.................................................. The Sun and Moon

Introduction. All the stars we see at night are big masses of hydrogen gas atoms that are reacting to each other and releasing a lot of energy. This atomic energy source, called nuclear fusion, as derived from the union of the cores of hydrogen atoms, is the energy of the universe itself and, therefore, the most natural energy. Today it is also what sustains life on our planet, which allowed millions of years ago to live the plankton which led to oil and trees that led to coal, which moves water which, then, will enable operation of hydroelectric plants , which causes the wind which can obtain wind, etc. Unlike the known nuclear fission of uranium atoms, this energy leaves no pollutants residue, which is very important to avoid radioactive contamination of our planet.

Scientists are currently trying to get this energy on Earth to replace energy from the combustion of oil, which is declining much and as much causes carbon dioxide , which is a gas that hinders the output of heat from Earth to the space, and it is provoking excessive global warming (greenhouse effect). The big problem is that the Earth is very difficult to get the enormous pressures and temperatures found inside the Sun and that, for now, seem essential to obtain hydrogen fusion. Much needed support the scientific research on this topic because today almost all of humanity can not survive without a great source of energy. Again, knowledge of the Universe is the former that is giving us the key to find the solution of our problems.

1 . The Sun. The sun is a yellow medium sized star. Its superficial temperature is 5500 ° C and internal temperature of 15 million ° C. Its core is basically form hydrogen and the fusion of two atoms form helium and release a lot of energy, so called atomic or nuclear energy of the fusion of hydrogen. Its 109 times the Earth's diameter, which means a higher volume to a million times the volume of Earth. Since the gas consists only its weight is about 300,000 times the weight of the Earth.

The Sun has an internal structure constituted by the following three layers:

• Core. Is the central part. Presents a high density (158 times greater than water), high temperature (15000000 ºC) and is subjected to pressure. Therein are made the hydrogen fusion reactions.
• Radiation zone. It is the intermediate zone. Energy coming from the core, travels through the radiation zone.
• Convection zone. It is the top of the Sun, the boiling zone and that brings energy to the photosphere, which is the surface layer. This is only one layer thick 400 km to 5500 °C is what constitutes the disk and of the Sun. visible
  If the Sun was compared with an the photosphere would be equivalent with apple, its skin. In the photosphere are observed darker areas called sunspots, which are areas of lower temperature (4000 °C). The small areas last a day or two but the majority grow and can last from a few weeks to a few months.

Sun above presents the solar atmosphere which may be distinguished two layers.

• Chromosphe. It is the lower atmosphere. It is pink and almost transparent. 10 000 km It presents a thickness and a temperature ranging between 5500 ° C in its base and 1 000 000 ° C on the top. It presents a great activity. On one side are the spicules that are the hydrogen ejections that do not surpass the chromosphere, that last about ten minutes and that continuously produce many at once. On the other are solar prominences that are huge surgencies of hydrogen gas that can surpass the crown. Some fall back on the chromosphere,  forming an arch that lasts a few hours, other form thinner arches that last for years; and a third type called "flares" run vertically and last a few minutes.
• Corona (Crown). It is the upper atmosphere. It consists of a whitish halo that reaches a million kilometers in thickness and presents a temperature ranging between 1 million to 2 000 000 º C. Visible only in times of eclipse, is to say when the Moon covers the solar disk. In the corona also originate protrusions and much of the solar electromagnetic radiation longwave.

The Sun was formed approximately 4.6 billion years ago by condensation of of hydrogen, helium and cosmic dust (now presents a 75% hydrogen, 23% helium and 2% cosmic dust) and is believed to be off within other 4500 million years, when all its hydrogen be consumed.

2 . First scientific theories about how the solar system. There are two different major scientific theories, the oldest called geocentric and the modern called heliocentric.

Geocentric Theory. This theory proposes that the Earth is the center of the universe and that the planets and the sun revolve around her describing circles. It was based on daily observation of sunrise and sunset. Dating back to the third century BC. Subsequently, in the second century AD, the Greek Ptolemy (90-168 AD) noted that at certain times of the year were seen the brightest planets and the sun looked bigger. To explain all this without contradicting the orbits were circumferences, Ptolemy proposed that the Sun and planets were smaller orbits around a point which both described a great circle around the Earth. The more observations he made, more subordinate circles he had to imagine, and so he came to propose up to 39 of these circles. Is called Ptolemy model.

Heliocentric Theory. This theory proposes that the Sun is at the center and it is the Earth and the other planets that revolve around it. Dating back to the third century BC. and was recovered by Copernicus (1473-1543 AD) in the sixteenth century, then improved by Kepler (1571-1630) who demonstrated that the orbits were not circular but elliptical and finally confirmed by Galileo (1564-1642) with observations with the telescope he invented.

3 . The Moon.It is the only satellite of the Earth. It is believed to come from a planet revolving around it and was trapped by Earth's attraction. It is one of the largest moons in the solar system, it has a diameter of 3476 km, or slightly more than one-quarter the diameter of Earth. These small dimensions make this a force of gravity 6 times lower than that of Earth, by which a person on Earth that weighs 90 kg, on the moon would weigh only 15 kg and therefore be able to make great leaps. Another consequence of this small force of gravity is that the Moon is unable to retain gases. By not having atmosphere, the average surface temperature varies widely between day (107 ° C) overnight (-173 ° C). These conditions make impossible the existence of life.

With no atmosphere, every time a meteorite falls is very violent shock and produces a small earthquake. This has been used to study the internal structure of the Moon with devices that were installed by the first astronauts and have allowed to discover that, like Earth, it has a crust, a mantle and a core. On its surface may be observed large craters that are the result of the impact of large meteorites, vast plains known as the "Sea of Tranquility" and mountain ranges of thousands of feet high.

The Moon has a translational motion around Earth, which makes a complete turn every 27 days and 7 hours and a rotating motion on itself in which makes a complete turn on the same time. As a result the Moon always shows the same side the Earth (visible face). The other side is called "the dark side" of the Moon.

Due to translational motion the following phenomena occur: the lunar phases, the eclipses and the tides.

• The phases of the moon. These are the successive parts of the face of the Moon illuminated by the Sun that may be seen from Earth on a complete turn. If the Moon turn in the same plane of the ecliptic, is to say in the plane of rotation of the Earth respecting to the Sun, when the Earth were between the Sun and the Moon, the Earth would prevent the Moon to be illuminated by the Sun, but as the plan of rotation of respecting the Earth the Moon makes an angle of 3 ° respecting to the plane of the ecliptic, in this position the Moon is fully illuminated. This is changing throughout the month as the Moon goes around the Earth and thus produce different moon phases. These are:

• Full Moon. It is when the moon is in a position such that at night you can see all the Moon illuminated by the Sun.
• Waning moon. It is when the moon is in a position such that at night we can only see the left half of the Moon, illuminated by the Sun. The Moon begins to take the form of C.
• New Moon. It is when the Moon is in a position such that at night can not see it.
• Crescent moon. It is when the Moon is in a position such that at night we can only see the right half of the Moon illuminated by the Sun. the Moon has a D-shaped for

• The eclipses. It is the total or partial occultation of a star to the viewer due to the interposition of another star. According to the astro hidden are distinguished the Solar eclipses and Lunar eclipses.

• Lunar Eclipse. It's called Lunar eclipse when the Moon is the star that is eclipsed, is to say that it is covered up. This eclipse occurs when the Earth passes between the Sun and the Moon and prevents sunlight to illuminate the Moon. Obviously it can only occur when the moon is full. Depending if the Moon is completely covered or not, the eclipses differ between total and partial.

• Solar Eclipse. It is called Solar eclipse when the Sun is the star that is eclipsed, is to say that it is covered up. This eclipse occurs when the Moon passes between the Sun and Earth and prevents the Sun to be seen from the Earth. Obviously it may only occur when it is the New Moon . Depending if the Sun is completely covered or not, the eclipses differ between total and partial.

• Las mareas. The tides. Are the rise and fall of sea level caused by the gravitational attraction that the Moon and, in a lesser degree the Sun, have on the water. These stars cause a stack of water in the ocean next to the Moon, and also in the opposite zone, because at the minimal gravitational attraction, the centrifugal force causes another stack. This results in a rise in sea level, called high tide or high water. Simultaneously, in the two places in between, and the lack of water, a drop of sea level is called low tide or low water. Due to the rotation of the Earth., on the majority of the coasts occur every day two ascents (high tide) and two descents (low tides). Between a high tide and the following generally pass 12 hours. In the Mediterranean Sea there are variations of a few centimeters, but in the Atlantic and other oceans the high tide can be of several meters.