The Earth is old.  Ahem.  Let me try that again.  The Earth is OOOOOOOOOOLD.  If it had a birthday cake, there would be around 4.6 billion candles on it.  That's 4,600,000,000 candles!  When scientists talk about what happened in the Earth's past, they can't use minutes, hours, days, weeks, months, or even years.  There's just too much stuff that happened over too much time to hold it all in our heads.  When big things happen on Earth, they happen very slowly.  Plains turn into mountains.  Lakes turn into deserts.  Dinosaurs turn into birds.  We had to come up with new terms for the Earth's time.  Get ready to think big and think slow.

Before we start, we need a good way to see just how much of Earth's history stretches behind us.  Let's say you have a whole roll of toilet paper.  Don't blow your nose on any of it yet.  We need all of it.  This roll represents the history of planet Earth. That's all 4.6 billion years.  If we mark off the different eras and major events, the entire roll will be taken up by sections of all sizes.  Some eras are two or three pieces long and others are fifteen pieces, depending on the amount of time that passed during each segment of time.  Now let's unroll the whole toilet paper roll.  The time humans have spent on Earth would be the thinnest pencil line that you can draw on the very end of the roll.  Compared to the age of the Earth, we haven't been around very long at all.

When we talk about the life of the Earth, we talk about BIG things that happened.  We talk about when it was born, when land first showed up, when that land broke apart, when new living things showed up, and when they died.  The word geology means the study of rocks.  The geologic time scale uses events, or big things that have happened, instead of years to talk about the history of Earth.  Can you remember the first time you crawled?  Walked?  How about when you first lost a baby tooth?  Or moved into a new house?  Sure, you may count your age by years.  It's the things that happen that change your life, though.  That's how we look at Earth: by the big things that changed it over a long, long time. 

If I have to measure time on a geologic scale, I'm going to need a new clock.

Still, we need ways to measure these BIG things that happen.  So we have three important words to break up the history of Earth.  Eons are the biggest piece of Earth time, and they cover a lot of time.  Eons are so long, we haven't even come up with a number of years for them.  If we were to talk about your life in school using eons, one eon would be like your time from first to fifth grade.  Another would cover all of middle school.  There are not a lot of these times, and the numbers are different for each one.  Also, a lot of things happen to you during these times.  That's how it is with eons, too.  It only took three eons for Earth to make our air, make life, and have that life leave marks in the rocks.  Did all of that fit in your head?  I just hope it isn't too much.  I don't want your head to blow up. 

Now that your head's all stretched out, you should be able to think about times that are a little shorter.  Eras are the second biggest thing we use to measure the story of Earth.  These would be more like one year in your school life, like fourth or fifth grade.  Scientists choose eras when they find big changes in fossils.  When the plants or animals we dig up look really different than the ones a little farther down in the rock, that makes an era.  They are still hundreds of millions of years long.  Since these eras can be really long or only kind of long, we do not have one number for them either.  Maybe the Earth became really cold and a lot of living things died out, making way for new living things.  That would be an era.  What would make one for you?  Losing all your baby teeth?  I just hope that did not happen too quickly.

Now here's a different kind of calendar. I'll need some kind of interpreter for this.

Now we come to some names you might know.  Ever heard of Jurassic, as in the movie Jurassic Park?  Yes!  This is a period, or the third biggest way to measure the story of Earth.  As you might have already guessed, it's when most of the dinosaurs you know about lived.  There are two periods in each era, and they track the times when different plants and animals first showed up on Earth.  Because there are only two of them, you could think of these times as seasons in a school year.  The animals springing up, and then their fall.  Like the leaves in the trees . . . over millions of years.

Is this where you find the petting zoo?

The Earth is OOOOOOOOOOLD.  It's hard to wrap our heads around 4.6 billion years, so we have come up with new words to describe it.  Earth time is really long, so it helps us to compare its many, many years to the times in your life.  Eons, eras, and periods are kind of like changing schools, or years, or seasons.  And when we look at the many beautiful things the Earth has made--the mountains, the animals, and the seas--we can be a little less surprised. It had a long time to make them.  But that does not mean we are any less amazed.

References:  "Geologic Time Scale"  Esse, 2010.  <>

University of California Museum of Paleontology.  "The Proterozoic Eon"  UCMP, 2009.  <>

Enchanted Learning.  "Geologic Eons, Eras and Periods"  Enchanted Learning, 2010.  <>

National Geographic.  "Jurassic Period"  National Geographic, 2009.  <>

Windows to the Universe.  "Happenings During the Mesozoic Era"  Windows to the Universe, 2008.  <>