• Show where the stars, planets, sun, and moon are in the sky;
• Understand the motions of the heavenly bodies;
• View the sky from anywhere on earth, at any time in the reasonably near future or recent past;
• Learn the constellations and the names of the brighter stars.

User Guide

• The default view shows half the visible sky, with your horizon at the bottom and zenith (the point straight overhead) at the top. To change the direction you're facing, press and drag on the direction letters just below the horizon.

• Use the controls at top-left to adjust the time. The outer dial controls the time of day, while the inner dial controls the date of the year. The center buttons let you start and stop continuous time animation, forward or backward. Click these buttons repeatedly to change the speed. The animation will be in increments of half a minute when the outer (time) button is selected (as indicated by the white dot), and in increments of days when the inner (date) button is selected. To select the other button, stop the animation and then click on it.

• Use the controls at top-right to adjust your location on earth. The round dial sets your longitude while the vertical slider sets your latitude. Here's a web site that will tell you the longitude and latitude for any location on earth.

• On any of the time and location controls, clicking the white line to either side of the button nudges the setting by a small amount.

• Check the "Trails" box and then start the animation to see trails of the moving stars and other celestial bodies. You can use this feature to draw simulated long-exposure photos of star trails, and to depict more intricate (apparent) motions such as analemmas and retrograde loops. Uncheck the box to clear the trails. (And you'll probably want to uncheck it before changing the latitude, longitude, or direction you're facing.)

• This applet doesn't know about modern time zones or daylight saving time. Instead it displays local mean time (LMT), which is more or less "time by the sun". At 12:00 noon LMT, the sun will be near its highest point in the sky on that day. Unless you're located on your time zone's central meridian, LMT differs from standard time by a fixed amount that depends on your east-west location within your time zone (four minutes for each degree of longitude east or west of the central meridian). Try adjusting the longitude and watch what happens to the LMT value. Alternatively, you can check the box to make the applet display Universal time (UT), which is LMT at longitude zero (e.g., Greenwich, England).

• Most of the other checkboxes let you select what is and isn't shown in the applet's simulated sky. When "Sidereal days" is checked, any further changes to the date (using the dial or the animation buttons) are in increments of 4 minutes less than a full day. This is the time it takes for the stars, rather than the sun, to return to the same place with respect to our horizon. Use this setting when you want to see how the planets, moon, and sun appear to move with respect to the stars.

• Point the cursor at a bright star or planet to see its name.

• Click the "Full sky view" button to switch to a view of the full sky, with zenith at the center and the horizon around the edge. This can be a little disorienting at first, but it's more convenient than constantly dragging the view to a different direction.

FAQ

• Why do the stars and planets appear to move? The daily motion of the stars and other celestial bodies is actually caused by the spinning earth, which continuously shifts our horizon with respect to the heavens. The moon's eastward motion with respect to the stars is caused by its monthly orbit around the earth. The sun's apparent motion (with respect to the stars) is caused by earth's yearly orbit around the sun. And the planets' motions, as viewed from earth, are a combination of their orbital motions around the sun and our own orbital motion.

• Why are the constellations distorted near the chart's edges? Because it's impossible to map the domed sky onto a flat screen without distortion. To view a constellation without distortion, adjust the view so it's near the middle of the (half or full) circle.

• What are all those lines and grids? They help us understand and describe where things are in the sky. The altitude-azimuth grid is like a latitude-longitude system based on our local horizon, with the "north pole" straight overhead. The declination-right-ascension grid is similar, but it's fixed with respect to the stars and has its north pole over earth's north pole. The meridian is an imaginary line running from north to south and passing straight overhead. The celestial equator is just the equator of the Dec-RA coordinate system, directly above earth's equator. And the ecliptic is a great circle around the sky that traces the sun's apparent path among the stars. (The ecliptic passes through the twelve constellations of the zodiac, and the moon and planets are always found near it.)

• How accurate is this applet? Accurate enough for most casual naked-eye observing, but not for high-precision predictions. The applet ignores atmospheric refraction, so predictions of rise/set times will be a few minutes off. The applet also ignores precession of the equinoxes, so its depiction of the stars will become noticeably inaccurate if you run the time forward or backward more than about a century. And it's possible that the applet's code contains bugs or bad data; I haven't done extensive testing.

• Why won't it let me set my exact latitude and longitude, to the nearest fraction of a degree? This applet isn't intended for high-precision work; see the answer to the previous question.

• Are the star colors real? More or less. They're based on the color tables found here, with a few simplifications that your eye is unlikely to notice. The colors of stars indicate their surface temperatures, with the hottest stars being blue-white and the coolest stars orange-red. However, the human eye sees faint objects in black and white, so under the real sky we ordinarily notice the colors only for the brightest stars.

• Why are some stars visible in the daytime? So you can see where they are and better understand the motions of the sky. This applet isn't intended to be realistic in every respect.

• Does this applet show all the visible stars? No. This applet uses a database of about 1600 stars to magnitude 5.0, based on the Bright Star Catalog. This is more stars than you would normally see from a light-polluted city, or under a full moon. But under ideal conditions a person with good vision can see several times as many stars. The full Bright Star Catalog contains about 8400 stars with magnitude 6.5 or less.

• Are the planets, sun, and moon shown accurately? Yes and no. The brightnesses of the planets, compared to the stars, are reasonably accurate. Note that some planets vary quite a bit in brightness, with Mars becoming brightest when it is opposite the sun (that's when it's closest to us) and Mercury almost disappearing when it passes between us and the sun (because the side toward us isn't illuminated). Uranus and Neptune (and Pluto) aren't shown, because they aren't visible to the naked eye. The planets' colors are not especially accurate. The actual sun and moon are of course much brighter, relative to the stars, than can be shown on a computer screen. To make the sun and moon more recognizable, the sizes of their disks are exaggerated in this simulation.

• Will this applet show eclipses? This applet does plot the moon taking your position on earth into account, so you can use it to make approximate predictions of the time and location of solar eclipses. But the precision isn't high enough to determine exactly when an eclipse will begin/end, or where it will be total (or annular). The applet makes no attempt to show lunar eclipses, though I'd like to add that feature in the future.

• What other features might you add? I'd like the applet to display a little more information about the stars, and to show the Milky Way and a few naked-eye galaxies, clusters, and nebulae. I also wish the custom time and location controls were prettier. And I'd like to allow more parameters (besides latitude and longitude) to be set from the html page, so others could easily customize the applet for their own purposes. But I have no intention of adding any zoom capability or anything else that would be needed for binocular or telescope observing. For those purposes you should switch to a more professional program.

• Why did you write a new planetarium simulation when there are so many already? Good question! In fact, I considered calling this one YAPS, for Yet Another Planetarium Simulation. There are many excellent sky simulations out there, but for my astronomy students I needed one that's (a) free; (b) easy to access (nothing to download or install); (c) easy for beginners to understand; and (d) convenient for showing the motions of the stars and other objects with respect to earth's horizon. Sky View Cafe almost fits the bill, but its time and location controls are a little awkward and its default full-sky view can be disorienting for beginners. (For a more complete explanation of why I created this applet, see this blog article.)

• This applet doesn't have the features I want. Where can I find a better sky simulation? Lots of places. Here's a Google Directory list and here's another list of microcomputer programs, including some excellent free programs like Stellarium. There are also many handy mobile planetarium apps like these for the iPhone and iPod Touch. If you're looking for web software, Sky View Cafe and AstroViewer are both excellent, while Your Sky is a very flexible server-based program that lets you create a highly customized star chart. Finally, don't dismiss the idea of using a simple low-tech planisphere.

• This applet is too large to fit on the screen of my netbook/projector/wristwatch. What should I do? Try this smaller version.

• How can I run this applet from my own server or hard drive? Check back soon; I intend to post a copy of the .jar file that you can download and run from wherever you wish.

• I want to modify this applet. Will you release the source code? Yes, after I've had a little time to make sure it isn't too buggy.

• Where did you get your information/inspiration? It's a pleasure to acknowledge the following:
  • Sky View Cafe for general inspiration and some guidance on how to draw stars
  • The Yale Bright Star Catalog
  • H. A. Rey's Find the Constellations for making it clear that a half-sky projection is easiest for beginners to understand
  • Carlos A. Furuti for explaining how to do a half-sky projection
  • Fiona Vincent for explaining how to convert to horizontal coordinates
  • Paul Schlyter for explaining of how to compute planetary positions
  • Mitchell Charity for showing how to draw star colors on a computer
  • Konica-Minolta, whose Mediaglobe system inspired the "trails" feature
  • GoSkyWatch for showing how to design an intuitive time/date control

• Are all of these questions really "frequently asked"? No, I made most of them up.

Other applets and software

Astronomy course home page

Daniel V. Schroeder home page

Last modified on 17 February 2010