Watching during the day the movement of the sunthe sky, a man from ancient times tried to find an explanation for this. Dependence on the luminaries in the definition of time, in the organization of economic and daily life generated the attitude towards him as a deity.
That it is not the Sun that orbits the Earth, buton the contrary, it became finally clear only by the middle of the XVI century. Moreover, one of the people who expressed this idea (Copernicus) was so sure that it contradicts the obvious - what can be observed daily, that it prefaced its publication about the heliocentric system with the words about its obvious absurdity.
Now and the schoolboy knows that in factthe movement of the solar disk over the sky is the result of the Earth’s rotation around its axis, that one full revolution our planet makes per day, that the position of the sun above the horizon at a particular time depends not only on the angle of rotation to the star. It is influenced by the position occupied by the Earth during its annual circulation around the Sun. In winter, in the northern hemisphere, our star rises at noon lower than in summer.
In order to accurately determine when the positionthe sun is the tallest in the sky, it is important to understand how this position is measured. If the highest point of the center of the solar disk at midday is located exactly above the head (then the pole, stuck into the ground strictly vertically, casts a zero shadow), then the observer is at the equator.
In the Northern Hemisphere, the zenith point is the greatestthe declination angle of the star relative to the horizon during the day, or the position of the sun at noon, shifts northward; in the southern hemisphere it shifts downward southwards during the year. The main reason for the annual change in the height of the sun above the horizon at noon is the inclination of the axis of rotation of the globe to the orbital plane. It is constant and equal to 23.5 °.
This slope explains the change of seasons toour planet. The starting points for an astronomical winter or summer are the solstice points. In the Northern Hemisphere, the summer solstice is June 21 and 22. It is on this day that the sun is above everything else in the sky. The angle of sunlight on the surface of the planet is such that it heats up best. Gradually warming up a large mass of water and land and the air temperature rises. The northern hemisphere at this time turned to the sun.
In the winter of December 21 and 22, north of the equator inNoon, when the position of the sun in the sky is the highest, it rises above the horizon to the minimum height, if we take the change of this indicator during the year. At the same time, our region of the earth's surface is turned away from the main star of the solar system, receiving the least amount of beneficial heat. In the Southern Hemisphere, everything happens exactly the opposite.
Появление таких единиц времени, как год, месяц и The day is associated with astronomical observations and is directly dependent on the interaction between the only habitable planet and the central star. The division into hours, minutes and seconds is arbitrary. The main factor for the emergence of just such numbers is the six-decimal number system, which was traditional for Ancient Babylon - it is from there that the modern units of time are from.
The period that passes between the moments whenThe position of the sun in the sky is the highest — what is called the day — it is not constant throughout the year. The fluctuation of this difference reaches 30 minutes and is explained by the elliptical shape of the orbit along which the Earth moves around the star. If real time were used in practice, one would have to create a clock with an uneven course. Technically, now it is possible, but precisely in our time, when high accuracy of synchronization of events occurring in different parts of the planet is required, such a system seems to be particularly impractical.
In order to level the annual difference inthe duration of astronomical days, astronomers introduced the so-called "average Sun". This purely theoretical concept denotes a point that determines the position of our luminary in the sky, the movement of which during 365 days is uniform. Twice a year, its position coincides with the center of the present Sun. This occurs on the days of the spring and autumn equinox.
The introduction of the "average Sun" led tothe use of more convenient in practice of the average time, otherwise called civil. So, on a day other than the days of the equinox, the twelfth stroke of the clock at noon does not mean the time of day when the position of the sun is highest. To calculate a similar moment with an accuracy of fractions of a second, use the "equation of time" in which the data derived from the geographical coordinates of a particular place are laid.
