26.HOW WAS THE LIGHT-YEAR DISCOVERED?
While we can’t fully explain light, we can measure it quite accurately. We have a pretty good idea of how fast light travels. Since a light-year is merely the distance that a beam of light will travel in a year, the real discovery had to do with measuring the speed of light.
This was done by a Danish astronomer named Olaus Roemer in 1676. He noticed that the eclipses of one of the moons of Jupiter kept coming later and later as the earth moved in its orbit to the opposite side of the sun from that occupied by Jupiter. Then, as the earth moved back into its former position, the eclipses came on schedule again.
25.HOW WAS THE GRAND CANYON FORMED?
The Grand Canyon is one of the greatest spectacles on the face of the earth. At some points it appears like a magic city of rock, with temples, towers, and castles of dazzling colors.
One of the most amazing things about it is that the Grand Canyon was made by a river! The waters of the Colorado River cut out this great gorge in the course of thousands of years. When you consider that it was cut out of solid rock in many places, you begin to appreciate the tremendous force of these waters. Even now, year by year, the rushing Colorado continues to cut deeper into the bottom of the gorge.
24.HOW OLD IS THE EARTH?
This is a question to which we may never have the exact answer. Man has wondered about the age of the earth since ancient times, and there were all kinds of myths and legends that seemed to have the answer. But he couldn’t begin to think about the question scientifically until about 400 years ago.
When it was proven at that time that the earth revolved around the sun (in other words, that the earth was part of our solar system), then scientists knew where to begin. To find the age of the earth, it was necessary to explain how the solar system was born. How did the sun and all the planets come into being?
One theory was called the nebular hypothesis. According to this theory, there was once a great mass of white-hot gas whirling about in space and getting smaller and hotter all the time. As the gas- cloud grew smaller, it threw off rings of gas. Each of these rings condensed to form a planet, and the rest of the mass shrank into the center to become the sun.
Another explanation is called the planetesimal theory. According to this, millions and millions of years ago, there was a huge mass made up of small, solid bodies called planetesimals, with the sun at the center. A great star came along and pulled on the sun so that parts of it broke away. These parts picked up the tiny planetesimals the way a rolling snowball picks up snow, and they became planets.
Whichever theory is right, astronomers have figured out that it all probably happened about 5,500,00O,000 years ago! But other scientists besides astronomers have tackled this question. They tried to find the answer by studying how long it took for the earth to become the way we know it. They studied the length of time it takes to wear down the oldest mountains, or the time needed for the oceans to collect the salt they now contain.
After all their studies, these scientists agree with the astronomers: The earth is about 5,500,000,000 years old!
