The earth


The Earth formed 4570 million years ago (mya) but the first animals with shells and bones appeared less than 600 mya. It is mainly with the help of their fossils that geologists have learned about the Earth’s history since then. We know very little about the 4000 million years before, known as Precambrian Time. Just as days are divided into hours and minutes, so geologists divide the Earth’s history into time periods. The longest are eons, thousands of millions of years long. The shortest are chrons, a few thousand years long. In between come eras, periods, epochs and ages .The years since Precambrian . 

 Time are split into three eras: 

 Mesozoic and 
Different plants and animals lived at different times, so geologists can tell from the fossils in rocks how long ago the rocks formed. 

Fossils and earth's history

Using fossils, they have divided the Earth’s history since Precambrian Time into 11 periods. Layers of rock form on top of each other, so the oldest rocks are usually , at the bottom and the youngest at the top, unless they have been disturbed. 
The order of layers from top to bottom is known as the geological column. By looking for certain fossils geologists can tell if one layer of rock is older than another. 
 Fossils can only show if a rock is older or younger than another; they cannot give a date in years. Also, many rocks contain no fossils. To give an absolute date, radiocarbon dating is used. Radiocarbon dating allows the oldest rocks to be dated. After certain substances, such as uranium and rubidium, form in rocks, their atoms break down into different atoms. This sends out rays, or radioactivity. 
By assessing how many atoms in a rock have changed, geologists work out the rock’s age. Breaks in the sequence of the geological column are called unconformities.The Solar System was created when the gas cloud left over from a giant supernova explosion started to collapse in on itself and spin. 

 Formation of earth 

creation of earth

About 4.55 billion years ago there was just a vast, hot cloud of dust and gas circling a new star, our Sun. The Earth probably began when tiny pieces of space debris (called planetesimals) began to clump together, pulled together by each other’s gravity.These were like asteroids which collapse the earth oftenly. As the Earth in the universe formed more space debris kept on smashing into it, adding new material. 
This debris included ice from the edges of the Solar System. About 4.5 billion years ago, rock the size of Mars crashed into Earth. Splashes of material from this crash clumped together to form the Moon. The collision that formed the Moon made the Earth very hot. Radioactive decay heated the Earth even further.For a long time the surface of the Earth was a mass of erupting volcanoes.Iron and nickel melted and sank to form the core.
 Aluminium, oxygen and silicon floated up and cooled to form the crust The Earth is the third planet out from the Sun, 149.6 million km away on average. On 3 January, at the nearest point of its orbit (called the perihelion), the Earth is 147,097,800 km away from the Sun. On 4 July, at its furthest (the aphelion), it is 152,098,200 km away. 
The Earth is the fifth largest planet in the Solar System, with a diameter of 12,756 km and a circumference of 40,024 km at the Equator. The Earth is one of four rocky planets, along with Mercury, Venus and Mars. It is made mostly of rock, with a core of iron and nickel. No other planet in the solar system has water on its surface, which is why Earth is uniquely suitable for life. It is the only planet known to support life. 
Over 70 percent of Earth’s surface is under water.The water is present in rivers which come from the mountains when ice melts on it, and in oceans and seas,The Earth’s atmosphere is mainly harmless nitrogen and life-giving oxygen, and it is over 700 km deep. The oxygen has been made and maintained by plants over billions of years. The Earth formed 4.65 billion years ago from clouds of space dust whirling around the Sun. The planet was so hot that it was molten at first. Only slowly did the surface cool into a hard crust. 
The Earth’s orbit around the Sun is 939,886,400 km long and takes 365.242 days. The Earth is tilted at an angle of 23.5°. Even so, it orbits the Sun in a level plane, called the plane of the ecliptic.The Earth is made up of the same basic materials as meteorites and the other rocky planets - mostly iron (35 percent), oxygen (28 percent) magnesium (17 percent) silicon (13 percent) and nickel (2.7 percent). Most of the Fourth's rocky crust is drowned beneath oceans, formed from steam belched out by volcanoes early in the planet’s history. 
The Earth is just the right distance from the Sun for surface temperatures to stay an average 15°C, and keep most of its water liquid. The Earth is protected from the Sun’sradiation by a magnetic field.I which stretches 60,000 km out into space. The study of the shape of the Earth is called geodesy.
 In the past, geodesy depended on ground-based surveys. Today, satellites play a major role. The Earth is not a perfect sphere. It is a The ancient Greeks realized that the Earth is a globe. Satellite measurements show that it is not quite perfectly round. The Earth spins faster at the Equator than at the Poles, because the Equator is farther from the Earth’s spinning axis. 
The extra speed of the Earth at the Equator flings it out in a bulge, while it is flattened at the Poles.Equatorial bulge was predicted in 1687 by Isaac Newton. The equatorial bulge was confirmed 70 years after Newton - by French surveys in Peru by Charles de La Condamine, and in Lapland by Pierre de Maupertuis. The Earth’s diameter at the Equator is 12,758 km. This is larger, by 43 km, than the vertical diameter from North Pole to South Pole. The official measurement of the Earths radius at the Equator is 6,376,136 m plus or minus 1 m. 
The Lageos (Laser Geodynamic) satellite launched in 1976 has measured gravitational differences with extreme precision. It has revealed bumps up to 100 m high, notably just south of India. The Seasat satellite confirmed the Axis ocean surfaces are South pole geoid. 
It took millions m North poie of measurements of the height of the ocean surface, accurate to within a few centimetres. The Earth rotates around its axis, the imaginary line running through the centre of the planet from pole to pole at an angle of 23.5°.

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