetary rings), but mathematical accuracy is extreme, allowing to simulate interesting phenomena. As an example you can simulate the transit of Phobos (Martian satellite) on the Sun disk, as visible from the Gale crater (where the Curiosity rover landed), obtaining the same image the rover observed by itself (for instance on September 13, 2012, at 5:15:30 UTC, from Mars location 137° E, 5° S). A second mode allows to see the Solar System from 1 AU above the north ecliptic pole. For this mode press Ctrl+double click with the left button of the mouse, without selecting a planet. As an exercise you can (after Ctrl+double clic) follow these steps: select ecliptic coordenadas for the visualization, go to the south pole and double clic on the Sun to center it, select search object and clic on ok without any name if the search box (so that the Sun is centered now but in later updates of the image nothing is centered automatically), activate the visualization of comets and space probes, set the date to June, 1, 2003, and select to update the sky in intervals of 10 days forward. With these steps you can follow the evolution of several space probes as they leave the Earth and get to their destinations in other planets. When using any of these modes it is not recommended to use local time in the simulations, since in these cases it is assumed that local time is the same as UTC (no time zone or daylight saving time are considered).

During the visualization from the south ecliptic pole it is possible to move the observer from that point to any other. Use the up/down cursor keys to move forward/backward, and the right/left cursor keys to increase/decrease the spatial step of the movement with each hit to the up/down keys. With some practice you can get any perspective you want in the visualization of the Solar System.

    Possible execution problems and workarounds

    The program doesn't start due to a lack of Java in the system.

    First, this program requires Java 1.6 or later. In case you don't have it installed you should install it. In the first execution of the program (or when an update is available) the program will automatically download all files required to run. The amount of download is above 30 MB, so the first execution could require some minutes of wait, depending on the speed of your connection. Be patient!

    Some users have reported problems using the Safari browser under a Mac, while others can execute the applet without problem using Safari. In case of problems, be sure to have Java 1.6 or above installed, and check in the browser configuration that both Java and javascript are enabled. In case the applet doesn't work please try with Firefox or Chrome.

    The program doesn't start because the browser blocks all Java applets.

    Support for Java applets is being limited in the last versions of some browsers. Currently Java works in IE, Safari, and Firefox, although it may become unsupported also in Firefox in a future. To make applets work in Chrome use version 41 or lower in Windows/Mac, and 34 or lower in Linux. In some versions of Internet Explorer you may need to add to the list of secure or trusted internet sites in the configuration of the browser.

    The program doesn't start due to security limitations.

    In recent versions of Java applets can be completely blocked by default due to system security holes. A workaround is to launch the Java Control Panel and set the security level to Medium in the security tab (reload the applet later). This is probably required also if you launch the applet locally after downloading it to your hard drive. It is currently not clear if this workaround will continue to work in the future and how long. In case the applet stops working the only workaround I can suggest is to completely uninstall all your Java (Oracle) versions and to install an old version of Java (Sun's Java 6, not 7 neither 8). Oracle now requires all Java applets to be certified, which costs money and exposes your PC to security risks since with a certification all Java applets you run can modify your PC and get information from user documents. Decide yourself if you prefer or is more secure to run Java programs in a limited environment or with full access to your PC. In addition, this project has been done at 0-cost with free source code and it is not acceptable to buy anything. Another option is to use OpenJDK, which works on Linux/Mac and should also work on Windows (unofficial installers available from and

    The program doesn't start and in the Java console there is a message mentioning an 'out of memory' or 'java heap space' error.

    An out of memory error should only happen in old computers, since this program requires about 40 MB to run. To solve it you can configurate Java to execute applets requiring a lot of memory. To do that open the Java 6 plugin configuration program, located in the Windows control panel as 'Sun Java 6 Plugin Control Panel'. In that window go to the 'Java' tab, click on show/see, and for all Java versions installed in your system add '-Xmx100m -Xms30m' (without ') in the tab 'Parameters of the execution environment'. Click on Accept, Apply, and reinitialize the browser and load again this page.

    It is not possible to export images to the hard disk.

   The easiest workaround is to download the .zip file containing the program and to use it locally from your hard disk, so that the security restrictions dissapear (usually). To export images to the hard disk it is necessary to configure Java properly. If you have problems and want to fix them, find the java.policy file inside the directory ..jre/lib/security (in Linux it is usually located at /usr/lib/jvm/java-6-sun-1.6.0.xx/jre/lib/security or /etc/java-6-sun/security/), and add the lines that appear below. As a general rule, to do this is not recommended due to security risks, if you do this you assume that this web page is secure.

    In case of error the program will try to copy the image to the clipboard, which can also fail due to security restrictions. To allow only the access to your clipboard (recommended, although any other applet you execute will be allowed too). Inside the section

grant {
    add the line

permission java.awt.AWTPermission "accessClipboard";

    To allow everything: access to clipboard, to directly write the image from the program, and the automatic update of orbital elements. Add following lines to the beggining of the file (this could be not enough to give priviledges to the applet).

grant codeBase "*" {