USGS: Science for
a Changing World


Scientists who study the past try to put events in their proper order. When we discuss events that happened in historical times, we often use dates or numbers, but we do not have to do so.

Layered rocks at St. Stephens, 
In layered rocks like these at Saint Stephens, Alabama, geologists can easily determine the order in which the rocks were formed.
Consider six historical events: the Wright brothers' flight, the bicentennial of American independence, the First and Second World Wars, the first astronaut landing on the moon, and when television became common in homes. First, let's try to put these events in order. Our knowledge of the words first and second tells us that the First World War came before the Second World War. We may know or may have been told that the landing of Neil Armstrong on the moon was seen by many people on television, but there was no television around when the Wright brothers flew at Kitty Hawk. Thus, we can order these three events: first Wright brothers' flight, then television common in homes, then the landing on the moon. By a process of gathering evidence and making comparisons, we can eventually put all six events in the complete proper order: Wright brothers' flight, First World War, Second World War, television common in homes, landing on the moon, and American bicentennial.

Sixth historical events of the 20th 
Century in relative and numeric order
The box at the top shows six events that occurred during the twentieth century. The bottom shows these events in relative order and numeric order.
Because we have written records of the time each of these events happened, we can also put them in order by using numbers. The Wright brothers' flight occurred in 1903, the First World War lasted from 1914 to 1918, and the Second World War lasted from 1939 to 1945. Televisions became part of our homes in the 1950's, Neil Armstrong walked on the moon in 1969, and America celebrated 200 years of independence in 1976.

Written records are available for only a tiny fraction of the history of Earth. Understanding the rest of the history requires detective work: gathering the evidence and making comparisons.

This page is URL:
Last updated 10 June 1998 (krw)
Maintained by John Watson