E A R T H

EARTH is the third planet from the Sun. From vantage point of space, the Earth appears as a blue disk that is due to the oceans, intermixed with the characteristic cloud patterns and continents peeking out from places between those clouds.

Earth compared with all the other planets is moderately small. Our home planet does have the advantage in its placement from the Sun and means it is neither too hot nor too cold for life. If the Earth were closer to the Sun, like the inner planets of Mercury and Venus, our oceans would soon quickly evaporate into space. If our world were among the outer planets, the oceans would soon freeze into solid ice. We happen to be in the narrow intermediate habitable zone, where liquid water can exist for life. If we were 10% closer to the Sun (15 million kilometres) the Earth would be too hot, and if 15% farther away (23 million km.), it would become too cold. As the orbital eccentricity of the Earth is almost circular, our planet moves completely within this narrow habitable zone.

During the year the closest approach to the Sun, or perihelion, occurs on either January 3rd or 4th at the distance of 147,088,067 km. Aphelion is placed furthest from the Sun and occurs on July 3rd to 6th at 152,104,230 km. We commonly call the Earth’s mean solar distance an astronomical unit, measured as 149,597,870 km. Since light travels at a finite speed of about 300 000 km each second, it takes sunlight to traverse this gulf between the Sun and Earth about 499 seconds or 08 minutes 19 seconds. This figure varies only by 8.3 seconds throughout the entire year.

Earth has an axial tilt of 23½° to the ecliptic, being tilted always towards the same direction in space as seen as the celestial poles. Each year, observers will see the day to day solar positions in continuous change in its daily apparent height or elevation above the horizon. This produces the annual seasons each 365¼ days. At low Sun altitudes, the slanting of the sunlight is more obliquely As these rays have to passing through more atmosphere this reduces the amount of received energy, which we recognise as the season of winter. When the Sun is at higher angles, the sunlight is more direct to the ground, making the warmer months of summer. Between these two opposing seasons are autumn - called fall in some countries (winter to summer), and spring (summer to winter).

All seasons do not happen exactly at the extremes of the winter or summer solstices every year, because there is a noteworthy lag for the atmosphere to absorb the heat. For example, the time of the Summer Solstice (southern hemisphere) occurs on 21st to 22nd December, yet the hottest month can be in late-January or early-February. During the winter solstice (southern hemisphere) on 21st to 22nd June, the coolest month is more often in early to mid-August. (In the northern hemisphere the reverse of the seasons are experienced.) Recognising the lag means the defined astronomical seasons are not the mid-point of the date of the soltices (or equinoxes) but start at the time of the equinox or solstice. Hence, the beginning of southern summer is not mid-February but begins on 21st to 22nd December each year. (See Earth Phenomena : 2005-2010)

Structure of the Earth

We understand much about the nature of the Earth, from the highest edges of space to the depths below the ground.

The Atmosphere of the Earth

Our Earth’s atmosphere offers very useful protection and sustainability for all life on the Earth. It shields us from meteors, dangerous energies like ultra-violet or X-rays electromagnetic radiation, and stops most of the high energy or high velocity particles coming from the energetic Sun or the depths of space.

Lying just above the surface is the lower atmosphere, whose 7 to 17 km. thick region sustains all the breathable air for life on Earth. Known as the troposphere, this habitable zone contains 80% the atmosphere, which acts like a giant blanket by absorbing much of heat and light from the intense solar radiation. This captured energy warms the planet and drives the weather. In addition, this energy ultimately creates the winds and tidal currents, and continues the so-called important water cycle - the transpiration (evaporation) of the oceans and the falling of water as snow, rain or precipitation.

Each of the gases in the atmosphere that comprises air is mainly composed of Nitrogen (78%), Oxygen (21%) and the inert Noble gas, Argon (0.9%). Other smaller traces of other gases exist, including about 0.04% of Carbon Dioxide (CO₂), Neon, Helium, Methane (CH₄), Krypton, Sulphur Dioxide (SO₂) and Hydrogen. Saturating the atmosphere is water vapour, either as clouds or in clear air, making on average about 0.5% of its volume.

Climate Change

Some gases, especially CO₂ can have drastic influences on the climate by the so-called greenhouse effect, which can absorb and trap the energy from the Sun and prevent it escaping back into space. Its effects can make the surface and troposphere tempertures increase to adversely affect the weather, the climate and the water cycle. cycle. If unchecked, the continuance of plants and animal habitation as we presently know it could end. Such observed changes seen only in recent decades have almost certainly been caused by human activity who have collectively been increasingly using or burning more and more fossil fuels for their industries, food production, cars and electricity - all just to maintain their lifestyle and exploding populations. Consequences for these actions have slowly changed the careful balance existing between the solar radiation and the atmosphere - commonly now expressed as climate change. By simply burning and releasing the copious amounts of carbon once usefully trapped in oil and living trees, creates the converted by-product of CO₂.

As the greenhouse effect takes hold, the usual average Earth temperature of about 15°C will begin to rise by several tenths of degrees, but is sufficient to cause melting of the ice caps, and in time, rising sea levels. In turn, the effect cascades and accelerates, causing the ocean temperatures also warm and release the once dissolved C0₂. Water vapour is also a notable greenhouse gas, and if the Earth does heat up, will only increase the greenhouse effect.

Over periods of decades, centuries or many millennia, significant effects on climate can generated by small changes in solar energy output or activity. Small variations could easily cause subtle but slow changes in both the local and global climate, possibly beginning the onset of Ice Ages or periods of warmer conditions. We know that over the last 150,000 years that the Earth has experienced such changes many times, and some have been quite rapid. Obtained evidence has come from geological and anthropological studies, where we can document the rising and fall of the seas and the effect on the terrain of glacial formation and the history of the advancing or preceding ice caps. Even in the last 5,000 years, we have seen places of human settlement being occupied for centuries before the effects of the local climate forces them to find more suitable locations. An extreme example was the arrival of the Australian Aborigines some 40,000 years ago, when a land bridge existed between Asia, through Papua New Guinea, caused by sea levels being 200 metres below where they are currently today. Even the origins of the original prehistoric human migrations across the countless millennia may have principally caused of climate change. Some suspect the end of the last ice Age produced the extinction of the cold-adapted Neanderthals

The Upper Atmosphere

Above the troposphere, the remaining 20% of upper atmosphere divides into several different layers of temperature and pressures. Next in height is the stratosphere, where the air becomes very cold, and so thin, that humans cannot breath in it. Above the stratosphere is the middle region of the atmosphere called the mesosphere, and roughly spans 50 to 80 km. This is where the atmospheric density and pressure is still sufficient to cause resistance for returning spacecraft or from the incessant and constant meteor bombardment falling towards the Earth. As such, the friction generated by the high velocities of re-entry causes the body to glow or form a bright visible streak across the sky. The mesosphere therefore provides a useful safeguard that would otherwise not stop destructive meteorites hitting the ground. On the edge of the upper mesosphere is the thermosphere. Lying 80 to 80 km. in height, the temperature begins to quickly rise, mostly caused by energetic ionised particles.

Higher again is the ionosphere, where the temperature continues to rise. Here some of the air molecules are readily ionised by charged particles from the Sun and Earth’s magnetic field, or even space - where some atoms have their electrons stripped away At the height of about 200 km. the number of atoms decreases to a virtually perfect vacuum. We really do not know where the atmosphere ends, but it is somewhere above about 250 km - and lies where most orbiting satellites are unimpeded. Beyond this is known as space or technically interplanetary space.

The Interior of the Earth

Below the atmosphere lies the oceans or hydrosphere whose depth averages 3.8 kilometres. Below this, lies the true internal structure of the Earth. Only partly understood, and most of this information was obtained by earthquakes by the science of seismology. Under the surface lies the crust which varies between about twenty-five and ninety kilometres thick. The Earth’s crust divides into many plates that include the seven continents. Closer to the centre is the mantle, which contains higher pressures and temperatures than the crust. It is about 3000 km thick with the continents ‘floating’ on the mantle. Further down is the core, divided into the inner core and outer core. The outer core is liquid rock and is about 2,100 km thick. The inner core is about 1,300 km thick and is all solid material. Temperatures in this region are around 4,000°C at the pressure of 100,000 atmospheres. A recent map of the surface of this solid sphere surrounding the core shows many valleys and mountains, while radioactivity may keep the inner Earth hot. The Earth itself is most composed of minerals containing elements like Oxygen (O), Silicon (Si), Aluminium (Al) and Iron (Fe).

The Magnetic Field of the Earth

In the inner core, there is a higher content of Iron, Cobalt and other heavy elements. This is the Earth’s magnetosphere. Our Earth acts as a giant bar magnet, originating from the electric currents that flow through the metallic material. At present, the axis of magnetism is 13° from the true poles of rotation. The north magnetic pole is in Greenland, while the south magnetic pole lies on the Australian side of the coastline of Antarctica. This field is important for life as it protects the Earth from the large numbers of high velocity charged particles, like electrons and protons - the solar wind. This is especially important during strong solar activity, when the magnetic field captures the solar wind along the field lines entrapped in the concentric rings called the Van Allen Belts. Trapped electrons, excite the thin gas 100 to 1000 kilometres in the upper atmosphere or, so that the sky literally begins to glow. The generated electrical current is huge, akin to a very large fluorescent light and produces the aurora. Most aurorae appears as coloured glows but some show either rays or curtain-like structures. In the northern hemisphere, this is the Aurora Borealis, in the south, the Aurora Australis. . Various attributed colours during some displays are due to different gases, mainly oxygen and nitrogen in the upper atmosphere. (See the Aurora Australis Article).

Diclaimer

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Last Update : 2nd September 2008

Southern Astronomical Delights © (2008)

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