Earth, the third rock from the Sun

Earth is unique. As far as we know it is the only planet in the entire universe that supports life. Nearly seventy percent of the surface is covered with water and the land surfaces are in motion through tectonic plates.  

The third rock from the Sun is called Earth. It is also sometimes referred to as Terra, after the Roman Goddess of the Earth. This planet is unique since it is the only planet in our solar system which lies near the centre of the ecosphere, the volume of space around the Sun where conditions are suitable for life to exist. Venus is too close to the inner limit and has a hot, poisonous atmosphere, while Mars is too cold with only a tenuous atmosphere. It is also the only planet we know of with liquid water.

Structure of the Earth 

The Earth is around 4.57 billion years old, having formed from the pre-solar nebula just like the other bodies in the solar system. In the beginning the Earth was a ball of molten rock. In millions of years the planet cooled down and a crust formed on the surface. Our Earth now consists of a solid inner core of iron, surrounded by a molten layer of iron and nickel, the outer core. The core generates a magnetic field that creates a magnetosphere around the planet, which protects it from the radiation of the Sun. The core is surrounded by the mantle, a layer of semi-solid rock on which the solid crust “floats”. Earth’s solid surface is broken up into a number of segments called “plates”, which move in relation to each other. The movement of the plates and collisions between them create earthquakes, volcanic activity and raise mountain ranges. The highest point above the mean sea level is the Mount Everest, at 8,848 metres (29,000 feet). The lowest point in the ocean is the Mariana Trench in the Pacific Ocean, 10,900 metres (35,761 feet) below the mean sea level. About seventy percent of the Earth’s surface is covered with salt water. These seas and oceans absorb a lot of the warmth of the Sun, and currents within these vast oceans circulate the warmer and cooler flows of water to all different parts of the globe. 

Third rock from the Sun

Earth's diameter at the equator is 12,756 kilometres (7,926 miles). The distance between the poles, from north to south, is marginally less, 12,713 kilometres (7,900 miles), making our planet almost perfectly round. The Earth goes around the Sun in an almost circular orbit at an average distance of 150 million kilometres (93 million miles). One orbit takes one year, just over 365 days. Since our year only has 365 days, every fourth year is a leap year where one extra day is added to compensate for the other years being a quarter of a day too short. Earth’s orbit is prograde, counter-clockwise around the Sun when viewed from above. One rotation of the Earth around its own axis is one day or 24 hours. Actually it takes the Earth almost four minutes less to spin around its own axis, but since the planet moves through space in its orbit around the Sun, it takes a little more than one full rotation in order for the Sun to be on the meridian again. Earth’s axis is tilted at an angle of 23.4 degrees, in respect to the plane of its orbit around the Sun and that is what causes the seasons. We have one natural satellite, our Moon.

The seasons

Since the Earth’s axis is tilted, the northern hemisphere is facing the Sun when Earth is on one side of the Sun, while the southern hemisphere is not. The northern hemisphere now has summer, while the southern hemisphere has winter. When the Sun reaches its most northerly position on 21 June, over the tropic of Cancer, it is called summer solstice in the northern hemisphere. Six months later the northern hemisphere will be facing away from the Sun, while the southern hemisphere is now facing towards it. Now the northern hemisphere has winter and on 21 December the Sun reaches its most southern position above the tropic of Capricorn. In between the Sun passes over the celestial equator and this is called an equinox, marking the start of spring and autumn.

The atmosphere

The Earth’s atmosphere consists of about 78 percent nitrogen, 21 percent oxygen and some small amounts of other gasses. The gaseous mix has not always been the same. In the past, the oxygen level has changed from only a few percent to its current level in the last half billion years. It is believed that it has even been as high as 35 percent at one time. Some of the gasses, like water vapour, carbon dioxide and methane, prevent surface heat from radiating back out into space. This is called the greenhouse effect. In the upper regions of the atmosphere a thin layer of ozone helps to shield the planet from harmful ultraviolet radiation from the Sun. The atmosphere is loosely divided into several layers. The first layer around the Earth is called the troposphere and that is where almost all of our weather takes place. This layer is followed by the stratosphere, the mesosphere, the thermosphere and finally the exosphere.

Sources: www.todio.nl