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Archive Astronomy Questions of the Week - International Year of Astronomy 2009
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Why study astronomy?

Week 13
Astronomy is one of the oldest sciences. It studies the celestial bodies: planets, moons, meteorites and stars; it explores galaxies and investigates cosmic radiation, and it tries to shed light on the origins and evolution of the universe.
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Why does the Sun have (black) spots?

Week 12
The concept that a heavenly body such as the Sun should not be flawless: this sacrilegious idea bordered on blasphemy and got the natural scientist Galileo Galilei (1564–1642) into trouble with the Catholic church. However, he was not the first to concern himself with the dark areas on the Sun's surface. The black spots had already been described in antiquity. They had been systematically observed since the invention of the telescope at the beginning of the 17th century.
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How fast is the Earth moving?

Week 11
In order to answer this question, we must first make sure we are aware of what is know as the frame of reference. A surfer hardly moves at all, relative to the wave he is on, but seen from the shore he is rushing towards the observer. Cosmic background radiation provides an absolute reference system for speed in space (see the astronomic question from week 10: Where is the coldest point in the Universe?). It is measured coming almost completely uniformly from every direction of space with a temperature of approximately minus 270 degrees Celsius.
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Where is the coldest point in the Universe?

Week 10
The entire Universe is saturated with what is known as microwave background radiation, a remnant of the Big Bang. Microwaves are electromagnetic waves – just like visible light. However, microwaves have wavelengths between one metre and one millimetre (= one thousandth of a metre), while the wavelength of light lies between 380 and 780 nanometres (= one thousandth of one millionth of a metre)?
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What is the story behind the 'dark side' of the Moon?

Week 9
The rock band Pink Floyd named an album after it: 'The Dark Side of the Moon'. In reality this satellite of the Earth does not have a dark side – over the course of one month the Sun shines upon the entire surface of the Moon. The Moon is not visible from the Earth with the naked eye when it is positioned between the Earth and the Sun (new Moon), but its reverse side is fully lit.
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Lagrange-Punkt L2

What is the best place to 'park' a satellite?

Week 8
The mathematician Joseph-Louis Lagrange (1736-1813) could not yet have known anything about space exploration and satellite technology. Still, he made a mathematical discovery that is of great importance to modern space research: If two differently sized bodies, such as the Earth and the Sun, orbit around each other, there are five special points in their gravitational field.
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Satellit GOCE

Do satellites still need rocket propulsion when they are in orbit?

Week 7
When a satellite has entered into its orbit around the Earth, it moves so fast that the centrifugal force caused by its motion is equal to the Earth's gravitational pull. (See also the Astronomy Question for week 6: How fast does a rocket have to travel to reach space?) In the absence of disturbing influences the satellite would continue along its trajectory forever.
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Kosmische Geschwindigkeit

How fast does a rocket have to travel to reach space?

Week 6
If a rocket is launched from the surface of the Earth, it needs to reach a speed of at least 7.9 kilometres per second in order to reach space. This speed of 7.9 kilometres per second is known as the orbital velocity, referred to in German as the 'first cosmic velocity' – it corresponds to more than 20 times the speed of sound. At the start of the space age, Russian scientists applied the term 'cosmic velocities' to certain velocities that are important for space exploration. The 'first cosmic velocity', known as the orbital velocity, will bring a rocket or other projectile into orbit around the Earth. A slower projectile will fall back to Earth.
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Erde - Mond

Will the moon crash into Earth one day?

Week 5
No, even under the most adverse circumstances that is not something to be reckoned with. It is true that two bodies with large masses, such as the Earth and the Moon, exert a strong gravitational pull on each other. If this were the only active force, the two celestial bodies would indeed crash into each other. However, the centrifugal (outward) force of the Moon's motion counteracts this gravitational pull.
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Olympus Mons

Which is the highest mountain in our solar system?

Week 4
The Olympus Mons (Latin for Mount Olympus) rises 26 kilometres above the Martian surface. This makes it the highest and, with its diameter of 600 kilometres, also the largest known mountain. As a giant volcano, it spewed molten rock from deep within the Red Planet for billions of years. Scientists suspect that it is still active today.
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Habitable Zone

In which parts of the universe can life develop?

Week 3
Living organisms as we know them need water to survive. In order for there to be liquid water on the surface of a planet, it should not be too far from its central star, that is to say its "sun". Otherwise, the water will freeze. However, the planet should also not get too close to its source of light and heat - if it does, the precious liquid will evaporate.
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Erdumlaufbahn (schematisch)

Why wasn't it nice and warm on 4 January?

Week 2
Every year, the Earth actually makes its closest approach to the Sun in early January, when it is winter here in the northern hemisphere. This year, it happened at 16:23 Central European Time on 4 January - at that time the Earth and the Sun were "just" 147 million kilometres apart (at the top right of the image). In early July, the Earth reaches the most distant point in its orbit around the Sun (at the bottom left of the image). The distance between the Earth and the Sun is then about 152 million kilometres, while at the same time it is midsummer in these latitudes. This means that the seasonal differences in temperatures cannot be caused by the separation between the Earth and the Sun.
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Why does a week have seven days?

Week 1
Even though it would be perfectly possible to imagine a week having five, six or even eleven days, most cultures in the world have seven-day weeks. The reason for this is that seven celestial bodies were known to the ancients: the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturn. These celestial bodies are visible to the human eye and people in ancient times were able to observe how they moved across the sky. This is also where the term 'planet' comes from - it is derived from a Greek word meaning 'wanderer'.
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