Students can sense the magnitude of change in estimations of Earth's age by making a graph with string and tape. Affix a strip of tape (about 1 m long) vertically to a suitable wall. Mark off the years 1600 to 2000 to create a time line. Divide the class into small groups and give each a historic estimate of Earth's age from those provided. Have students cut a piece of string to represent their historic estimate of Earth's age (let 4 million years = 1 cm). Then, have students tape the string to the time line horizontally at the appropriate date and add the written description. (Note: The group with Ussher's estimate will find that it cannot even be shown using the given scale. Have these students try to represent 5,662 years in a different way. The group with the most recent estimate will need nearly 12 meters of string; they may have to tape it into a corner and onto the next wall.)

Ask students why people might have once thought Earth was so much younger. What changes in science, technology, or society might have encouraged scientists to question Ussher's estimate? Students can research methods used by different scientists, and enter Relevent information on the time line. If they discover other estimates of Earth's age from that 400-year period, they should enter them as well.

Estimates of Earth's Age

• 1658 Bishop James Ussher publishes calculations, based on biblical accounts, that Earth's history began in 4004 B.C.E.
  
  
• 1897 Lord Kelvin argues that Earth is between 20­40 million years old, based on the rate at which he believes Earth cooled from a molten state to its present temperature.
  
  
• 1900 John Joly figures that if sodium washes into the oceans at a steady rate, the proportion of sodium in the oceans shows that Earth is 90­100 million years old.
  
  
• 1907 B. B. Boltwood applies knowledge of radioactive decay rates to estimate the age of Earth at 2.2 billion years.
  
  
• 1913 Arthur Holmes uses radioactive dating to show that Earth is 4 billion years old.
  
  
• 1953 Clair Patterson analyzes meteorites to determine that Earth is 4.65 billion years old.

Earth and Life Sciences Program Contents

Seeing and Believing
DNA "Fingerprinting"
In-Depth Investigation: The Dating Game