The Universe in the Classroom

An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time


Scientific observations have not only revealed that the universe is very old, they have also shown that it changes over time, or - to use the word that has stirred so much controversy - that it "evolves". These cosmic changes are often very difficult to observe, because they happen so slowly. We have been studying the sky with powerful telescopes for only about a century, but astronomical changes can take millions to billions of years. We must therefore combine observations of many different objects out there and use our deductive powers to uncover evidence of cosmic evolution. Luckily nature has left a wide range of clues about evolution for us - at every scale of the universe - which we can uncover with some good astronomical detective work.

a) Changes in the Solar System

Because we have explored our solar system (with people landing on the Moon, and robot spacecraft landing on or flying by most of the planets), we have a lot of information about the history of our neighbor worlds. It is clear that all the planets have undergone profound changes with time and have a common origin in the great swirling cloud that made the Sun some 5 billion years ago.

We can calculate when the materials of the Earth's crust congealed from molten lava to hard rock (the geological, not the musical kind). As we discussed above, we can look at radioactive elements in the rock, and see how much of the radioactive parent and how much of the stable daughter elements are there. Our laboratory work shows that the process of radioactivity is not affected by temperature, pressure, or other outside factors, and proceeds at a rate set only by the little natural clocks built into the nucleus of the atom. Since many rocks have more than one radioactive element, they actually have several nuclear clocks running at the same time. These can be compared to check our results. Individual rocks on Earth have measured ages that go from last week (for rocks that just congealed from lava flows in Hawaii) to more than 4 billion years ago.

The Earth from Space
The Earth from Space

If you and your students take a good look at a world map, you can see that the continents "fit into" one another like pieces of a jigsaw puzzle. The coastline of Africa, for instance, neatly fits into that of South America. This is because these continents used to be joined, but have been drifting apart. Far back in the past, the very face of our world was different. Today, scientists can actually measure the rate at which the continents are moving - a few centimeters per year - and estimate how long it has taken them to move apart to their present positions.


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An Ancient Universe - Table of Contents

Home | Introduction | The Universe: An Overview | The Process of Science | The Ancient Universe - The Age of the Expanding Universe - The Age of the Oldest Stars - The Age of Light From Distant Galaxies - The Age of the Chemical Elements | The Changing Universe - Changes in the Solar System - Changes in Stars - Changes in the Universe | Science and Religion | Resource Guide | Activities

© Copyright 2001, American Astronomical Society. Permission to reproduce in its entirety for any non-profit, educational purpose is hereby granted. For all other uses contact the publisher: Astronomical Society of the Pacific, 390 Ashton Ave., San Francisco, CA 94112.

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