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 Earth were closer to the Sun, like the inner planets of Mercury and Venus, our oceans would quickly evaporate into space. Alternatively, if our world were placed among the outer planets, our oceans would soon freeze into solid ice. We are placed 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 kilometres), it would be too cold. Earth’s orbital eccentricity is nearly circular and moves completely within this 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 kilometers. Aphelion is placed furthest from the Sun and occurs on July 3rd-6th at 152 104 230 kilometres. We commonly call Earth’s mean solar distance of an astronomical unit, measured as 149 597 870 liometers. Since light travels at a finite speed of about 300 000 kilometres each second, it takes sunlight to traverse this gulf between the Sun and Earth about 499 seconds or 8 minutes 19 seconds. This figure varies only by 8.3 seconds throughout the entire year.


Satellites : One
Orbital Velocity : 29.79±0.26kms-1
Eccentricity (e) : 0.0167
Inclination (i) : 0.000o
Mean Solar Distance (AU) : 149.601 x 106 km
Radius : 6 370 km
Diameter (Equatorial) : 12 756 km
Diameter (Polar) : 12 714 km
Mass : 6.1x1027 g. or 6.1x1024 kg.
Mean Density : 5.52 g.cm-3
Distance Observer to Horizon : 8 km
Mean Solar Velocity : 29.8 km.sec.-1
Acceleration Due to Gravity : 9.8 m.sec.-1
Escape Velocity : 11.18 km.sec.-1
Year (Sidereal days) : 365.256 360 42d
Day (Sidereal) : 23h 56m 04.09s
Day (Solar) : 24h 00m 00.00s
Surface Temperature : -80oC to 50oC
Mean Surface Temperature 19.8oC
Highest Point (Mt. Everest) : 8 800 m.
Lowest Point : -10 430 m.
Land Area : 148.8x106 km.2
Water Area : 361.3x106 km.2
Albedo (Reflectivity) : 37%

The Earth’s axial tilt is at 23½o to the ecliptic, and tilting always towards the same spacial direction in space. Each year, the day to day solar position means observers will see a continuous angular change for the height of the Sun, producing the seasons. If the Sun is at a low altitude, the sunlight is slanted more obliquely. As these rays have to passing through more atmosphere this reduces the received energy producing the season of winter. Were the angle high, the sunlight are at more direct producing the season of summer. Between these two seasons are autumn and spring.

We understand many details about the nature of the Earth, both above and below its surface. Above the surface lies the atmosphere which is made up of different layers. Only a tiny fraction, some 5 km thick, is the troposphere where all life exists. Above this is the stratosphere where the air becomes very thin. Higher again is the ionosphere. The atmosphere absorbs much of the solar radiation. Air molecules here are sometimes ionised where electrons are stripped away from each atom, and these charged particles produce a useful protection for life on Earth. Earth’s atmosphere also protects us significantly from the constant bombardment of meteors. At the height of about 200 km the number of atoms decreases to a virtually perfect vacuum. We really do not know where the end of the atmosphere is, but it is above about 250 km - as most orbiting satellites are unimpeded. The area beyond this is known as space or technically interplanetary space.

Below the atmosphere lies the oceans or hydrosphere whose depth average 3.8 kilometres. Below this, the true internal structure of the Earth is 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. Composed mainly Oxygen (O), Silicon (Si), Aluminium (Al) and Iron (Fe), the 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 000oC at the pressure of 100 000 atmospheres. A map of the surface of this solid sphere surrounding the core has been made that shows many valleys and mountains. Radioactivity is thought to have keep the inner Earth hot.

In the inner core there is a high content of Iron and other heavy elements that produce the magnetic field of the Earth that surrounds us well into space. This is the magnosphere. Our Earth acts as a giant bar magnet and originates from the electric currents that vigorously flow through the metallic material. At present the axis of magnetism is 13o from the true poles of rotation. The north magnetic pole is situated in Greenland, while the south magnetic pole lies on the Australian side of Antarctica, just off Antarctica’s coastline. 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 gas 100 to 1000 kilometres in the upper atmosphere, so that the sky literally begins to glow. The electrical current is huge, akin to a very large fluorescent light and produces the aurora. Most aurorae appears as coloured glows but some show rays or curtain-like structures. In the northern hemisphere, the aurora is named the Aurora Borealis, while in the south it is named the Aurora Australis. Various colours are produced during these displays which is attributed to the different type of gases, mainly oxygen and nitrogen in the upper atmosphere. (See Aurora Article here at Southern Astronomical Delights.


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Last Update : 21st May 2005

Southern Astronomical Delights © (2005)


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