Introduction to using Celestia
To start Celestia, you should double-click on its icon. As it starts, it first shows the Sun, and then takes your viewpoint to the sunlit side of the Earth. When you get there, you can tell Celestia to show you other interesting sights.
Initially, Celestia opens in a window on your screen. For a more immersive experience, you can tell Celestia to take over the entire display. Open Celestia's menu Render/Select Display Mode and choose an appropriate screen resolution. When it's in full-screen mode, if you move the cursor up to the top of the screen, you should see Celestia's menu bar again, where you can choose to go back to windowed mode.
Some may think Celestia is a game. In a way it is but it is much more than that. Like most programs (with graphics and objects) of today the mouse is the primary way of getting around. Celestia does use the mouse to rotate/panned the observers POV with a combination of left and right mouse click and drag functions. However to truly get the most out of Celestia the keyboard shortcut letters bring the full power and beauty of the program into play. People who are more game enthusiasts may find the interface of Celestia boring after a few minutes if they only use the mouse. So, start learning and memorizing the keyboard shortcuts. You can find a quick look of the keyboard shortcut commands in the Help/Control pulldown while the program is running. You can also find them in the Celestia User's Guide.
Below are some introductory events and places you can explore in Celestia, using the keyboard and mouse. You will notice below some letters and words are in bold face. These are the keyboard shortcuts that can be typed to control Celestia.
Earth's rotation and the pole star
This needs a fairly wide field of view – use the comma (,) and dot (.) keys to adjust the window to around 60°. If you don't have the Earth on the screen, press the three keys H3G to go there. Then back away a little with the End key or your mouse's wheel. Adjust the number of stars with the [ and ] keys, setting a limiting magnitude of around 5.
Adjust the rate of time to 1000 times faster (LLL) so the Earth can be seen slowly turning. The Earth spinning is what gives us day and night. Type a "y" to hover over one point on the Earth and see it enter light and shadow (day and night) as the Earth turns. Use the arrow keys Shift-← and Shift-→ to move round the Earth and see where night and day start and end. You can put the mouse cursor over a piece of land and just watch that point as day and night pass over it. This is best done with the clouds turned off (I). To turn the "sync orbit" off again, you can press F to return to "follow" mode.
With the synchronous orbit on, back away from the Earth a little using the End key (or the rotary wheel of your mouse if it has one) so you can see more stars. You can press the Home key to move closer again. As you watch the stars go past, you might see distinctive patterns of stars go by. People used to imagine people and animals in the patterns in the stars. Press / to turn on the constellation lines, and = to turn on their names. You can use the space-bar to pause time and take a longer look at any interesting shapes.
With the mouse, right-click and drag upwards to turn to look more Northwards. By the time you are looking squarely at the South pole of the Earth, the pole star Polaris should be visible. It's the star that's at the end of the tail of Ursa Minor, the "Small Bear", often called "the Little Dipper". If you watch for a while, you will see that all stars seem to revolve around that point while Polaris itself doesn't move much at all.
Keeping this view, zoom closer to the Earth (Home) and notice the way day and night look on Earth from this view. Depending on the time of year, the Antarctic ice-cap might be on permanent daylight or permanent shadow. This can be easier to see if you speed up to 10000x (L). You can briefly speed up even more (use K to slow down again) to hop between seasons.
Start Celestia. You are looking at the Earth from the direction of the Sun. We want to look at the Earth from the other side. Back away from the Earth using the End key until the Earth is about the size of a pea, and then use Shift-← and Shift-→ to rotate around the Earth until the Sun is visible in the background. The Earth might be easier to see if you turn up the background illumination with the } key, although this is not realistic as it makes planets glow in the dark (only stars really do that). Now turn the time rate up to a million times faster by typing LLLLLL.
You should notice the Moon whizzing round the Earth. Move closer or further away (Home/End) until the moon uses the whole width of the screen. The Moon takes about 28 days to go round the Earth. If you slow down time a bit by pressing K, you should be able to count the number of days (turns of the Earth) it takes for the Moon to go round once. You can turn the Moon's name on and off by pressing M. If you see a giant mass of names appear, all together, it's probably the names of all Jupiter's moons in the distance – Jupiter has a lot of moons. Speed back up to a million times again.
You will notice that every year as the Sun goes past, it is accompanied by a collection of small bright dots. These are other planets. You can turn their names on and off with P. It's time to leave the Earth. Type HFC to follow and centre the Sun. Leave planet names on (P). Back away from the Sun a little and you will be able to see Mercury, Venus, Earth and Mars moving from side to side as they orbit the Sun. If you back away further you will be able to see 9 planets. The outer ones don't seem to be moving much, so press L speed things up a little. Time is passing at about a year every 4 seconds now.
Let's turn on the planet's orbit lines – press O. They're not very clear seen edge on, so use the right mouse button to drag downwards until the orbits look circular, seen from above. You can't see all the orbit lines at once – if you move far enough away to get Pluto's orbit in, then the inner planet's orbits disappear. Move in to see the inner planet's orbits and the outer orbits are off the screen. By right-dragging, you can get a good edge-on view that shows that all the orbits are in almost exactly the same plane except for Pluto.
You may notice as Jupiter goes past that it has a huge mass of orbit lines for its many moons. Four of them are big enough to be seen from the Earth by using binoculars.
Time for a quick tour. This is possible best done with the orbit lines switched on. First, to about 1000x time rate (K,L). Then we can visit the 9 planets by typing 1G, 2G etc. Each of the number keys corresponds to a planet. Use Home and End (or the mouse scroll wheel) to zoom in and out, and right-drag to change the viewpoint and get a good look at each planet.
While visiting the outer planets, it is worth looking from above to see just how much like a little solar-system their moons make. You may occasionally see a black spot move across the sunlit surface of Jupiter – this is the shadow of one of the moons as it passes between Jupiter and the Sun.
Not all stars are like the Sun. Some stars are hotter than the sun, some cooler, bigger, smaller, brighter and dimmer, redder, bluer. Actually the blue ones are hotter, and the red ones cooler.
You cannot tell how far away stars are by just looking at them. So when people invented the constellations, they drew lines using stars that might be nearby, or far away. A good demonstration of this is to see how distorted familiar constellations would look if seen from a different angle – from a different place.
Turn on the constellation lines (/) and use dot and comma to set the field of view to around 60°. Centre a star in the constellation of the plough (Ursa Major, also called the Big Dipper) by pressing the Enter (↵)key, typing the name megrez and enter again. Then type CF to centre and follow the star. Now hold down the right mouse key and drag around the screen. Your viewpoint is now many light years away from the Earth, orbiting around the distant star Megrez. Try both with and without the constellation lines. You can plainly see the 3D pattern of the stars. If you move right round the back of the Plough (or Big Dipper), you can see that all the constellation lines seem to point roughly in the same direction – the direction of Earth.
It is interesting to return to the Sun while maintaining your view on Megrez – press T to track Megrez (keep it centred), then HG to go home. You can see the constellations return to their familiar shapes as we get back to the Sun. Press T again (or Esc) to stop tracking Megrez.
Another good star to do this with is Arcturus in the constellation of Boötes.
The constellations are all drawn with stars that are visible from the Earth, and are therefore the nearest stars to us. Turn the constellation lines on and then back a long way away (hold End), until all the lines are visible on the screen. Turn on more stars by holding the ] key and raise the limiting magnitude to about 10. Back away a little more until the whole of the galaxy is visible on the screen. Then right-drag so the galaxy is seen edge-on. You can see that the stars in the Celestia program form a sphere round the Sun. This is because we don't know the distances of all the other stars in the galaxy well enough to put their positions in the program – so the rest of the galaxy is just shown as a 'mist' of stars.
Reduce the limiting magnitude to 10 or less again with the [ key.
You can fly to many other galaxies, much like our own. For example, pressing ↵M 31↵g takes you to the Andromeda Galaxy (aka M 31), the closest spiral galaxy to the Milky Way. You can always go home again by typing the keys H3G.