This is the most expansive part of the site for it covers how our basic understanding of the rest of the site came to be: Humanity's exploration of space.
The first section, "Ancient Times," covers what ancient people and civilization learned about the universe, how they viewed it, and some evidence for it (such as Stonehenge, calendar systems, and the alignment of the pyramids).
"Scientific Revolution through Pluto's Discovery" is something of an arbitrary section in time, but it effectively covers the transition period between ancient understanding and our current scientific processes. Included are some of the major players such as Galileo Galilei, Isaac Newton, and Johann Kepler. It also discusses some of the major uses of astronomy like how to determine longitude. Instrumentation such as the development of the telescope is also discussed. And also included are how we began to catalog the sky along with the discovery of the modern planets.
"Space Age" is the largest sub-section that discusses pretty much every scientific space probe or mission that has been launched. This includes huge space missions such as Sputnik, Gemini, and Apollo; it also includes planetary probes such as all the Mariners and Vikings, Voyagers and Pioneers, and modern ones like Galileo, MESSENGER, and Cassini. Specifically NOT discussed in this section are space stations, the US space shuttle, and Earth-observation satellites.
Finally comes the "Observational Techniques" section which is a broad category to summarize how telescopes work, different types of telescopes, the important field of spectroscopy, and how astronomical detectors work.