alt="The geologic time scale"/>
The geologic time scale
How old is the Earth?
The Earth is approximately 4.54 billion years old (+/- 1%). We know this because scientists use radiometric dating techniques to analyze samples in order to determine its age. One of the oldest materials found in the Earth is a sample of crystals of the mineral zircon, found in Western Australia, that is at least 4.04 billion years old.
How was the Earth formed?
Scientists believe that the Earth was formed, along with the rest of the solar system, from a massive gas cloud. As the cloud solidified, it formed the solid masses such as the Earth and the other planets.
What is the circumference of the Earth?
The circumference of the Earth at the equator is 24,901.55 miles (40,066.59 km). Due to the irregular, ellipsoid shape of the Earth, a line of longitude wrapped around the Earth going through the north and south poles is 24,859.82 miles (40,000 km). Therefore, the Earth is a little bit (about 41 miles [66 km]) wider than it is high. The diameter of the Earth is 7,926.41 miles (12,753.59 km).
Is the Earth a perfect sphere?
No, the Earth is a bit wider than it is “high.” The shape is often called a geoid (Earth-like) or an ellipsoid. The rotation of the Earth causes a slight bulge toward the equator. The circumference of the Earth at the equator is 24,901.55 miles (40,066.59 km), which is about 41 miles (66 km) greater than the circumference through the poles (24,859.82 miles [40,000 km]). If you were standing on the moon, looking back home, it would be virtually impossible to see the bulge, and the Earth would appear to be a perfect sphere (which it practically is).
What is a hemisphere?
A hemisphere is half of the Earth. The Earth can actually be divided into hemispheres in two ways: by the equator and by the Prime Meridian (through Greenwich, England) at 0 degrees longitude and another meridian at 180 degrees longitude (near the location of the International Date Line in the western Pacific Ocean. Zero and 180 degrees longitude divide the Earth into the Eastern (most of Europe, Africa, Australia, and Asia) and Western (the Americas) Hemispheres.
What is the equator?
The equator divides the Earth into Northern and Southern Hemispheres. There are seasonal differences between the Northern and Southern Hemispheres. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere. But there is no such difference between the Eastern and Western Hemispheres.
Western (left) and Eastern Hemispheres of our planet
How was the circumference of the Earth determined?
The Greek geographer and librarian at the Great Library of Alexandria, Eratosthenes (c. 273—c. 192 B.C.E.), was aware that the sun reached the bottom of a well in Egypt only once a year, on the first day of summer. The well that he studied was near the city of Aswan and the Tropic of Cancer (where the sun is directly overhead at noon on the summer solstice). Around the year 240 B.C.E., Eratosthenes estimated the distance between the well and the city of Alexandria based on the length of time it took camel caravans to travel between the two places. He measured the angle of the sun’s shadow in Alexandria at the same time that the well was lit by the sun and then used a mathematic formula to determine that the circumference of the Earth was 25,000 miles (about 40,000 km)—amazingly close to the actual figure, which at the equator is 24,898 miles (40,070 km).
What are the Arctic and Antarctic Circles?
The circles are imaginary lines that surround the North and South Poles at 66.5 degrees latitude. The Arctic Circle is a line of latitude at 66.5 degrees north of the equator, and the Antarctic Circle is a line of latitude at 66.5 degrees south. Areas north of the Arctic Circle are dark for 24 hours near December 21 and areas south of the Antarctic Circle are dark for 24 hours near June 21. Almost all of the continent of Antarctica is located to the south of the Antarctic Circle.
If the Earth is so large, why did Christopher Columbus think that India was close enough to reach by sailing west from Europe?
The Greek geographer Posidonius did not believe Eratosthenes’s earlier calculation, so he performed his own measurement of the Earth’s circumference and arrived at the figure of 18,000 miles (28,962 km). Columbus used the circumference estimated by Posidonius when he argued his plan before the Spanish court. The 7,000-mile (11,263-km) difference between the actual circumference and the one Columbus used led him to believe he could reach India rather quickly by sailing west from Europe.
How fast does the Earth spin?
It depends on where you are on the planet. If you were standing on the North Pole or close to it, you would be moving at a very slow rate of speed—nearly zero miles per hour. On the other hand, those who live at the equator (and therefore have to move about 24,900 miles [40,000 km] in a 24-hour period) zoom at about 1,038 miles (1,670 km) per hour. Those in the mid-latitudes, as in the United States, breeze along from about 700 to 900 miles (1,126 to 1,448 km) per hour.
Why don’t we feel the Earth moving?
Even though we constantly move at a high rate of speed, we don’t feel it, just as we don’t feel the speed at which we’re flying in an airplane or driving in a car. It’s only when there is a sudden change in speed that we notice, and if the Earth made such a change we would certainly feel it.
Does the Earth spin at a constant rate?
The rotation of the Earth actually has slight variations. Motion and activity within the Earth, such as friction due to tides, wind, and other forces, change the speed of the planet’s rotation a little. These changes only amount to milliseconds over hundreds of years but do cause people who keep exact time to make corrections every few years.
What is the axis of the Earth?
The axis is the imaginary line that passes through the North and South Poles about which the Earth revolves. Because the Earth is tilted along this spin axis, as we make our way around the sun in an elliptical orbit for 365 days, we have our seasons (winter, spring, summer, and fall). As the Earth’s axis is pointed toward the sun for people in the Northern Hemisphere, we experience summer. At the same time, for people in the Southern Hemisphere, as the axis points away from the sun, we experience winter.
What is inside the Earth?
The Earth is comprised of several layers: crust, mantle, and core. The outermost part of the Earth, the crust, is divided into huge plates that float atop the mantle, and are always in motion. The crust is comprised of the elements iron, oxygen, silicon, magnesium, sulphur, nickel, and trace amounts of many other elements. Beneath the crust is the mantle, which is about 1,800 miles (2,890 km) deep and is composed of silicate rocks, which are heated and cooled by the core. The mantle, which makes up the bulk of the interior of the Earth, is composed of three layers—two outer layers are solid and the inner layer (the asthenosphere) is a layer of rock that is easily moved and shaped. At the very center of the Earth is a dense and solid inner core of iron and other minerals that is about 1,520 miles (1,220 km) wide. Surrounding the inner core is a liquid (molten) outer core that is about 1,355 miles (2,180 km) thick.
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