It is a fascinating story, but not so simple as single measurement. Our universe is a large, old, and complicated place. Earth and other bodies have endured a long and sometimes violent history, the events of which have frequently obscured the record that we seek to decipher. Although in detail the journey into Earth's past requires considerable scientific skill, knowledge, and imagination, the story is not so complicated that it cannot be explained to someone who wants to know and understand the basic evidence. This book, then, has been written for people with some modest background in science, but at a level that will allow the material to be useful and accessible to those without a deep knowledge of geology or physics or mathematics.