The Earth and Beyond
The Earth is just one of nine planets in orbit around the sun. The Earth's orbit is almost a circle. The Sun's gravity acts as centripetal force to pull the Earth into orbit. One orbit of the sun takes 365¼ days and this is why we have a leap year every four years.
Aristarchus of Samos was on of the first people to observe that the Earth seemed to orbit the Sun, but the majority of the Greek philosophers of the day believed that the Earth was a large motionless globe in the centre of the universe and threw out Aristarchus' theory.
The idea that the Earth was the centre of the universe was commonly accepted until the
16th Century when Copernicus critically examined the Earth-centred model and explained that the planetary motions can be explained just as well with the Sun as a more central body. However, this idea was still not generally accepted.
Later, Galileo saw his chance to prove Copernicus' work. He used a small refracting telescope, which had only recently been invented in Holland, to identify four moons in orbit of Jupiter, and to prove that Venus was is in orbit of the sun. These ideas were accepted in the time of Newton when he said that all bodies have an a attractive force known as gravity.
There are 8,000 stars visible from the Earth. Most of these stars form constellations which are 'imagined pictures' named by the Ancient religious or mythical figures.
The stars' positions seem to be fixed, but in actual fact they are moving at high speeds; however, due to the distances involved, the stars movement is only apparent over centuries.
Occasionally, we can see a planet such as Venus or Mars in the night sky. They look like stars but they move across the constellations. They are not naturally luminous, we see them because light from the sun reflects off them.
Satellites can be put into orbit around a planet (Earth) for a number of reasons.
Weather satellites can be used to take pictures of the Earth. As they are above the Earth they can see the positions of clouds and help meteorologists to predict the weather.
We can use satellites to relay radio signals around the world. This allows us to 'talk' to countries such as America which is out of our line of sight.
We also use satellites such as the Hubble Space Telescope to monitor the rest of the universe that would be impossible to make because the Earth's atmosphere would absorb a lot of the radiation that might need to be monitored.
The orbits of the satellite vary according to function. Television satellites are in high Earth orbit and they orbit the Earth at the same speed as the Earth is rotating. This means that satellite receivers on people's homes do not have to be constantly adjusted to keep up with the satellite transmitter. This is called a Geostationary Orbit.
In a similar way, weather satellites are in a lower Earth orbit. This allows them to monitor the whole of the Earth in a day or less. Often these types of satellite orbit around the poles. This is called a Polar Orbit.
The nine planets' orbit of the Sun are all, except Pluto, slightly elliptical. This happens because their speed increases as they travel towards the sun due to its gravity and decreases as they move away.
All of the planets have a certain amount of gravity which is related to their mass. For example, the Moon is 1/6 the mass of Earth so its gravitational force is 1/6 as big. For larger planets such as Jupiter the gravitational force is larger. As one moves away from the body, the gravitational field gets weaker and weaker - but it never reaches '0'.
Smaller bodies stay in orbit of larger bodies due to gravity. Centripetal force is provided in the form of gravity which is what is needed to propel the object into orbit. However, the speed of the smaller body must be high to maintain the orbit.
The mean distance of a satellites orbit is dependant on its speed. Travelling too slowly may result in a collision of a planet into the sun for example. The further away a planet is from the sun, the longer its orbit. This true of the Earth and Mars. 1 Earth orbit takes a year, Mars' year is almost twice that of the Earth's because it is further from the sun.
Comets have extremely large orbits. The orbit of Halley's Comet is about seventy years. When they come close to the sun, a tail develops. This is due to 'exhaust' gases being produced as the comet accelerates within the Sun's gravity. The term tail is perhaps inappropriate as it always faces away from the Sun.
Our Sun is just an ordinary star. It is around 4600 million years old and is expected to live for about another 4600 million years. It is not burning like a fire, but it is a huge (controlled) H Bomb. The nearest star to Earth is Alpha Centauri C, (often called Proxima Centauri) and it is 4.3 light years away. This means that it takes 4.3 Years for light to reach us from this star.
The sun along with millions of other stars make up The Galaxy, called the Milky Way. It is though that there is nothing 'special' about the Milky Way either. There are billions of galaxies that make up the Universe.
The distances between stars in a galaxy is often millions of times further than the planets in the solar system. The distance between galaxies in the Universe is also many times further apart than stars in the galaxy.
Cosmologists believe that the Solar System was formed when a huge cloud of dust and gas came together in space and began to shrink under its own gravity. As this happened, the dust and gas became hotter and hotter. Eventually, the hot centre became the Sun and the rest formed nine planets. Lots of left over pieces of dust formed the asteroid belt and other meteorites.
Stars have immense gravitational forces around them. It is though that these fields bear some relation to the way in which they were created. In addition, the very high temperatures which are a result of the hydrogen nuclei fusing to form helium cause the star to expand. During the majority of a star's 'life' the forces of heat and gravity are balanced. Our Sun is at that stage of its life now. This state of balance can last for billions of years.
Later, the Sun's expansion forces become stronger than its own gravity and the star begins to expand forming a Red Giant. However, the gravity of the star eventually takes over and the star begins to shrink forming a White Dwarf.
If the red giant becomes large enough, it may become a supernova and contract very quickly. A supernova throws dust and gas into space before the star itself forms an incredible dense star called a neutron star. Many stars go through their life cycle quicker than our sun - taking around five million years.
The planets closest to the Sun (Mercury and Venus) contain some of the heaviest elements and these are also present in the Sun. This suggests that the Solar System may have been formed from the remnants of an earlier star or solar system.
When cosmologists look at what started the Universe they have a number of things to think about. Firstly, light from distant galaxies is shifted towards the red end of the spectrum. This is called Red-Shift and secondly, the further away the galaxy is, the bigger the red shift.
Scientists believe that red-shift is to do with the Doppler Effect and suggests that the galaxies are moving away from us, with the galaxies further away moving the fastest. This leads us to important explanations of the Universe's beginnings. It suggests that at one point in time, all matter was concentrated at one point - the centre of the Universe. There was a large explosion and the galaxies were formed. This would explain why the galaxies are all moving away from each other.
This could also mean that one day, the galaxies will lose momentum and begin to fall in to each other forming a 'Big Crash'. The results of which could be another Big-Bang and another Universe.
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