Life on earth could never exist were it not for a series of very fortunate "coincidences,some of which were unknown or poorly understood until the 20th century. Those coincidence include the following:
Earth's location in the milky way galaxy and the solar system,as well as the planets orbit,tilt,rotation speed,and unusual moon.
A magnetic field and an atmosphere that serve as a dual shield.
Natural cycles that replenish and cleanse the planets air and water sulply.
As you considered each of these topics,asked yourself, 'Are earth's features a product of blind chance or of purposeful design?
Earths perfect "address"
When you write down your address,What do you include? You might put in your country,city and street.By way of comparison,let's call the milky way galaxy earth's "country,"the solar system that is,the sun and it's planets-earths "city" and earth orbit within the solar system earth's street.Thanks to advances in astronomy and physics,scientists have gained deep insight into the merits of our special spot in the universe.To begin with,our "city" or solar system,is located in the ideal region of the milky way galaxy not too close to the center and not too far from it.This "habitable zone,"as scientists call it,contains just the right concentration of the chemical elements needed to support life.Farther out,those element are too scarce;farther in,the neighborhood is too dangerous because of the greater abundance of potentially lethal radiation and other factors."We live in the prime real estate," says Scientific American magazine.
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( the earth be located in a better position to host life?
The ideal street:No less "prime" is earth's street,or orbit within our solar system "city".About 93 million miles from the sun, this orbit lies within a limited zone that is habitable because life neither freezes nor fries.More over,earth's path is almost circular,keeping us roughly the same distance from the sun year round.
The sun,meanwhile,is the perfect "powerhouse".It is stable, it is the ideal size,and it emits just the right amount of energy.For good reason,it has been called "a very special star".
The perfect neighbor: if you had to choose a next door neighbor for the earth,you could night improve on the moon.It's diameter measures just over a quarter of that of the earth.Thus,when compared with other moons in our solar system,our moon is unusually large in relation to its host planets.Mere coincidence? It seems unlikely.
For one thing,the moon is the principal cause of ocean tides,which play a vital role in earth's ecology.The moon also contributes to the planets stable spin axis.Without it's tailor made moon,our planets would wobble like a spinning top,perhaps even tipping right over and turning on it's side,as it were! THE resulting climatic,tidal and other changes would be catastrophic.
Earths perfect spin and tilt:Earth's tilt of about 23.4degrees causes the anal cycle of seasons,moderates temperatures,and allow for a wide range of climatic zones.Our planets tilt axis seems to be just right, says the book Rare earth-Why complex life is uncommon in the universe.
Also "just right" is the length of the day and night,a result of the earth's spin.If the speed of rotation were substantially slower,the days would be longer and the side of the eartg facing the sun would Bake while the other side would freeze.Conversely,if the earth were to spin much faster,the days would be shorter,perhaps just a few hours long, and earths rapid spin would cause relentless gale force winds and other harmful effects.
Earth's protective shields
Space is a dangerous place where lethal radiation is common and meteoroids are an ever present danger.Yet,our blue planet seems to fly through this galactic "shooting gallery" with relative impunity.Why?Because earth is protected by amazing armor a powerful magnetic field and a custom made atmosphere.
Earth's magnetic field:The center of the earth is a spinning ball of molten iron,which cause our planet to have a huge and powerful magnetic field that stretches far into space.This shield protect us from the gull intensity of cosmic radiation and from potentially deadly forces emanating from the sun.The latter include the solar wind,which is steady stream of energetic particles;solar flares,which I n minutes release as much energy as billions of hydrogen bombs;and explosions in the outerbregion,or corona,of the sun,Which blast billions of tons of matter into space.You can Dee visible reminders of the protection you received from the earth's magnetic field.Solar flares and explosions in the sun's corona trigger intense Aurora's,colorful displays if light visible in the upper atmosphere near earth's magnetic poles.
Earth's atmosphere:This blanket of gases not only keeps us breathing but also provides additional protection.An outer layer of the atmosphere,the stratosphere,contains a form of oxygen called ozone,which absorb up to 99 percent of incoming ultraviolet (UV) radiation.Thus, the ozone layer helps to protect many forms of life including humans and the plankton we depend on to produce much of phrase oxygen from dangerous radiation.The amount of stratospheric ozone is not fixed.Rather, it changes,growing as the intensity of UV radiation rises.So the ozone layer is a dynamic,efficient shield.
The atmosphere also protects us from the daily barrage debris from space millions of objects ranging in size from tiny particles to bouldersBy far the majority of these burn up in the atmosphere,becoming bright flashes of light called meteors.However, earth's shield do not block radiation that is essential to life,such as heat and visible light.The atmosphere even helps to distribute the heat around the globe, and at night the atmosphere acts as a blanket,slowing the escape of heat.Earth's atmosphere and magnetic field truly are marvels of design that are still not fully understood.The same could be said of the cycles that sustain life on this planet.Is it only a coincidence that our planet is protected by two dynamic shields?
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