Four billion and 600 million years ago, in a small corner of the universe, within the Milky Way, it happened something absolutely normal for the life of the cosmos but absolutely extraordinary for the consequences that would have for us. A supernova, a star next to die, he suffered a massive explosion triggering a series of events that lead to the birth of our solar system: the Sun and its nine planets that revolve around a series of more “accessories” such as satellites and asteroids. The formation of the Solar System (a name taken from our star, the Sun) was not, therefore, an exceptional phenomenon, so much so that, recently, the new computerised cameras have uncovered other “cradles of planets,” as it was ours. Regarding the formation of the sun does not seem to be many more doubts: the shock waves from the supernova explosion propagates in a interstellar nebula, causing an imbalance of its density. A portion of the nebula becomes so dense that I could not support its severity. So the interstellar nebula, consisting of gas and dust begins to contract and the nebula is born of the primordial solar system. Following the contraction, the temperature of the central portion rises gradually, triggering the first thermonuclear reaction and increasing the brilliance. By the supernova's explosion 10 million years are passed but you will have to wait for it to pass another 700 million before the whole process of creation leads to the solar system as it is today. At this point, after which the core of the Sun is almost completely condensed, it's up to the planets. There are two theories about this procedure, one definite classic and another is a new concept. The first step, according to the classical model, was to accretion to collision: Following the cooling of the primordial nebula of the planetary system, the molecules of metal material are separated from the gas and precipitate in the central part of the disc. In addition, many planets are formed with a diameter of about 10 km. The planets, which are subject to repeated and mutual collisions, develop as a primordial planets. In the second step, the dust would settle first in a thin disk, then where there is more, cakes forming the nuclei of future planets. The third step consists in still another series of collisions between planetoids that begin to give pierces the planets real. Aside from some other asteroid or comet collision with some very eccentric orbit, everything is ready and peaceful to ensure that the third planet in order of distance from the Sun, he began the long process that will lead to life. For the new theory, however, instead of the asteroids, the cloud of gas and dust begin to rotate at different speeds in the various regions. Would be formed by the rotation of the diverse reels of cosmic dust: the smallest inside, the largest in the outer regions of the solar system. The third step is not yet clear; eddies which are formed are of course the nuclei of future planets, but as these are real close to the planets is still being studied.

Our solar system does not end with Pluto, it is surrounded by two bands of “cold bodies”: the Edgeworth-Kuiper belt and the Oort cloud

The Belt of the Sun

“It was the night of 30 August 1992, when, with my colleagues, we had just finished recording four photographs of a portion of the sky, obtained at a later date. Looking at the photographs we noticed a faint star that changed position from one image to another. We were speechless. The comparison with those taken on successive nights took us to determine the distance of that object would be about 40 AU and that its average diameter was around 280 km. ” So Jane Luu, an astronomer at Harvard University (USA), recalls the discovery of the first planetoid beyond Pluto in what today is called the Edgeworth-Kuiper belt, from the names of the scientists who first hypothesised, and that is the most nearest of the two major areas of the cosmos wedges of icy objects (the other is the Oort cloud) from whom all the comets that we observe from Earth. From 1992 to date have been discovered beyond the orbit of Pluto 64 new bodies, but despite being relatively few, are enough to calculate that beyond the last planet there are at least 70,000 objects with a diameter greater than 100 km, and beyond a billion larger than a kilometer. Much farther there is another concentration of cosmic objects, this time in the form of a shell instead of a donut: the Oort cloud. Within these two systems, billions of comets embryos are ready to leave their nest to dive between the orbits of the planets. Today it is also found that, in addition to traditional comets, there are objects that come from the Edgeworth-Kuiper which are a cross between comets (consisting mainly of ice) and asteroids (mostly made of rock). Until now they have been observed are, but there could be in the millions. One of these, Chiron, was mistaken for a newly discovered asteroid, but then, approaching the Sun, performed the hair typical of comets.


According to the most widespread hypothesis, the solar system was formed in the same period in which the Sun was born, about 5 billion years ago, the center of a cosmic cloud formed by hydrogen gas and dust. The cosmic cloud, with the sun at the center, has contracted towards the center where he began to thicken and turn around on itself faster and faster, warming, inside the powders were thickened, forming other nuclei increasingly large and dense, which will give rise to planets. In addition to revolve around the Sun, these nuclei revolve around themselves. At the same time, in the core, fusion reactions begin and the sun slowly takes on the current characteristics.


The sun appears to move from W to E with respect to the system of fixed stars and only after a year back in the starting position. The path that the Sun does in its annual motion with respect to the fixed stars is called the ecliptic constellations and follow each other. Copernicus proved that it was possible to interpret the motions of the heavenly bodies assuming that the sun was still in the center of the universe and that all the planets revolved around a uniform circular motion.

Kepler's Laws

The spectacular order and symmetry of the solar system have moved some men to devote much of their lives to investigating the motion of the planets. One such man was a German astronomer of the 16th/17th centuries by the name of Johannes Kepler. Interestingly, he was motivated in his examination of the planetary movements by a firm belief in a Creator, a Master Architect, and the more he studied these movements, the stronger his faith became. His discoveries, which paved the way for Isaac Newton in uncovering the law of universal gravitation. Johannes Kepler was born in the year 1571 in Weil, a small town in Germany. In spite of a lowly background and a sickly nature, he was able to graduate from Tübingen University, one of the foremost colleges in Europe. Originally Kepler intended to enter the Protestant ministry, but his talents in mathematics and astronomy led him in a different direction. Kepler became a mathematics teacher in the city of Graz, Austria, in 1594, but only six years later he was forced to leave, due to pressures from the religious leaders of the Catholic Church. Kepler and his wife then moved to Prague, where he became associated with the eminent Danish astronomer Tycho Brahe. About a year after Kepler's arrival, Brahe died and Johannes Kepler was appointed his successor in the office of Imperial Court mathematician to Emperor Rudolf II, and subsequently to Emperor Matthias. While serving at this post, Kepler discovered the three principles actually made by the Creator to govern planetary motion. Consequently, they became known as “Kepler's Laws.” For centuries astronomers had felt that planetary orbits involved some form of circular motion. This belief, however, had not proved true in actual observation, and scientists were led to extremely complex diagrams and equations to explain the discrepancies. Kepler, after years of calculation, primarily with regard to the planet Mars, arrived at the conclusion that this planet's orbit was not circular but a geometric figure called an ellipse. Most of the planets travel in orbits that are nearly circular, the earth's orbit being almost a perfect circle. A few planets, however, have elliptical paths that are quite eccentric, that is, they are flatter or less round. Pluto and Mercury are the most eccentric of the major planets, but some comets, such as the famous Halley's Comet, have extremely eccentric orbits. Kepler deduced from a study of the orbit of Mars that all planets travel in elliptical paths. Moreover, he concluded that in every case the sun is at one of the focal points of the planet's orbit. These conclusions have since been verified, and constitute what has come to be known as Kepler's first law of planetary motion. What a remarkable law this is! It shows that the planets do not move in some strange, irregular, and random pattern. Rather, their paths are a smooth mathematical curve. From Kepler's first planetary law it can easily be seen that planets are closer to the sun at certain times than at others. In fact, the Earth, at its closest point to the sun, is 91 million miles (146.450 million kilometers) away, whereas at its farthest point it is over 94 million miles (151.278 million kilometers) away. Halley's Comet, with its eccentric orbit, is 56 million miles (90.123 million kilometers) from the sun at its nearest approach but over 3,200 million miles (5,149.900 million kilometers) when farthest away. From the time of the ancient Greeks it was thought that all planetary motion was uniform. In other words, they believed that a planet's speed was the same at every point in its path. Once again, however, observed facts proved otherwise, and scientists had extreme difficulties in explaining the differences. Johannes Kepler, after combing through mountains of observations made by Tycho Brahe, made another fascinating discovery. Planetary motion is not uniform; a planet travels faster when it is closer to the sun and slower when farther away. Furthermore, Kepler showed that a very curious law holds true: the line drawn between the sun and any planet will sweep out equal areas in equal periods of time. This is somewhat easier to understand by the following illustration: suppose it takes one month for a planet to travel from point T1 to point T2. Suppose it also takes one month from T3 to T4. Then, by Kepler's second law, the area of the two shaded sections will be equal. From this it can be seen that a planet would travel faster when it is nearer the sun, in order for an equal area to be produced. Accordingly, we see that the speed of the planets is not some unpredictable, chaotic, jerking motion. While they do move more rapidly at certain times and less rapidly at others, the changes of velocity are smooth and stable and in accordance with mathematical law. Each planet goes swinging back and forth in its orbit in graceful motion. How we marvel at this beautiful design! Surely we must also marvel at its Designer. By means of his first two planetary motion laws, Kepler had derived formulas for the shape and the speed of a planet's orbit. The answer to another perplexing question remained: What relation is there between a planet's distance from the sun and the time it takes to complete a circuit? He knew that planets that are closer to the sun travel at greater speeds than those farther away. After nearly 10 years of labor he discovered a formula that expressed this relationship. This came to be known as his Third Law. This law states that the squares of the periods of revolutions of any two planets are in the same ratio as the cubes of their average distances from the sun. An example of this relationship can be seen in the case of the planet Jupiter. Jupiter is approximately 5.2 times as far from the sun as is the Earth. Correspondingly, it takes Jupiter about 11.8 earth years to make one orbit around the sun, which is one Jupiter year. Let us prove the accuracy of the Third Law by applying it in the case of the planet Jupiter. To square a number is to multiply it by itself; to cube a number is to multiply this result again by the original number. So going back to the example of Jupiter, what do we find? If we square the period (Jupiter's period of orbit around the sun is 11.8 earth years), we get 11.8 times 11.8, which equals nearly 140. Now, if we cube the distance, we get 5.2 times 5.2 times 5.2, which also equals approximately 140. This equality holds true for each one of the planets. Kepler called his third law the “harmonic law” because he believed that it revealed the harmony that the Creator had manifested in the solar system. After discovering this law, Kepler exclaimed: “I feel carried away and possessed by an unutterable rapture over the divine spectacle of the heavenly harmony.”

Summary of the three low

  • The first: the planets revolve around the Sun along elliptical orbits with the Sun at one focus. The planets in their revolution motion are not always at the same distance from the Sun, but along the orbit there is a position, in which the perihelion are closer to them, and they are further away, the aphelion. Perihelion and aphelion are joined by the so-called line of apses.
  • The second: the radius vector joining the center of the Sun with the center of the planet sweeps out equal areas in equal times. The planet, so when it moves close to the perihelion must traverse a longer orbit than when it moves near aphelion and then moves along its orbit at speeds ranging. (Perihelion: maximum aphelion: low)
  • The third: the squares of the periods of revolution of the planets are proportional to the cubes of their mean distances from the Sun And this means that the angular velocity of revolution of the planets decreases with increasing their distance from the Sun.

The three laws of Kepler, in turn, are the consequence of the law of universal gravitation: two bodies attract each other with a force whose intensity is directly proportional to the product of their masses and inversely proportional to the square of their distance.


It is called in this way the motion that makes the Earth rotating around the Sun using a year to carry out his one whole. The point of closest approach to the sun is the perihelion (January 3) and the maximum distance is the aphelion (July 3), in fact the Earth's orbit is an ellipse slightly flattened. Its total length is about 940 million km and the average speed of the Earth is 29.8 km / s (30.3 perihelion and aphelion 29.3) The orbit traveled by land lies in a plane which is called the ecliptic plane, the Earth's rotation axis is not perpendicular but at an angle of 66 ° 33 '(this causes the seasons) Evidence of this revolution motion are:

  • the analogy with the other planets of the solar system
  • the apparent movement of the Sun with respect to the fixed stars
  • the periodicity in the year of some groups of stars falling (because the Earth crosses the same positions swarms of meteorites, some of which are attracted by the Earth's gravitational field, and falling through the atmosphere to glow, giving rise to the phenomenon of shooting stars).

The aberration of the light coming from the stars (the amplitude of the angle between the true direction of the star and the apparent is affected by the variation of the speed of revolution of the Earth. This aberration of light depends on the fact that, while the speed of rotation of the ground is less than 1 km / s, the speed of revolution of the earth is not negligible compared to the speed of light.)


The Earth makes the motion of revolution in a year, considering the duration of the year as the time it takes for the sun to return to the same position with respect to the system of fixed stars (sidereal year 365d 6h 9m 10s) but if the year is measured as the time that elapses between two successive equinoxes, we have the calendar year or tropic (365d 5h 48m 46s).

Solstices and equinoxes

At any time of the motion of revolution around the sun, only half of the Earth's surface is illuminated by the sun, while the other half is in shadow. The two sides are separated by the circle of illumination. Because of the atmosphere, the circle is not a sharply defined line. If the Earth's axis were perpendicular to the ecliptic plane, the circle of illumination always pass through the poles and the length of the day would be the same as that of the night to every point on Earth every day of the year. The inclination of the axis relative to the perpendicular of the plane of the ecliptic makes that there are only two positions the anoint Earth orbit in which the circle of illumination pass through the poles. In his movements the circle of illumination always cut in half the equator, which is why in places along the equator and the day and the night always last 12 hours. Equinoxes: Two days of the year when the circle of illumination is tangent to the meridians, the duration of day and night are equal. There is the spring equinox (March 21) and the autumnal equinox (23 September). In places along the equator at noon the sun culminates perpendicular to the plane of the orbit, and in all locations of the Earth is exactly E and sets in W. Solstices: summer solstice (June 21) at right angles culminates in all locations that are on the way to parallel 23 ° 27 ' north latitude = Tropic of Cancer. On this day the Arctic Circle is completely dark, having the Sun in the plane of the horizon. While the Antarctic Circle has the sun above the horizon plane for the duration of 24 hours. on the day of the autumnal equinox (September 23) back to culminate again vertically on the equator, then onward to the south where the day of the winter solstice (December 22) perpendicular culminates in all places along the parallel of latitude 23 ° 27 ' south = Tropic of Capricorn.

Different lengths of day and night

In winter the light period (day) is much shorter than that in summer. The phenomenon is related to the position of the circle of illumination on the Earth's surface because the length of each parallel is distributed unevenly to the circle of illumination. In the period between spring equinox and autumn equinox, in the north of the parallel on which the sun culminates vertically the illuminated part is greater than the shade, during the six months from autumn equinox to the spring equinox the situation is reversed .

Sidereal day and solar day

When measuring the length of the day you get different results depending on the reference system: fixed star sidereal day period of time between two successive culminations of the star) 23h 56m 4s, sun solar day (time span between two noons) 24h ca. The reason for the longer duration is that the Earth as she makes a full circle around itself, it also moves along its orbit around the sun, and the two movements are in the same direction, toward E. After making a complete revolution around its own axis which corresponds to the length of a sidereal day, the Earth must rotate again, about 4 minutes, before the sun highlights again on the same meridian.

Change the length of the solar day

The duration of the solar day is not constant due to the motion of revolution.

The seasons

The phenomenon of the seasons is caused by the inclination of Earth's axis and the motion of revolution of our planet around the Sun, from west to east. In fact, the Earth, orbiting along a trajectory of elliptical shape, of which the sun occupies one focus, describes basically a plane which in turn is called the ecliptic. In this way it keeps the axis of rotation parallel to itself, the length of time it takes for the earth to make a complete revolution takes 365 days 5 hours 48 minutes and 46 seconds. The alternation of the four seasons is a consequence of the astronomical revolution motion and tilt of the Earth (the Earth's axis is tilted by 66 ° 33 ' with respect to the plane of the orbit and 23 ° 27' with respect to the normal to such a plan. The magnitude of the inclination varies cyclically between about 22.5 ° to about 24.5 ° with a period of 41,000 years, and currently is 23 ° 27 '. Moreover, the axis of the Earth slowly revolves around the perpendicular to ecliptic, describing a double cone and making one revolution every 25,800 years (52 “ per year). This motion is called precession of the equinoxes and is due to the tidal force exerted by the Moon and the Sun finally, there are the oscillations of the axis minor (about 20 ”) with a shorter period (about 18.6 years). Touching, so at certain times of the year, those four basic points that mark the beginning of each season, and which correspond to the same number and similar points of the apparent solar path since this is the celestial projection of Earth's orbit. The duration of the seasons lens undergoes changes over time for the precession of the equinoxes and the perturbations produced by other planets on the motion of the Earth's revolution.


  • Apparent motions of the heavenly bodies. The line along which the sun moves in the course of a year is called the ecliptic. The strip ideal, large 8 degrees above and below the ecliptic is the zodiac (circle of animals). Sun, Moon and planets are always found within this strip is divided into twelve parts.

The apparent motion of the planets on the celestial sphere is even more complicated. For as the sun and moon rise and set, and they are on the east to the west, and normally seem to flow eastward but in some period of their sliding motion is reversed (retrograde) to continue in the right direction (forward motion) after some months describing a kind of loop.

  • Alternation of day and night (at least the sun is high in the sky, the greater the thickness of atmosphere you have to cross, and for this the dawn and dusk are longer in winter and in the circumpolar regions. The atmosphere affects the sunlight through refraction and diffusion.)
  • Change in the acceleration of gravity with latitude (regularly increases going from the equator to the poles, also influenced by the decrease effect of the centrifugal force that is maximum at the equator to the poles and nothing. Being the angular speed equal for all points of the surface, the centrifugal force depends on the distance from the axis of rotation and then decreases from the equator towards the poles)

Deviation of the direction of moving bodies on the earth's surface due to the fact that all the points that lie on the same meridian rotate integral with the earth with the same angular speed, but with tangential speed decreasing going from the equator to the poles. The body is subject to the Coriolis force: the bodies which move from the equator toward the poles trying to follow a meridian have a higher initial speed of all points that meet and so will move towards E. This phenomenon is described by the law of Ferrel.

The medium solar day

If we measure the length of the day with respect to the stars, we have the sidereal day (23h 56m 4s). If we measure the day as time between two successive culminations of the sun, we have the solar day (about 24h, changes during the year and up to 40s deviates more or less than the average). The reason is the change in velocity with which the earth moves in its orbit around the sun. The increased duration of the solar day corresponds to the time necessary because the earth performs a rotation of 360 ° as it rotates around the sun, and then, to return to the same position relative to the sun, must fulfil a small rotation in more around its axis. This angle further than 360 ° is equal to the angle that the radius vector has swept in its motion of revolution.

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