Sky Motion Applet
Use this applet to:
- 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
- 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
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,
- 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.
- 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
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 doesn't have the features I want. Where can I find a better sky simulation?
Lots of places.
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
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
- 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
- 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