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The appearance in the sky of this beautiful comet is listed for centuries. But it is only since his last visit in 1986 that knows more about this exotic solar system.
Halley's Comet is one of the known comets of the public. His spectacular pass in 1910 has long marked the spirits.

History of comet

In 1705, the English astronomer Edmond Halley announced that comets that had appeared in 1531, 1607 and 1682 were in fact one and the same comet. He predicted, thanks to the laws of Newton, his return in 1759. The comet actually came back in 1758 with a few months in advance, 16 years after his death. The comet was named in his honor, even if now, comets are baptized in the name of their discoverer.

This triumph of the late astronomer allowed to permanently sit laws of Newtonian mechanics.

famous passages

The most famous appearance of Halley's comet was undoubtedly at the Battle of Hastings in 1066, which marked the invasion of England by the Normans. This famous event is represented on the 32nd stage of the famous Bayeux Tapestry. This appearance on the eve of the battle was interpreted as a bad omen. The English defeat that ensued gave them ample reason.

More recently, last appearances dating from 1910, which was an impressive sight and winter 1986 when several probes went visits to the comet. The Giotto probe afforded some impressive photos of the comet nucleus and learn more about these strange bodies of the solar system.

The accurate prediction of the next passage of the comet is difficult because it undergoes gravitational influence of the giant of the solar system that may cause it to radically change its trajectory. However, with the data collected, astronomers estimate that the comet will return in 2061 near the Earth.

Specifications

Orbit of the comet: The comet's orbital period varies around 76 years. With perihelion located 88 million kilometers from the Sun and an aphelion located 5240 million kilometers, its orbit is highly elongated. The comet orbits in retrograde direction (east to west) and is inclined 18 ° to the plane of the ecliptic. Only when the comet is less than 200 million kilometers that we can begin to see on Earth.

With all these features, astronomers think it would come from the large tank, located in the outer solar system called the Oort Cloud. However, comets coming from this reservoir has a period greater than 200 years. Now Halley's Comet to a period of 76 years. This anomaly could be explained by the gravitational perturbations of the giant that would have shortened its orbit.

A dark core: With an albedo of only 3%, Comet Halley is even darker than coal. With the Giotto probe, astronomers have seen surprised by the darkness of the sun so bright when it first appeared. This albedo because of Comet Halley one of the darkest planets of the solar system.

Low density: The density of the comet nucleus of Halley's comet is much lower than previously thought: 0.1 mg / cm3. We must therefore imagine a porous foam made of water ice, dry ice and hydrocarbons rather than a block of ice. Astronomers have in fact estimated the mass of this star range only between 20 and 100 billion tons. The comet, with each pass, losing about 0.1% of its mass which represents some 100 billion kilos! It was estimated that the icy geysers of Halley's Comet expelled at their maximum activity around 30 000 liters of water per second!

Saturn (alchemy and astrology) .- The planet Saturn has under its patronage lead, among Greek alchemists and those of the Middle Ages. The same sign designates the star and metal. The dark color of one was compared to that of amber, and the slow apparent motion of the planet likened to gravity (density) of lead.

Saturnus (Roman Religion). - One of the most ancient gods of Latium and central Italy. A tradition collected by Pompey Trogus and Justin represented as a king of the Aborigines, ie d. primitive populations. The ancient, eg Varro and Augustine after him, had attached the word Saturnus at the root sat contained in satum (verb sero, sow) sator. This is an etymology that seems always admissible. Saturn was therefore, at least originally, the god who presided over seeding, which protected the seeds entrusted to the earth; but he did not remain confined to these special powers. As far as we can judge by the Greek additions later modified the physiognomy of the god, Saturn, husband of Ops was for the inhabitants of Latium agricultural deity par excellence. His main attribute was a sickle, or rather a curved knife, which he used to cut the harvest, pruning trees, to prune vines

Finally, under the epithet Stercutus or Sterculius, he was the god of fertilizers that increase soil fertility. His character was clearly chthonic; in him the ancient Romans worshiped the subterranean forces that germinate seeds buried in the ground; it seems to have been a close relationship between Rome and Saturn Dis Pater. Macrobius says that the altar of Saturn adjoined a chapel Dispater (Saturnalia, I, 10, § 48).

The month of December was dedicated to Saturn, because the sowing were then completed, and because it was the time of year beginning in the womb of the earth, the work of germination, distant prelude to the future harvest. Saturn was very formerly worshiped on the same Rome location. There was, in historical times, on the southern slope of the Capitol, while a quarter of the city which preserved the memory of this cult: there was the temple of Saturn and Ops, which were deposited the Treasury, public and the state archives; not far opened in the enclosure called Servius Tullius, a door Saturn, Porta Saturnia; Varro adds that the Capitoline hill was originally called mons of Saturn, and that before the founding of Rome there was this height a town that bore the name Saturnia.

After the Sun, the Moon is certainly the star who most needed his presence in the skies of Earth Our satellite also has something "more": the complexity of its cycles .This gave him a special significance both in mythologies that in the oldest astronomy.

As was the case for all the stars, from the seventeenth century, the use of raw glasses, and the advent of Newtonian mechanics, a new way of viewing the moon is made daily. The complexity of its movement only became more evident, and its study has now found new tools provided by celestial mechanics. But above all, its surface has now been studied in detail. The first maps of the moon were drawn and she was born selenography, which runs from mid-nineteenth century through photography.

Along the way, astronomers have watched on the surface of our satellite variations, real or imagined. This questions is found intimately linked with that of the possibility of this geologic activity on the Moon. In particular, the question arose whether the craters and lunar seas were volcanic or whether to invoke other causes. If astronomers have finally opted for the meteorite hypothesis

picture

It took them to wait for the exploration of the Moon from 1960 to be able to base this conclusion on a solid argument.

Key dates

640 BC. AD: Thales recognizes the lunar phases a result of the reflection of sunlight.

1609: First observations of the Moon by Galileo using a telescope.

1830-1837: Publication by Beer and Maedler their map of the Moon.

1833 To calculate the inequalities of the Moon, Fish provides a process that is incomplete for a theory of our satellite, but which could be applied with advantage in the case of the planets.

1787 Herschel believes observe a volcanic eruption on our satellite (Aristarchus crater).

1840: First photographs of the Moon.

1894 -1910: Photographic Atlas Publication of the Moon Loewy and Puiseux.

1959: First shot from the opposite side of the Moon by the probe Luna 3.

1969-1972: Twelve human trample on the moon in the Apollo program.

The phases of the Moon.

(According Hevelius)

The moon before telescope

Moon played a big role in all mythologies. Its cycles, often put in relation with the idea of fertility, were all closely monitored. From the seventh century BC, the Ionian philosophers also began to speculate on the nature of this star. Is this a fiery or earthy body? A perfect crystal sphere or another habitable earth? These issues will be discussed, in different variations, until the appearance of the first telescopes in the seventeenth century.

Steps selenography

As its etymology suggests, selenography is to the moon, what geography is to the Earth. The principles are the same, but measurements, remained for a long indirect, made this term synonymous primarily lunar mapping. The observation in 1609 by Galileo and his contemporaries of the Moon's surface with the first glasses revealed the presence of mountains associated with clear our satellite regions. But the question that immediately arose was whether the dark regions, in contrast, were not seas. We quickly abandoned this idea. The current nomenclature of the Moon, the first steps back to the cards Hevelius and Riccioli, however, still bears traces of these questions. Following these seminal works, it should be noted cartographic efforts Cassini, Tobias Mayer, Schroeter, to Lohrmann and Schmidt, and especially Beer and Maedler whose card, completed in 1837, represents the last great completion of selenography before the use of the photograph.

At the age of photography

The first photographs of the Moon date back to 1840. Although this new technique is not able to compete with the direct observation with regard to the fine detail that can be detected, it brings a certain objectivity, allowing its use for achieving new maps of our satellite. Photographic Atlas of the Moon, directed by Loewy and Puiseux between 1894 and 1910 is probably the culmination of this approach. But photography has other early recognized benefits. And Will it be used for photometric studies, or to explore the beaches of the electromagnetic spectrum located beyond the visible range. Today, traditional photography is hardly used by astronomers. However, it has known a last moment of glory during the exploration of our satellite program in the 1960s, during which we could take pictures of the Moon "on site".

The variations on the Moon?

Observers such as William Herschel, Schroeter Gruithuisen, Littrow, believed distinguish our satellite traces of constructions "made by human hands." Kepler himself in the Dream says that lunar craters are too regularly trained for nature is responsible. A closer look obviously every time then proved that these constructions (walls, trenches, canals and roads supposedly) were not artificial but purely natural formation. However, these detailed observations, if misinterpreted, also brought to light the possibility of changes to the surface of the Moon, also called transient lunar phenomena (LTP). Episodic lights, craters that change shape. This showed it to the existence of current geological activity on our satellite? Could they claim the existence of a volcanic earthquakes or the pest on the Moon? Nobody believes today. But astronomers of the past had their reasons to think otherwise.

The origin of the craters and seas

The generations of astronomers who have followed in an attempt to detect changes on the surface of the Moon often had in mind the possibility that the Moon is the seat of volcanic eruptions. Indeed, such a type of geologic activity has long been privileged to explain these formations called craters. It showed craters of volcanoes, extinct in the main, but some of which could still give some signs of activity. It was the volcanic hypothesis of the origin of lunar craters and seas. But another option began to be seen around 1895 and eventually prevailed from the 1960s is the meteorite hypothesis. Thus, astronomers now recognize that the craters of the moon have nothing volcanic, but instead were formed during a large meteorite bombardment that took place in the early days of the Solar System

Space exploration

Long destination imaginary journeys, the moon has begun to be considered seriously as a goal reached by few engineers of the early decades of the twentieth century engaged in the development of the first rocket engines (Robert Goddard, Robert Esnault-Pelterie, etc.) . But it was not until after the Second World War, to the availability of technologies developed for military purposes (mainly the German V1 and V2 rockets), added to the context of the Cold War place in 1953 to priority programs access to space. Reach the moon being then very quickly perceived as a sign of technological expertise required to launch intercontinental missiles, and the most obvious symbol of the victory of one or the other side in this new conflict so strongly tinted propaganda.

The first firing of rockets towards the moon date from 1958, and the following year, the first automatic probes effectively managed until our satellite. Quickly, these test shots called the emergence of human spaceflight programs, an American on the Soviet side. It is essentially to prepare for the arrival of the first man on the moon that will be scheduled the launch frenzy that constantly occur over the following decade. launch. Finally, three years after the first soft landing of a Soviet probe, the first American crew of the Apollo program in 1969 set foot on the Sea of Tranquility

Since then, one is tempted to say that the whole thing summed up in one sentence: we walked on the moon and came back. A total of sixty trips to the moon will take place. Nearly 400 kilograms of moon rocks will have been reported to Earth to be studied there. Only eight trips (all belonging to the Apollo program) have been inhabited, six moon landings, which have allowed twelve humans to set foot on our satellite. Yet even as the Cold War that justified all this no longer appears as a spasm of our history, the first lunar exploration program have fundamentally changed the vision we can have today world. They marked a turning point not only in the history of knowledge, but in human history.

Between the time of Ptolemy and that of Copernicus, a period of over a thousand years, astronomy knows no notable development in Europe. In the Islamic world, by cons, important progress will occur between the ninth and eleventh centuries, both in the mathematical tools of astronomy in observing the sky.

This golden age of Muslim astronomy will begin in the reign of Caliph Al-Rashid and his son al-Mamun, both focusing on promoting scientific and cultural work in their empire. During his reign in Baghdad between 813 and 833, the Caliph Al-Mamun and will establish the largest library from that of Alexandria, the House of Wisdom, and set in 829 the first permanent astronomical observatory in the world.

Al-Khwarizmi

The most notable scholar of the ninth century Persian Al-Khwarizmi. He wrote the first book on algebra, al-Jabr Hisab w'al-muqabala and founded the same time this discipline. He introduced and spread the use of numbers we use today (they are called for Arab although they are actually from India). Its main direct contribution to the astronomy book will Sindhind zij, based on Hindu astronomy, in which he sets the tables on the position of the Sun, Moon and planets, and studying a variety of subjects such as eclipses or visibility of the moon.

Al-Farghani

Around the same time, the Persian Al-Farghani writes Astronomy Elements (Kitab fi al-Harakat al-Samawiya Jawami wa al-Ilm Nujum), a book based on the Ptolemaic astronomy. It also introduces new ideas, for example that the precession must affect the apparent position of the planets, not just the stars. This book will play a significant role in Western Europe when it was translated into Latin in the twelfth century.

Al-Battani

Around the late ninth century, the dominant figure is the Arab astronomer Al-Battani will observe the sky from Syria and make measurements of outstanding accuracy for its time. It will thus determine the length of the solar year, the value of the precession of the equinoxes and the obliquity of the ecliptic. He would also like to establish a catalog of 489 stars.

From a theoretical point of view, his main work, Kitab al-Zij is of fundamental importance as it introduces for the first time trigonometry in the study of the celestial sphere. This new approach will prove more powerful than the geometrical method of Ptolemy. This book will be translated into Latin in the twelfth century and influence many major European figures of the sixteenth and seventeenth centuries.

Al-Khujandi

In 994, the astronomer Al-Khujandi, from the current Tajikistan, built a huge mural sextant to Ray observatory near Tehran, the first instrument for more accurate measurements than the arc minute. He uses it in particular to determine a more precise value of the obliquity of the ecliptic.

Al-Biruni

Around the same time appears another scholar, al-Biruni, from near the Aral Sea. Like its predecessors, it is interested in many subjects such as mathematics and geography. In astronomy, he distinguished himself by his observations of lunar and solar eclipses, but also by a more modern approach to the experimental method, especially when analyzing errors which undermine its actions and those of Al-Khujandi.

Omar Khayyam

In the eleventh century Persian Omar Khayyam, now best known for his poetry, is interested also in various subjects, particularly algebra and astronomy. It creates new astronomical tables, but is especially remarkable in determining the length of the solar year with extreme precision for the time.

This golden age of Islamic astronomy will end in the twelfth century. The works of this prosperous period will gradually be translated into Latin, particularly in Toledo, Spain, and spread in Europe. It will be through these translations that European scholars of the late Middle Ages rediscover the theories of Ptolemy and will take note of progress made in the Muslim world.

From a historical point of view, Greek astronomy was dominated by two characters, Aristotle and Ptolemy, who introduced ideas and incorrect models that would dominate scientific thought for almost two millennia.

Aristote

The key figure is Aristotle, a philosopher of the fourth century BC, who leaned on the ideas of one of his predecessors, Plato. According to him, the world must have a spherical shape and movement of all celestial bodies must be circular and uniform, ie at constant speed.

In the system of Aristotle, as in that of Pythagoras, the Earth was motionless at the center of the world and surrounded by a series of crystalline spheres. The problem of the Pythagorean model was the fact that each planet was associated with a single sphere, which could not explain the irregularities of the apparent motions.

Aristotle overcame this problem by creating a more complex system containing 55 spheres nested into each other. Each planet was then associated with a group of spheres whose movement is superimposed. Combining different rotations allowed to give each planet a complex movement that could be adjusted to match the one that was observed in the sky.

With a combination of 55 spheres, Aristotle came relatively well to reproduce the apparent motions of the planets. His system was nonetheless a major flaw: it was unable to explain the apparent changes in brightness of the planets.

We now know that these changes are due to the change in distance between the Earth and each planet. But in the system of Aristotle planets were at a fixed distance from the Earth and the brightness changes remained unexplained.

Aristotle also introduced a more philosophical concept that came to be accepted until the sixteenth century: the distinction between the Earth and the heavens. For him, the interior of the lunar orbit, which included the Earth and its atmosphere, represented the realm of imperfection and change. Beyond the Moon, was the kingdom of perfection and immutability.

Ptolemy

The main flaw of Aristotle's system was its inability to explain the brightness variations of the planets. For this reason, an Alexandrian astronomer Claudius Ptolemy, changed the system in the second century AD, but without undermining the principles enunciated by Plato and Aristotle.

For Ptolemy, the heavenly bodies were not related to crystalline spheres centered on the Earth. Indeed, each planet was moving on a small circle, called epicycle, the center itself was moving in a great circle centered on the Earth, called deferent.

By adjusting the size and position of all circles involved, Ptolemy got a system that can accurately reproduce the apparent motions of the heavenly bodies. He was more able to explain the variations in brightness of the planets, since they now saw their distance from Earth varies.

This double success explains that the Ptolemaic system, which improved the Aristotelian in form but not the spirit, was accepted until the sixteenth century.

Heracleides

Despite the dominance of Aristotle and Ptolemy, two other Greek philosophers proposed much more realistic systems. At the time of Aristotle, Heraclitus advanced that the Earth was not stationary, but actually turned on itself. The apparent rotation of the sky 24 hours then explained much more natural way. The explanation was good, but it was not accepted.

Later, to explain the specific movements of Mercury and Venus, which seemed to hover around the Sun, Heracleides forward these two planets do not turn around the earth but around the sun. He thus obtained a more realistic description, even if he thought the rest of the heavenly bodies, including the Sun, still revolved around the Earth.

If only the Library of Alexandria had withstood wars and conquests! Unfortunately, our knowledge of astronomy of ancient Egypt are very limited and we just few papyrus and a few inscriptions on tombs or temples.

In Egyptian mythology, the goddess Nut represents the sky, the air Shou and Geb the earth. This image shows part of the Greenfield Papyrus (1025 BC) where we see the body of Nut, held in place by Shou, and that of Geb, the ground.

A solar year of 365 days

At the time of ancient Egypt, The Nile flood occurred every year around July 19. Coincidentally, it is also at this time that the brightest star in the sky, Sirius, called Sothis in Greek and Egyptian Sopdet had its helical rising and thus made his first appearance of the year. As the Nile flood would fertilize the land and feed the people, observing the helical rising of Sirius, and more generally of the night sky, became a key element of the Egyptian civilization.

Based on their time measuring the apparent movement of the Sun, rather than the cycles of the moon, the Egyptians invented the solar calendar. As the helical rising of Sirius occurred approximately every 365 days and nights, they divided the year into 365 days. As the cycle of the moon lasted about 30 days and nights, they divided the year into 12 months of 30 days each month is still divided into three decades of 10 days.

Finally, to reach a total of 365, they added five additional days, called epagomenal days, which became day celebration of the gods Osiris, Seth, Isis, Nephthys and Horus.

As the astronomical year does not last exactly 365 days, the Egyptian calendar gently drifting over the cycle of the sky, about one day every four years. The Nile flood therefore coincided with the official start of the year that every 1,460 years, a length of time we named the Sothic period.

It was not until Julius Caesar established the Julian calendar and leap year in 45 BC that the timeframe be better aligned with the stars.

A 24-hour day

Egyptians also invented division of the day into 24 hours. To better get into the sky and measure the passage of time, they cut the sky in small groups recognizable stars who got up one after the other during the night. To coincide with the decades of 10 days, each group of stars was chosen so that its helical rising is separated from the previous 10 days. So we had 36 groups of stars, which named the decans.

Since the length of the night depends on the season, the number of observable decanates overnight varies. But at the beginning of the summer, at the time of the heliacal rising of Sirius, the night lasts about 8 hours and only 12 decans are observable. This number was taken - in somewhat arbitrary - as the basis of the new system. The principle was extended to the day itself divided into 12 hours. Thus the Egyptians established the 24-hour day that we still use.

Egyptian myths

The Egyptians had a very rich and often associated with celestial phenomena mythology. In some texts, the world was a big rectangular box whose north and south sides were longer. This box was a flat ceiling supported by four pillars. These were connected by a mountain range and a celestial river flowed quietly in a projection along this chain. Boats plying the celestial river and carried the Moon, Sun and planets.

In another interpretation, it was the body of the goddess Nut, deployed on top of the world, arms and legs spread, which formed the heavens. Her uterus engendered every morning the sun is at its mouth and swallowed the sunset to the west. Geb, god of the Earth, was lying under Nout, his wife and sister.

All celestial bodies were usually associated with deities. The Sun representing different gods depending on its position in the sky, Khepri at dawn, noon and Atum Re at night. Moon represented herself as several deities, Aah, Thoth and Khonsu.

The constellation of Orion was of particular importance and evoked Osiris, the first child of Nut and Geb, god of death and renewal. The death of Osiris and his rebirth were powerful symbols of annual drought Egypt always followed the Nile River and fertilization of the land.

UNIVERSE

Halley's Comet

History of comet

Specifications

Saturn and mythology

Moon: History of discoveries