In some ways, it is more like a book, and the rocks are its pages.
Rocks that are now on top of a mountain may once have been at the bottom of the sea.
Thus, to understand the world we live on, we must add the dimension of time. 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. We keep track of time with a marvelous invention, the calendar, which is based on the movements of Earth in space.
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.
We study our Earth for many reasons: to find water to drink or oil to run our cars or coal to heat our homes, to know where to expect earthquakes or landslides or floods, and to try to understand our natural surroundings.
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.