Category The World Around us

          The surface of the Earth, the crust, makes up a very small part of the whole planet. While it is relatively straightforward to find out about the Earth’s surface, investigating deep within the Earth is part science, part guesswork. What is known is that there are three main layers: the crust, the mantle and the core, and that these consist of rocks and metals in various states and forms.

          The Earth started out as a ball of very, very hot liquid. This liquid was mostly made of two elements called oxygen and silica. But there were small amounts of other elements too. In fact, it was a mixture of almost every element in existence. This all happened around 4.6 billion years ago – that’s a really long time, so long that we can’t even imagine it.

          Over time, Earth began to cool down. The heavier elements, like iron and nickel, sank into the centre of the planet (the core). And it’s hot: the Earth’s core is as hot as the surface of the sun, so hot that we wouldn’t be able to go near it, let alone touch it. But you don’t have to worry about getting too close. Wherever you are, whether in Kenya, China or Brazil, the core is around 1800 miles below your feet. This means we will never be able to visit it.

          Even though we can’t actually go to the Earth’s core, we know some things about it. We know, for example, that the core is full of iron, because Earth acts like a giant magnet, drawing some elements to it. This magnetic core is very useful: it means we can use a compass to find our way, like sailors in the ocean.

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          The rotation of the Earth causes it to bulge slightly in the middle. Centrifugal force makes the Earth’s material move away from the centre — the faster the spin, the greater the force. As places at the Equator are moving faster than places at the poles, the centre of the Earth pushes out slightly more than the rest.

          The simplest model for the shape of the entire Earth is a sphere. The Earth’s radius is the distance from Earth’s center to its surface, about 6,371 kilometers (3,959 mi). While “radius” normally is a characteristic of perfect spheres, the Earth deviates from spherical by only a third of a percent, sufficiently close to treat it as a sphere in many contexts and justifying the term “the radius of the Earth”.

          The concept of a spherical Earth dates back to around the 6th century BC, but remained a matter of philosophical speculation until the 3rd century BC. The first scientific estimation of the radius of the Earth was given by Eratosthenes about 240 BC, with estimates of the accuracy of Eratosthenes’s measurement ranging from 2% to 15%.

          The Earth is only approximately spherical, so no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km (3,947 – 3,968 mi). Several different ways of modeling the Earth as a sphere each yield a mean radius of 6,371 kilometers (3,959 mi). Regardless of the model, any radius falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km (3,950 – 3,963 mi). The difference 21 kilometers (13 mi) correspond to the polar radius being approximately 0.3% shorter than the equator radius.

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          The earth spins as a result of things colliding with each other when the Solar System was formed. Some scientists believe that the Earth started spinning after a direct collision with the Moon. Kept moving by the force of momentum, the Earth takes one day to make one full rotation.

          It can’t be a coincidence. Look down on the Earth from above, and you’d see that it’s turning in a counter-clockwise direction. Same with the Sun, Mars and most of the planets.

          It’s the conservation of angular momentum. Think about the individual atoms in the cloud of hydrogen. Each particle has its own momentum as it drifts through the void. As these atoms glom onto one another with gravity, they need to average out their momentum. It might be possible to average out perfectly to zero, but it’s really unlikely.

          Which means, there will be some left over. Like a figure skater pulling in her arms to spin more rapidly, the collapsing proto-Solar System with its averaged out particle momentum began to spin faster and faster. As the Solar System spun more rapidly, it flattened out into a disk with a bulge in the middle. We see this same structure throughout the Universe: the shape of galaxies, around rapidly spinning black holes, and we even see it in pizza restaurants.

          Over the course of a few hundred million years, all of the material in the Solar System gathered together into planets, asteroids, moons and comets. Then the powerful radiation and solar winds from the young Sun cleared out everything that was left over. Without any unbalanced forces acting on them, the inertia of the Sun and the planets have kept them spinning for billions of years.

          And they’ll continue to do so until they collide with some object, billions or even trillions of years in the future. The Earth spins because it formed in the accretion disk of a cloud of hydrogen that collapsed down from mutual gravity and needed to conserve its angular momentum. It continues to spin because of inertia.

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          The size of the Earth depends upon how you measure it. If you were to circumnavigate the world (on land and sea) following the Equator, you would travel 40,075km (24,902 miles). Starting at one pole and visiting the other, you would travel 67km (42 miles) less. The Earth’s diameter, pole to pole, is 12,714km (7900 miles), while the distance through the Earth at the Equator is a further 43km (27 miles).

          The radius of Earth at the equator is 3,963 miles (6,378 kilometers), according to NASA’s Goddard Space Flight Center. However, Earth is not quite a sphere. The planet’s rotation causes it to bulge at the equator. Earth’s polar radius is 3,950 miles (6,356 km) — a difference of 13 miles (22 km).

          Earth’s density is 5.513 grams per cubic centimeter, according to NASA. Earth is the densest planet in the solar system because of its metallic core and rocky mantle. Jupiter, which is 318 more massive than Earth, is less dense because it is made of gases, such as hydrogen.

          Earth’s mass is 6.6 sextillion tons (5.9722 x 1024 kilograms). It volume is about 260 billion cubic miles (1 trillion cubic kilometers).

          The total surface area of Earth is about 197 million square miles (510 million square km). About 71 percent is covered by water and 29 percent by land.

          Mount Everest is the highest place on Earth above sea level, at 29,028 feet (8,848 meters), but it is not the highest point on Earth — that is, the place most distant from the center of the Earth. That distinction belongs to Mount Chimaborazo in the Andes Mountains in Ecuador, according to the National Oceanic and Atmospheric Administration (NOAA). Although Chimaborazo is about 10,000 feet shorter (relative to sea level) than Everest, this mountain is about 6,800 feet (2,073 m) farther into space because of the equatorial bulge.

          The lowest point on Earth is Challenger Deep in the Mariana Trench in the western Pacific Ocean, according to the NOAA. It reaches down about 36,200 feet (11,034 meters) below sea level.

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          It takes the Earth one whole year to make one full orbit of the Sun.

          Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi), and one complete orbit takes 365.256 days (1 sidereal year), during which time Earth has traveled 940 million km (584 million mi). Ignoring the influence of other solar system bodies, Earth’s orbit is an ellipse with the Earth-Sun barycenter as one focus and a current eccentricity of 0.0167; since this value is close to zero, the center of the orbit is close, relative to the size of the orbit, to the center of the Sun.

          As seen from Earth, the planet’s orbital prograde motion makes the Sun appear to move with respect to other stars at a rate of about 1° eastward per solar day (or a Sun or Moon diameter every 12 hours). Earth’s orbital speed averages 29.78 km/s (107,208 km/h; 66,616 mph), which is fast enough to cover the planet’s diameter in 7 minutes and the distance to the Moon in 4 hours.

          From a vantage point above the north pole of either the Sun or Earth, Earth would appear to revolve in a counterclockwise direction around the Sun. From the same vantage point, both the Earth and the Sun would appear to rotate also in a counterclockwise direction about their respective axes.

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