Universe

The planets of the Solar System revolve around the Sun, each with unique characteristics and orbit. They are divided into rocky planets (or inner) and jovian planets (or outer). The first (in order of proximity to the Sun we have Mercury, Venus, Earth and Mars) are relatively small and consist mainly of rocky material. The seconds (Jupiter, Saturn, Uranus and Neptune) are composed of light elements such as hydrogen and methane that wrap probably a solid center core. Between Mars and Jupiter (then in the area that separates the inner planets from the external ones) there is the asteroid belt composed of a myriad of rocky bodies of irregular shape and size of less than 1000 km (but most of them do not come to 100 km). As our Earth, all the planets of the Solar System have an elliptical orbit. The orbits are almost all on the same plane called ecliptic and, for that reason, if you happen to see more planets in the sky at the same time, they all seem aligned.

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Sun is similar to all the other stars in the sky; it appears larger due to its proximity, about 150 million kilometers (compared to many billions of kilometers corresponding to different light-years of other stars). It's therefore a privileged study object since it let us closely observe various phenomena. Energy produced by the Sun, thanks to internal nuclear reactions, generates a electromagnetic radiation continuous spectrum, from gamma rays to radio waves, which illuminates whole Solar System and Earth's surface, that is filtered through our atmosphere.

Sun has an high density and temperature (about 15 million degrees) inner core with a radius of about 150000 km where nuclear fusion of hydrogen (the "engine" that illuminates our star) into helium occur.

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Moon, the only natural satellite of Earth, is easily visible with naked eye during day and night and it looks like when fully lit, as large as the Sun (size in the sky about 0.5°). Seen with binoculars or a telescope, it shows a surface covered by a multitude of impressive craters (more visible in lighter areas calls highlands) alternating with areas darker and smoother called seas. From geological point of view, highlands are what remains of its primitive crust (with about four billion years of age) while seas are areas of lava material leakage in ancient times (more than three billion years ago) probably as a result of the fall of a large asteroid on the lunar surface.

The age difference between two formations also explains lower number of craters in seas: there are less because seas were born a billion years after highlands.

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Identify constellations and stars is crucial to understand how to orientate in the Universe. One of methods is to observe particular stellar alignments: the main ones are Big Dipper and Orion constellation.

Big Dipper, for observation places to above 40 ° latitude, is circumpolar ie is visible throughout the year. If we extend the line joining two stars opposite the curve of cart shaft, we find the North Star. If we extend wheel curve, we come to star Arcturus.

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An astronomy introduction for the benefit of those unfamiliar with the Universe: we face now themes are more familiar to amateur astronomers such as the nature of celestial objects, their names and recognition in firmament. In a pause to observe starry night sky from a place with little light pollution, we can see hundreds of stars. Most are real stars, some are planets like Jupiter or Saturn. Moon, the only natural satellite of Earth, can even be annoying with its intense light, but majestic and fascinating because of the details that can be observed with naked eye. If we have a pair of binoculars or a telescope and we know where to look, we can discover nebulae, star clusters and galaxies of various shapes, sizes and brightness.

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