For this article, I want to look at the largest thing possible, the whole universe. At least, that's the claim made by Celestia, the software package I'm introducing here. In all seriousness though, Celestia is a very well done astronomical simulator, similar to other software packages like Stellarium. Celestia is completely open source and is licensed under the GPL.
If Celestia isn't available via the package management system for your favorite distribution, you always can get the latest stable version from the Celestia's website as an installable binary package. If you really need the absolute latest version, you can grab it from the GitHub repository. Binaries also are available for Windows and Mac OS X, in case you need to travel on the dark side of computing.
Once you have installed Celestia, starting it provides a view of the Earth from space.
Figure 1. Celestia begins your exploration of space with a 3D view of Earth.
You're first placed on a track that follows the Earth through space. This is necessary, because Celestia is actually a real-time simulation. If you were in a fixed location in space, any object you were looking at quickly would leave your field of view. You can pause the simulation by pressing the spacebar. Once you are following an object, you can rotate your view by clicking the left mouse button and dragging left/right or up/down.
If you're more interested in observing a centered object, you can click the right mouse button, and then dragging will move you around the object instead, allowing you to see the object's details. You can zoom in or out by using the mouse wheel. All of these navigation actions also have keyboard shortcuts, for those who prefer that to using a mouse.
But, how do you select which object you are centered on? The easiest option is to click the Navigation→Solar System Browser menu item to pop up a selection window.
Figure 2. You can use the solar system browser to select objects to center on within the solar system.
From here, you can choose from planets, moons, asteroids and other solar system objects available by default within Celestia (I'll explain how to add even more items shortly).
If you're looking at items beyond the solar system, you can click the Navigation→Star Browser menu item to open a new window.