We have all heard people talk about something that happened a long time ago.  For instance, maybe someone has said to you, “That house was built in 1850.”  Or perhaps your teacher has discussed a civilization that lived 3000 years ago.  When you hear these dates or numbers, what do you think?  You probably think, that sounds like a long time ago!  What exactly does “a long time ago” mean?  How much time has passed since that house was built?  Or, what year was it when that civilization lived?  These are the kinds of the questions we will learn how to answer in this lesson.

How do we measure time?  When we think about important events that have happened throughout history, we often think of certain years.  For example, in the year 1969, man first walked on the moon.   Does that seem like a long time ago to you?  Let's go back even farther.  The Declaration of Independence was signed in the year 1776.   That was a really long time ago, wasn't it?  What if someone told you that 4500 years ago, the Egyptians were building pyramids?  Now, that was a really really long time ago!  Could you figure out what year that was?

In order to understand the passage of time, we must understand our calendar.  Our calendar tells us what year it is.  We know that it is the year 2003.  As we go back in time, we subtract, or take away years.  If you are nine years old and you have had your birthday this year, then we can go back, or subtract, nine from 2003 and find out that you were born in the year 1994.

But what happens when we go back to that really really long time ago when the Egyptians were building pyramids?  We can not take 4500 away from 2003.  Yet the world was a very busy place 4500 years ago, and those were very real times.

The calendar we use starts with the year 1.  The year 1 is labeled with the letters AD following it.  Every year after 1 AD, including our year 2002 years later, is also labeled with the letters AD.  So our year is called 2003 AD.  That means that two thousand and three years have passed since our calendar started.  Lots of things have happened in 2003 years! But, lots of things happened before that year 1 AD, too.

The year before 1 AD, when our calendar began, is also called year 1.  We use different labeling letters however, so we can tell the two years apart.  The year before the calendar began is called 1 BC.  The year before that is 2 BC.  As we go farther back in BC time, we add more years.  We can go back to 100 years BC, 1000 years BC, 5000 years BC, and more!  So, whenever we see “BC” we know that were talking about more than 2003 years ago.

Now, let's go back to our question that asked what year it was 4500 years ago, when the Egyptians were building pyramids.  First, we'll round off 2003 to the year 2000.  This will make it much simpler.  Since we are talking about a period of time so long ago, and building the pyramids took many years, it won't matter if we round off 3 years.  Now, let's subtract 2000 from 4500 to see how many years we have left over.  4500 - 2000 = 2500.  That means that there were 2,500 years before the calendar began.  So, something that happened 4500 years ago, happened in the year 2500 BC.  That was the time when the Egyptians were building the pyramids.

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