Gregorian calendar

First, the Julian calendar had no complaints, but gradually found that year lasts for 11 minutes less, than counted. This error was corrected in 1582 by Pope Gregory XIII, who decreed the day 5 October 15 Oct. Otherwise the calendar date more and more would not coincide with certain seasons. To exclude such error in the future, he also gave the order century years (at the turn of the century) to be leap-year only when they are divided by 400 without a remainder. This Gregorian calendar is still used in Western countries.
Night and day was the shortest periods of time known to our ancestors. A more accurate calculation of time appeared about 4000 p. n. e., when ancient Egyptians divided the time of day on the clock. The first timepiece was the sundial, consisting mainly of the column which is stuck into the ground so that it cast a shadow on the dial with hour divisions. When the sun moved across the sky, the shadow of the column moved on a scale and showed time.

Time

The rotation of the Earth and moon determines the duration of years, seasons, months, and days. The cycle of rotation of the Earth around the Sun is one year. During this time, the stars are replaced by other in the night sky, and in weather there are seasonal changes.
Once was discovered this sequence of events, people learned to extract from their observations of practical use. For example, the appearance of a certain star meant that the time has come for Seva. The ancient Egyptians knew; the annual flooding of the Nile always began soon after the appearance on the horizon of a star Sirius.

The Phases Of The Moon

For the month characterized by the mode of change in the visible part of the moon during its passage along the orbit around the Earth. These different stages are called phases. Within a month the light of the stars falls on the surface of the moon at different angles. When the Sun lights a Month back, it's actually not visible from Earth, this phase is called the new month (the new moon). A half - cycle of the observed full month (or full moon) as seen from the Earth a portion of its surface facing the Sun. The basis of the Roman calendar was based on lunar month the time during which the Moon goes through a complete cycle of changes.
The length of a day is determined by Earth's rotation around its axis, and it seems to us that the Sun rises and sets. The difficulty in compiling the calendar is that the year is not a whole number of days, but about 365.25 days. 45 G. B. E. Julius Caesar solved this problem and introduced a calendar in which after three years of 365 days each was a 366-day leap year.

Hourglass

The main drawback of the solar hours was the fact that it was impossible to use at night and on cloudy days. This problem was eventually solved around 1500 to n. e, when the Egyptians invented the clepsydra, or water clock. The simplest form of such a clock consisted of a vessel with a small hole at the bottom. Filled in a vessel of water slowly flowed through a hole and the water level relative to the marked scale indicates the time that has passed.
The Sunny hours came the sand in which the sand is poured through the narrowing part of the glass vessel, showing after a certain period of time, often a half hour or an hour. Today the miniature hourglass is used in the home while cooking eggs.
To time also used a candle. Notches on wax showed how long hours from time of inflammation. Candles were widely used for timekeeping, but was not an exact instrument, because different wicks, wax varieties and thrust affect the burning rate.

Chronometer Harrison

The problem was so serious that in 1714 the British government established the award in 20 thousand pounds to anyone who can offer an accurate method of determining longitude during navigation. The accuracy should be so high that, if we were talking about the clock, its daily error was not to exceed three seconds for six weeks. The award went to the carpenter John Harrison who designed several timepieces - accurate clocks at sea. In 17б0 in the competition won its fourth model. During one voyage the accuracy of his watch was only 54 seconds for 156 days. Harrison found a way in which temperature fluctuations change the effective length of the terminal curve of the spring, providing automatic compensation for previous errors.

Missiles

Rocket racketi was originally used as a weapon. Today, these powerful, giant machines used for human flight in outer space and the orbit of artificial satellites and various equipment. However, missiles with warheads continue to pose a threat to life on Earth.
The first missile was launched about 800 years ago. At the beginning of the XIII century it was used by the Chinese against the Mongols. As in modern fireworks, the driving force of the Chinese missiles was the powder charge. Attached to the spears or arrows missiles was a terrible weapon. The Mongols were shocked, but recovered and set up their own rocket for the war with the Arabs. Until the middle of XIII century of a rocket were and Arabs. French crusaders brought them to Europe.

Missiles in Europe

In 1429 French troops under the command of Jeanne d'arc with the help of rockets to defend Orleans in the battle against the British. Subsequently, the missiles were replaced by more accurate weapons - guns.
Starting from the XVI century rockets were used in celebratory fireworks, first in Italy and then in other European countries. And only in the late eighteenth century, they were again used in combat. In 1792, British troops in India came under fire small metal missiles, their effectiveness was so great that Colonel Congreve decided to create a missile weapon for British troops. In 1804 he made a simple rocket to extremely destructive weapons with explosive or incendiary warhead. But the accuracy of this weapon remained low until about 1844, when the Englishman William Hale invented stabilization method: the curved vanes in the nozzle forced the missile to spin during flight, which gave her stability.

Rocket fuel

In most rockets using solid or liquid fuel is burned in a closed space, and the resulting gases released through one or more nozzles of relatively small diameter. The necessary combustion oxygen can be obtained from chemical compounds, for example potassium nitrate. In modern rocket engines, liquid oxygen is commonly used for burning such fuels as kerosene, liquid hydrogen or hydrazine (nitrogen-hydrogen compound).
Rocket engine solid fuel (SRM) are widely used due to its simplicity and reliability. They are installed on most military missiles are some boosters of space vehicles and sometimes as the engine parts of multi-stage rockets. However, for complex space missions give preference to LRE because they create thrust is easily adjustable. In addition, for the same weight of fuel a rocket engine provide more thrust and acceleration than solid propellant motors.

Artificial satellites

These missiles gave the Soviet scientists to display small objects in Earth's orbit. In October 1957, the audible signals from a tiny artificial satellite launched by USSR "Sputnik-1" has announced the beginning of the space age.
After four months in the United States under the leadership of von Braun was made the corresponding run. The influence of the ideas of this scientist continued until the introduction of the program "Apollo" which involved a giant three-stage rocket "Saturn-5", to deliver American astronauts to the moon in 1969
According to the third Newton's law (the law of action and reaction) to every action corresponds an equal and opposite reaction. This means that if you jump from a small boat on shore, the energy of your jump pushes the boat from the shore. Rockets use the same principle. They move due to the ejection flow of the material (usually gas). The effect of the energy causes a reaction gas relative to the rocket and causes it to fly. Unlike jet engines, which need air for burning fuel in rockets has everything you need for movement is a self - contained apparatus able to move in space.

Motors at the molecular particles

Other proposed variants include the fusion of hydrogen in the plasma gas stream of charged particles. Using a magnetic field, the plasma is displaced from the engine and creates thrust. Another idea is to use the electric field for emission from the engine of ions (charged atoms) of mercury or cesium. The tests confirmed the efficiency of this system, although created with this pull incredibly had only 1 kg for every 4 million watts of electricity consumed. However, with the gradual acceleration over many months, such a missile in the end can develop huge speed.
Previously considered another possibility, which came to mind science fiction writers, photon engine produces thrust by radiation of the flux of light quanta. However, just a focused beam of light produced by the photon thrust will not be able to compete even with minimal thrust ion engine.