David Levi: a Guide to the starry sky
Comets, the “tailed stars”. From time to time in the starry sky appear of the comet, which got its name from the Greek word “kometes” “tailed”. Often, the comet took over some otherworldly forces and beings, in the Christian era, they attributed divine the meaning and was considered God’s messengers – angels. The Slavs we, the Germans existed beliefs – seeing a shooting star to pray, convinced that any desire expressed at the short time, while star rolling, will come true. Comet was brought in to the stars also called tailed stars or sirkali with a broom, the Germans schweifstern, haarstern (hairy star) and pfauenschwanz (peacock tail) ; Lithuanian songs mention the comet, as old men with long beards, whose appearance portends something extraordinary. Thus, on the basis of different similes light strip cast by the nucleus of the comet, the language gives it a tail, a broom or a beard. Rare comets made him unusual for people to connect with these tailed stars the idea of divine predvidenie of future troubles for their sins and the mortal call to repentance. Interesting review, made by Patriarch Nikon in-out, in 1664 at the monastery of the resurrection. Sitting at night in the sleigh, he began to atrasti the dust from your feet, remembering prominent Evangelical words: “the IDA also if you do not accept, basedfrom the hail and the dust stuck to your feet, shake off as a testimony against nya”. Streltsy Colonel, dressed escort of Nikon, said: we are the ashes we’ll sweep! – Let razmyat the Lord God you trade only a divine broom, who is on many days! – objected Nikon, pointing to the burning in the sky the comet. Humboldt in his “Cosmos” points to the poetic view, which in ancient times was connected with the comets: they saw shaggy stars and flaming swords.
Comets have the appearance of a small hazy spot with a light tail, and sometimes even several. Inside misty spot, called the head of the comet, sometimes shows a relatively bright nucleus, like a star. In fact the nucleus of the comet is a big lump of frozen gases, within which solid particles and a variety of sizes, from fine dust to large rocky masses.
Various sizes comets nucleus, head and tail are also different. If the diameters of the nuclei of comets are measured in several kilometers, the size of the head can be from 25 thousand km in weak comets to 2 million kilometers at bright comets, and tails stretch for hundreds of thousands and millions of miles, reaching the very bright comet length 300 million km. Despite the huge size of comets, their mass is negligible, tens of millions and billions of times smaller than the mass of the Earth. and the density of the tail is so insignificant, that through him the weak Shine of the stars.
Comets are observed only near the Sun. Most of the observed comets belong to the Solar system and orbit the Sun in very elongated orbits, and some comets, which passed with great speed at several hundred kilometers per second near the Sun, forever leaving the Solar system into interstellar space.
The motion of comets on the eye is not noticeable, they appear motionless in the sky, but if every day to note their position relative to the stars, using star charts, this movement is easy to detect.
Comets belonging to the Solar system, from time to time (with different periods from 3.3 years to several thousand years) pass near the Sun and are therefore called periodic. Away from the Sun the comet is dimly illuminated by his rays, has no tail and is inaccessible to observations. But as it approaches the Sun, its illumination increases, the frozen gases in the nucleus, heated by sunlight, evaporate and envelop the core of a gas-dust shell, which forms the head of the comet. Under the action of light pressure from the sun’s rays, and particles ejected from the Sun, gas and dust away from the head of the comet, forming its tail, which is always directed away from the Sun and, depending on the nature of its constituent particles may have different shapes, from almost perfectly straight (the tail consists of ionized gas molecules) to strongly curved (tail heavy dust particles). Sometimes some comets are observed small anomalous tail, directed toward the Sun. The closer the Sun approaches the comet, the more it warms up the core and increases the pressure as the total light of the comet by the Sun. Therefore increase the brightness of the comet and its tail. When you remove from the Sun the comet’s brightness weakens, and the tail again decreases to complete disappearance.
The more the comet approaches the Sun, the faster it loses its substance. Therefore, those periodic comets which are moving away from the Sun a relatively close (for example to the orbit of Jupiter or Saturn ) and often return to him (short-period comets) are not bright; they are not visible to the naked eye and are observed only through telescopes, and more often found in Photographs of the sky.
On the contrary, long-period comets, with long periods of revolution around the Sun, near a very bright and visible to the naked eye. One of these comets is called Halley’s comet (named after English scientist, established in 1705 its frequency), returns to the Sun every 75-76 years. Last time it was visible in 1910 and her next appearance is expected in 1985