Spherical Earth

Eratosthenes was one of the first ones to imagine spherical Earth. He observed the shadow of two objects situated in two places, Syene and Alexandria, June 21st (summer solstice) at the local solar noon. It is at this precise moment of the year when in the north hemisphere the Sun holds the highest position above the horizon. Now, Eratosthenes noticed that there was no shadow in a well to Syene (city situated about on the Tropic of Cancer); so, at this precise moment, the Sun was vertical and went directly in the heart of the well. Eratosthenes noticed however that an obelisk of Alexandria formed a shadow.

By calculations of trigonometry, Eratosthenes deducted that the angle between sunbeams and vertical line was 7,2 degrees. Two hypotheses were then imperative, that is:
The Earth is flat, but the Sun is rather close so that the difference of beams is significant. The Earth is curved, and sunbeams reaching the Earth are any parallels, then it is the sphericity of the Earth that creates the difference between Alexandria and Syene. Eratosthenes calculated then the distance between Syene and Alexandria by appealing to a bematiste Un Bematiste is a land surveyor of antique Egypt which had the load to measure distances in number of steps ( bêma ). Eratosthenes used a bematiste to measure the distance between Alexandria and Syene (Assouan) to measure the circumference of the earth.  which based itself at the time in the days of walking of camel enter

both cities: the obtained distance was 5000 stadiums, that is 800 km, measure very close to the reality, a stadium (length used in the stadiums of Olympia or Delphos) being approximately worth 157,5 m. Eratosthenes proposed a figure of brilliant one simplicity: it consisted of a simple circle having an angle in the center of 7,2 degrees which intercepts a bow (connecting Syene in Alexandria) of 800 km. By connections in circles, he obtained that the circumference of the Earth was 39 375 km (5000*157,5*360/7,2/1000), extraordinarily precise measure for period.

The life is a mysterious and universal tendency of the matter to join, to get organized, to complicate. The alive is characterized by the fact that it draws some energy from the outside middle, uses this energy, throws back the waste and enriches its order, its organization. On the scale of the sorts, the alive does not stop the complexity since 4.5 billion years.

The not alive evolves always in time in the sense of their disorganization. It is the growth of the entropy (from Greek review) It is a function thermodynamics. Thrown in a turbine, the water of a dam transforms its gravitational energy into electrical energy. Later, we shall make a movement in an electric engine or of the heat in a radiator. Throughout these transformations, the energy degrades in other words, its entropy increases. The entropy of a system remains constant when it returns to its initial state by a reversible transformation. The total entropy of an isolated system always has to increase, its disorder always has to grow, it is the second principle of the thermodynamics. (measure of the disorder). Not alive produced of the entropy and alive produced of the negative entropy, or still the neguentropy. The life is nothing else than a commonplace mechanism, that a particular shape of the matter, the secret of which we are certainly going to drill.

Satellite photo: notice on the enlarged photo, frail membrane of the ground atmosphere which protects the life.

Planets are supposed to be constituted by successive layers of increasing density. Materials are in the order of their density, the iron in the center then iron sulphide, silicates, water, nitrogen, carbon dioxide, ammonia, methane, helium, hydrogen. There are nevertheless two identical planets in their structures. Each has the appropriate characteristics. The native, first iron condensat Solid grains of condensed chemical and mineralogical compounds be born in nebulas, following that we call: the sequence of condensation. The first compounds which condense in 1300°C, are oxides rich in titanium, aluminum and calcium. Towards 1050°C condenses massively the metal iron, then towards 950°C, the first silicate in this particular case the silicate of magnesium and iron. Towards 800°C, form silicates with more cowardly structures, feldspars and iron sulphide. In even lower temperatures condenses a silicate containing some water and in 0°C the water condenses icy. Plentiful, is the constituent of the ground nucleus. The silicon, the silicate of magnesium and iron constitute the essential constituents of the ground coat. The feldspar, condensat which gives the basalt constitutes the floor of the ground oceans. The internal structure of the Earth is thus distributed in several successive envelopes, the earth's crust, the coat and the nucleus. This representation is very simplified because these envelopes can be decomposed themselves. To track down these layers, the seismologists use seismic waves, as soon as the speed of a seismic wave changes brutally, it is that there is change of middle, thus layer. This method allowed, for example, to determine the state of the matter to big depths (deep coat, nucleus). These layers are bounded by the discontinuities as the discontinuity

of Mohorovicic, that of Gutenberg, either that of Lehmann. The Earth formed by accretion of meteorites and during this formation, the various layers organized by respecting the density of its constituents. The theory of the plate tectonics is widely admitted now since the end of the 60s and is widely imperative in the scientific world. In 19th century we had difficulty in believing that whole continents can derive. We know now that the solid coat is animated by immense currents of convections which circulate since millions of years. The image which we have now is that of an active and complex planet the crust of which consists of oceanic and continental patches, different mineralogical compositions, ceaselessly in movement under the action combined by currents of convection intern and of the ground gravity. Continental blocks form by collisions of continental patches and tear, according to a cycle of 400 million years. Heavier, oceanic patches, participate in this ceaseless ballet since several billion years and often eventually plunge, inside the Earth by subduction so participating in the recycling of earth's crust.

Related subjects

category: planets

Mercury

Jupiter

Venus

Saturn

Terre

Uranus

Mars

Neptune