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How Big Is A Billion?
('s definition - Merriam-Webster's definition)

If I gave you $1,000 a day, seven days a week, how long would it take you to collect 1 billion dollars?  (Assume that you spend none of the money, and collect no interest on it.)

  1. 1,000,000,000 / 1,000 gives us the number of days.  In this case, 1 million.
  2. Dividing 1 million by 365.25 (roughly the number of days per calendar year) gives us the number of years.

[A note on precision: Correctly using the formula above results in the "answers" on this page. However the number of days in a year is not precisely 365.25. While the length of the mean solar year is a matter of debate among astronomers, I gather that many now place it at 365.24218967 days (see for more information).  I have chosen to use 365.25 as it is an easier number with which to work. Also, the length of the year is constantly changing ever so slightly. Over time those changes add up significantly. Therefore, using 365.25 as a constant leads to answers that appear precise but are not. For example, using the Gregorian Calendar, I'm told that adding 1 million days to January 1, 0001 results in November 27, 2738 rather than the November 7, 2738 that the formula above generates. This is another example of the importance of teaching Process over Answers.]

the answer: 2.73785 years (2 years, 8 months, 26 days)

To complicate things a bit (but once again show the importance of stressing processes rather than answers), the answer to the starting question shown above is correct as long as one is in the U.S. where one billion is defined as a thousand million.  In Britain, however, one billion is defined as 1,000,000 million (one million million, or what we in the U.S. call a trillion).  So in Britain, the analysis would look like this:

Scale 7,235,848  years = 365  days
Event # of years ago Appears
the Ape-Human split 7,235,848 January   1   12:00:00 AM
Australopithecus Afarensis 6,235,848 February  20   10:38:20 AM
Homo Ergaster 4,435,848 May  22   05:47:21 AM
Homo Neanderthalis 2,935,848 August   5   09:44:52 PM
Homo Sapiens 2,855,848 August   9   10:35:56 PM
Writing is invented 2,740,848 August  15   05:49:20 PM
Common Era begins 2,737,850 August  15   09:27:06 PM
Today, May 18, 2002 2,735,848 August  15   11:52:32 PM
1 British billion accumulates 0 December  31   11:59:59 PM

Notice that on this calendar, almost all that we know of human history (commencing with the invention of writing approximately 5,000 years ago) occurs in just over six hours on August 15.  The nearly two and three quarter million years between today on the calendar and the final accumulation of one billion (British) dollars is nearly equivalent to the time span between the disappearance of Australophithecus Afarensis (the species represented by the "Lucy" skeleton discovered by Donald Johanson in the mid-70s) and today.  What might a species descended from us, given that amount of evolutionary time to develop, be like?  Of one thing we can be fairly certain - the dollar will no longer exist.
  1. 1,000,000,000,000 / 1,000 gives us the number of days.  In this case, 1,000 million.
  2. Dividing 1,000 million by 365.25 (roughly the number of days per calendar year) gives us the number of years.

If you like thinking about large numbers, don't miss Jim Holt's essay, Larger Than Life: Can numbers become too big?. (Unfortunately, Lingua Franca, the magazine that published Holt's essay, suspended publication at the end of 2001. His essay, and many other fine articles, disappeared from the web when they took down their site.  Thankfully, the Internet Archive saved it, as it has so much else.  Also, I assume that Holt's essay was published on paper, so perhaps you can find a copy in a library that subscribed to Lingua Franca.  I believe it appeared in the February 2001 edition (Volume 11, No. 1).)    You also might be interested in the notation for some very large numbers.  If so, have a look at this.

Of course, very large numbers can lead to discussions about infinity.  For ideas about introducing this concept to your students, check here.

Books that help put the very large and very small into perspective

Web sites that help put the very large into a manageable perspective

Related activities

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original web posting: Thursday, December 10, 1998
last modified: Tuesday, June 04, 2013