Saturday 31 December 2016 - 13:07:22 Posted by  Bobby

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 Earth can be seen as a living, breathing organism: it regulates temperature, burns energy, continually renews its skin, and experiences changes to its face as it ages with time. Researchers in the the field of astrobiology have found that Earth’s current conditions are temporary and that Earth’s stable climate is an anomaly that will end in the next billion years.

Traits of Earth

 Strong evidence suggests that the universe began with the Big Bang about 13 billion years ago, and two thirds of its history passed before our own Solar System was formed. During this time, stars evolved and died, supplementing the basic hydrogen, helium, and trace of lithium that the universe began with and providing more complex atoms such as carbon, oxygen, silicon, magnesium, and nitrogen. The way Earth was formed allows for its many unique traits that have developed and will develop during its life.

The Moon

 The Moon is the Earth's only natural satellite. It is so close to the Earth - at an average distance of only 238,900 miles (384,400 km) - that even binoculars will reveal features on its surface in fascinating detail. The Moon has no air or liquid water, and hence no weather, it is also lifeless and without geological activity. Its main landforms have all resulted from meteorite impact.

Cosmic Orchestra


 For almost the entire history of our species, we humans had it completely wrong. We thought that our planet, Earth, was the center of the universe, and that everything else revolved around us. One can quite understand why that view would have arisen. Simple observation of the changing seasons, and the nightly spectacle of the stars and planets, all conspire to give the impression that we live in the middle of a cosmic orchestra, with Earth as the band leader. But we now know that that is not the case. Earth revolves around the Sun in concert with the seven other major planets (and myriad smaller bodies) that together make up our Solar System. And our Solar System is only one of probably countless billion planetary systems throughout the wider universe.

 Earth is just one name for our planet, of course. Every language and culture has its own name, often woven in with mythologies and creation legends. To the ancient Greeks it was Gaia, the Earth goddess; to the Romans, Terra Mater or Tellus Mater (both of which mean Mother Earth). Science fiction authors have often used Tellus and Terra in place of the more mundane Earth; they have even used Sol 3, referring to the fact that Earth is the third planet of our particular star, Sol - Sun in Latin.

 Yet despite being just one planet of many, Earth is exceptional. It is the only one known to support life... life that has evolved to a point where it can examine itself and the cosmos around it. And all this is possible only because of the special confluence of conditions in which Earth finds itself.

Just the Right Conditions


  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. It is the third of the terrestrial planets, and is actually located just beyond the outer edge of the Sun's habitable or "Goldilocks" zone - the region where the temperature would be neither too hot nor too cold, but just right for liquid water to exist. The Earth, nevertheless, does have vast quantities of water and is testament to the heat-retaining properties of the atmosphere and oceans, as well as the protective effects of the planet's magnetic field, and the contributions made by heat sources such as volcanism. If it didn't have these, it would be an icy world with lots of frozen water - and it is liquid water, of course, that is one of the essentials for life on our planet.

 Earth goes around the Sun in an almost circular orbit at an average distance of 93 million miles. One complete orbit is one year.  One rotation of Earth so that the Sun returns to the meridian -  an imaginary line in the sky joining North and South - is one day, which our system of timekeeping breaks down into 24 hours. (This is almost four minutes more than it actually takes Earth to spin once on its axis. The difference comes because Earth is moving through space, and it takes a smidge more than one full rotation in order for the Sun to once again be on the meridian.) One full orbit around the Sun takes about 365.26 days, which is why we have to add an extra day every four years - a leap year - to keep our calendar balanced.

 Earth's axis of rotation is tilted with respect to the ecliptic by an angle of 23.4 degrees, and it is this tilt that gives us our seasons. The spinning mass of Earth acts as a giant gyroscope, keeping its axis fixed in space. For this, it can be said that when Earth is on one side of the Sun, its northern hemisphere will be facing the Sun, while the southern hemisphere will not - this is summer in the north and winter in the south. Six months later, the Earth's northern hemisphere will be facing away from the Sun, while the southern hemisphere and now faces toward it - winter in the north and summer in the south. The autumn and spring seasons are mid-way between these two extremes. The axial tilt does vary a little over time, but that doesn't have much effect on our seasons.

 If Earth didn't have an axial tilt - if the rotation axis were straight up and down - we would not have seasons. The equatorial regions would be directly beneath the Sun, and the polar regions would get very little warmth indeed.

Third Rock From The Sun


 Earth's diameter at the equator is 7,926 miles. The distance between the poles from north to south is marginally less, 7,900 miles, making our planet only slightly less than perfectly round. Earth is bigger than the other terrestrial planets, the dwarf planets, and all the small Solar System bodies. Only the four gas giant planets and the Sun are larger than Earth.

 In terms of internal structure, our planet has a core, a mantle, and an outer crust. The core is in two parts: a central portion about 1,500 miles in diameter of solid iron surrounded by a zone of liquid iron that extends for about 1,360 miles. The core is believed to also contain small amounts of nickel and sulfur. The core generates a substantial magnetic field that creates a magnetosphere - a magnetic "bubble" - around the planet, which serves to protect it from much of the radiation from the Sun and interstellar sources. The North and South Poles of this field are located very close to the planet's geographic North and South Poles.

 The mantle is semi-solid rock, and extends from the outer boundary of the core almost all of the way up to the surface. The upper, more solid part of the mantle, and the solid crust above it, "float" on the mantle below. The crustal thickness is least at the bottom of the oceans (around 3.7 miles in thickness) and greatest where the continents lie (up to 31 miles in thickness).

 Earth's solid surface is actually broken up into a number of segments called "plates," which comprise the hard upper layer of the mantle, as well as the crustal rocks. These plates move relative to each other - where they meet, they sometimes submerge one under the other, or crash head on. The movement of the plates and collisions between them create volcanic activity, earthquakes, and raise mountain ranges.

 The highest point above mean sea level is the peak of Mount Everest, at 29,000 feet elevation, on the border between China and Nepal. The lowest point on land is the Dead Sea, which is 1,371 feet below mean sea level in the Jordan Rift Valley. In the oceans, the lowest point is the Mariana Trench, a deep fissure that extends to an incredible 35,761 feet below mean sea level, in the Pacific Ocean, near the island of Guam.

 Most of earth - around 70% - is covered with salt water seas and oceans. The land area is comprised of several large continental masses and innumerable islands. The southern polar region is permanently frozen, with an enormous ice sheet overlying a large continental landmass, Antarctica. Much of earth's freshwater is locked up in this ice sheet.

Atmosphere and Climate


 The planet's present atmosphere is around 78% nitrogen and 21% oxygen, with small amounts of other gases such as carbon dioxide and water vapor. The gaseous mix has not always been this way. In the geological past the oxygen level has risen and fallen on many occasions, at one time being perhaps only a few percent, but reaching a peak perhaps as high as 35% within the last half billion years.

 Some of the gases - prime among them water vapor, carbon dioxide, and methane - prevent some surface heat from radiating back out into space, keeping the surface of the planet warmer than it would otherwise be. This is the greenhouse effect. In addition, in the upper reaches of the atmosphere there exists a thin layer of ozone - a molecule comprising three oxygen atoms - that helps to shield the planet from harmful ultraviolet radiation from the Sun.

 Air density falls with increasing altitude, with most of the atmosphere being constrained within the first 6 to 7 miles above mean sea level. The atmosphere is loosely divided into several layers. The troposphere is first - this is where almost all of our weather takes place - followed by the stratosphere, mesosphere, thermosphere, and finally the exosphere. Temperatures change at different rates in the different layers, ironically reaching a maximum high above the ground in the thermosphere.

 The reason for this is that even though there are very few atoms and molecules in the rarefied air at that altitude, the energy of motion of the few that are there is very high, and this is how their temperature is measured.

 Weather patterns around the globe are roughly separated into bands. There is a tropical or equatorial band that spans the region around the equator, adjacent to which is the subtropical zone. Then there is the temperate zone and finally the polar region. These bands run north and south of the equator.

The Oceans


 Earth has a dynamic atmospheric and climatological system that is mostly driven by the energy received from the Sun, and the cycle of water from liquid to gas and back again.

 Oceans cover most of the surface of our planet, in enormous depressions or basins where the crust is thin and there are no continents above. The oceans hold a huge amount of water - more than 312 million cubic miles - with much of them having a depth of over 1.8 miles.

 If the solid surface of Earth could be leveled and smoothed out, the water in the oceans would cover the entire globe to a depth of over 1.5 miles.

 The oceans are home to an enormous variety of animal and plant life. They also act as a huge heat sink, absorbing warmth from the Sun, and currents within these vast oceans circulate the warmer and cooler flows of water to all the different parts of the globe.

 Earth Statistics

Average distance from the Sun 92.9 million miles (149.6 million km) 1 AU
Equatorial diameter 7,926 miles (12,756 km)
Axial tilt 23.45°
Axial rotation period (sidereal) 23.93 hours
Orbital period 365.24 days
Mass 1.314 x 1025 pounds (5.974 x 1024 kg)
Volume 0.26 trillion cubic miles (1.08 trillion km3)
Surface gravity 1 g (9.8 m/s2)
Average density (water=1) 5.5
Surface atmospheric pressure 14.7 psi (101.4 kPa)
Escape velocity 6.9 miles/s (11.2 km/s)
Orbital eccentricity 0.017
Highest suface temperature 136°F (58°C)
Lowest surface temperature -126°F (-88°C)
Albedo (reflectivity) 31%
Number of satellites 1 (the Moon)

 Earth Timeline

4.57 billion years ago 
Earth forms out of the solar nebula, along with the other planets in our Solar System. The Moon forms soon after.
5 million years ago 
Volcanoes erupt and create the area of land that joins North and South America. Mammals from North America move South and caused Extinction of mammals there. Human ancestors speciate from the ancestors of the chimpanzees.
1.8 million years ago
Homo erectus evolved in Africa and migrates to other continents.
c. 400 BCE
Aristotle reduces that the Earth is round from the shape of Earth's shadow on the moon during a lunar eclipse.
c. 350s BCE
Heraclides proposes that the apparent daily motion of the stars is created by the rotation of Earth on its axis once a day.
c. 200 BCE
Hipparcos carries out a calculation of the dimensions of the Earth-Moon system.
3rd Century BCE
Eratosthenes, Greek mathematician, astronomer, and geography, devises a map of the world. He also estimates the circumference of Earth, and the distance to the Moon and the Sun, and constructs a method for finding prime numbers.
1 CE
Human population reaches 150 million.
Copernicus states that Earth revolves around the Sun.
Galileo makes telescopic observations of planets. He is jailed for his theory that the Sun is fixed, contrary to Holy scriptures. Released on house arrest, he later goes blind, presumably from looking directly at the Sun.
Human population reaches 1 billion.
Albert Einstein publishes his general theory of relativity.
Sputnik was launched by the former Soviet Union. It orbits Earth, sending back a beeping signal for 23 days.
The National Aeronautics and Space Administration - NASA - is formed in the USA.
Luna 1, an unknown Russian spacecraft, makes the first flyby of the Moon.
On May 22, the largest earthquake in the world, with a magnitude of 9.5, strikes Chile. Approximately 1,655 are killed with thousands of injuries.
On April 12th, the first human, Russian cosmonaut Yuri Gagarin, travels beyond the atmosphere of Earth.
Apollo 11 is the first manned mission to land on the Moon. The first steps made on another planetary body are taken by Neil Armstrong and Buzz Aldrin on July 20th. They also returned the first samples from another planetary body.
Early on April 12th, the world's first reusable spacecraft, Columbia, known as the shuttle, is launched. It carries two astronauts and completes 36 orbits, then glides to perfect touch down at Edwards Air Force Base, California, USA.
On April 24th, the Space Shuttle Discovery lifts off from Earth carrying the Hubble Space Telescope. The following day, Hubble is released into space where it continues to offer mankind unique glimpses of our Solar System.
Space Shuttle Discovery maneuvers to within 37 feet of Russian space station Mir, in preparation for a shuttle-Mir docking. This is the first shuttle mission to be flown by a female pilot.
On December 26th, an earthquake centered off Sumatra triggers a devastating tsunami that killed over 275,000 people across Asia. Measuring 9.3 on the Richter scale, it is the deadliest and recorded history.
Human population approaches 6.6 billion, with China being the most populated country on Earth, with close to 1.5 billion people.

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