Visual Encyclopedia

Book Preface

Looking into space
For thousands of years, humans have told stories to explain the lights  in the night sky. Even the earliest stargazers recognized that many of  the objects that they could see behaved differently from each other,  but it was in ancient Greece – from around the 6th century BCE – that  astronomy began to be systematized. Greek astronomers drew up  formal lists of constellations, developed a scale of magnitude to  describe the brightness of stars, and made attempts to model the  paths of the planets. Following the invention of the telescope in 1608,  physical differences between the various objects began to become  more apparent, leading to an explosion in scientific knowledge

Celestial objects
The Universe is full of objects,  large and small. Many of the  closest bodies in our Solar  System – asteroids, planets,  and moons – are made visible  by reflected sunlight. They  move against a seemingly fixed  background of more distant  objects: luminous stars, glowing  nebulae, and remote galaxies.

Our Solar System
The region of space governed by the Sun – and everything contained  within it – is known as the Solar System. It encompasses eight major  planets, at least five dwarf planets, and a wealth of smaller bodies.

How the Solar System formed
Our Solar System emerged from a collapsing disc of material that was in orbit  around the newborn Sun some 4.6 billion years ago. Mid-sized bodies called  planetesimals gradually formed, and eventually developed into today’s planets.

Kepler’s laws of planetary motion
Three laws, discovered by Johannes Kepler between 1609 and 1619, govern  the behaviour of planets orbiting the Sun, or any object in an elliptical orbit  around another. They reflect the changing influence of gravity with distance.

Solar System planets size and scale
Earth is the largest of the rocky planets in the  inner part of the Solar System, but it is dwarfed  in size by the gas giants found in the outer region.  These, in turn, are relatively tiny when compared  with the vast size of our Sun.