|A photographic collage depicting the diversity and evolution of life on Earth through the last 600 million years. The oldest fossils are at the bottom and youngest at the top. The size of each time interval is proportional to its duration.|
When we talk about recorded history, time is measured in years, centuries, and tens of centuries. When we talk about Earth history, time is measured in millions and billions of years.
Time is an everyday part of our lives. We keep track of time with a marvelous invention, the calendar, which is based on the movements of Earth in space. One spin of Earth on its axis is a day, and one trip around the Sun is a year. The modern calendar is a great achievement, developed over many thousands of years as theory and technology improved.
People who study Earth's history also use a type of calendar, called the geologic
time scale. It looks very different from the familiar calendar. In some ways, it
is more like a book, and the rocks are its pages. Some of the pages are torn or
missing, and the pages are not numbered, but geology gives us the tools to help
us read this book.