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Although we have lived in its light ever since we came into the world as a species, although we have studied it for thousands of years and, in the last decades, we have invented and put to work some state-of-art devices to research it, we are still far from knowing everything about it.
Sun and Earth are intimately connected. Everything that happens to mankind’s main source of heat and light is in close connection to the Earth, significantly modifying Terra’s environment. In this respect, various terrestrial processes exist in relation with the solar ones, then with the transmission of the solar radiations through the interplanetary space and, not in the least, with the manner in which solar radiations are absorbed by the atmosphere. The paper herein presents the special importance that Sun holds, both for Terra, and for the inhabitants of this planet.
In Prehistory, Sun was deemed to be a god: “Ra” for the ancient Egyptians (Fig. 1), “Shamash” in Mesopotamia, “Helios” in Greece.
Sun worship was celebrated in many civilizations throughout the world, such as the Inca civilization in South America and the Aztec civilization in Central and North America (Fig. 2).
In the Roman empire, the feast Sol Invincitus (Invincible Sun) was celebrated immediately after the winter solstice.
In the Geto-Dacian culture, Sun is the guarantee of justice and the source of wisdom (Fig. 3).
However, the inquisitive human being has been searching to find more about the Sun; and nowadays the Sun is known to be not a god, but only a celestial body, precisely a star, which represents the main source of warmth and light for the planet Earth.
The astronomers and philosophers of Ancient Greece admitted that Sun, much as the Earth and Moon, were spherical bodies that moved throughout the Universe. Aristotle (384- 322 î.H.) posited that Sun was a sphere at great distance, but that Earth was at the Universe’s core (Fig. 4).
Aristarchus (270 b.Ch.), was the first to submit a heliocentric system: ”Earth, not the Skies, daily rotates and moves round the Sun”. (Fig. 5)
The heliocentric theory was also developed in the V-th century by the Indian astronomer Aryabhatta (476-550 d.H.), who deemed Earth to be a spherical body that moves round the Sun.
Another Indian astronomer, Brahmagupta, during the 7th century, deduced Earth’s circumference to 5000 yojanas (1 yojana=7,2 km), thereby resulting a circumference of 36.000 kilometres, close to the one known today (40.000 km).
Sanskrit verses of those times would show that the stars visible at night were much like the Sun visible during daytime, being therefore admitted that Sun was in fact a star, which appears to be bigger due to its closeness to the Earth. Brahmagupta stated with reference to the gravitation:”Bodies fall towards the Earth, as this is Earth’s nature –to attract bodies, as it is natural for water to flow”.
The heliocentric theory could only be proved 1000 years later, in default of the necessary tools.
Using the telescope, Galileo Galilei (1564 - 1642) studied the movement of the stars, of the sunspots, estimated the distance between Sun and Earth and supported the heliocentric theory. However, the one who mathematically substantiated the concept of planetary system having the Sun at its core was Nicolaus Copernicus (1473-1543)(Fig. 6).
During the XVII-th – XVIII-th centuries, the astronomers recognized that Sun was the closest star to the Earth and they calculated its volume, mass, its rotation speed and direction of movement through space. Other details on Sun’s mass and density were given by Newton (1645-1727).
Before the XVII-th century, the scientists described the sunspots. The visualization of the sunspots between the years 1600-1700 was made with the camera obscura. (Fig. 7).
However, detailed information were only obtained after the invention of the telescope. The astronomer William Herschell (1738-1822) used a huge telescope for investigating the planets.
In the XIX-th century, availing themselves of better instrumentation, scientists described Sun’s eclipses and characteristics (chromosphere, photosphere, observation of the sunspot dynamics).
In the XX-th century, physicists and astronomers disseminated detailed information worldwide, as regards Sun’s influence upon Geomagnetism and radio waves. The identification thereby occurred of the thermonuclear radiations, of the hydrogen in the Sun, of Sun’s internal temperature and of the energy generation through nuclear reactions. In the geophysical year 1957-1958, several scientists made observations on solar and terrestrial phenomena in more than 2000 locations. In the same year, in USSR, Sputnik satellite was launched, followed by Explorer 1, 3 and 4; Vanguard and Pioneer satellites. During 1958, in USA, USA, National Aeronautics and Space Administration (NASA) was set up.
During the next period, several satellites were launched, in order to observe solar electromagnetic radiation and to obtain terrestrial information.
The varied effects of the solar radiations upon the climate were studied, many conclusions being contradictory. At the intensification of the solar wind, the gravitational waves deform, which brings about an agglomeration / dissipation of clouds, causing changes in the weather.
The relation Sun – climate is very complex and is also influenced by other factors: greenhouse-effect gas, strong winds, volcanoes.
The relation Sun – Earth is intrinsic. The Earth’s movement on the solar orbit determines the night and day, and settles the life cycle on Terra. Sun fields warmth and light. It heats the atmosphere, it vaporizes the ocean water, it directs the resulting clouds through air currents, also called winds, towards the continents, where they prove their usefulness, by causing rains and by maintaining the river flows.