Time is divided into years, months, weeks, days, hours, seconds. Historians count down time for centuries, geologists count down time for millions of years. But only three units of time are connected with celestial phenomena, it is a year, a month, a day. The change of day and night is especially important for living beings living on Earth. Already caveman knew that from sunrise to sunset or between the two moments of the sun's standing in the zenith is about the same time, and called it "day".
In ancient times, our ancestors noticed that the Moon does not look the same every night and that it disappears from the sky from time to time. Sometimes it turns into a thin sickle, and then becomes round again, between two such full moons passes about 30 days. This fact has also been known for many millennia and was the basis for the introduction of another important, natural phenomena related unit of time - the month. Very soon people understood that approximately every 365 days the vital phenomena of nature are repeated, such as the melting of snow in the north or the flood of the Nile in Egypt, and that these processes are associated with the regular lowest or highest standing of the Sun. Always the same time - a year - passed from the beginning of one spring to the beginning of another. However, for many millennia, people still had little idea of what was really happening in the sky every year, month or day.
The Earth's orbital speed is almost 30 km per second, more than 100,000 km per hour. Its orbit is 300 million km in diameter. In other words, our "spaceship" Earth travels almost 1 billion km a year. It seems to us that the Sun travels in a circle through 12 constellations in the sky during the year. Specifically, the full circulation of the Earth around the Sun takes 365,2564 average days. This period of time is a sideric, or stellar, year. The time from one beginning of spring to another is set slightly shorter (20 min. 24 sec.) for astronomical reasons, it is called a tropical year, and the calendar must correspond exactly to the tropical year.
There are no visible events in the sky that are repeated every 7 days. At the same time, it can be noted that between the first appearance of the Moon, after the new moon, and the first quarter of the incoming Moon is exactly 7 days. The same applies to the time between the quarter and the full moon. From the full Moon to the last quarter, it is also exactly one week. Another 7 days elapse between the last quarter of the Moon and its complete disappearance (the new moon). Some scientists believe that these phenomena have contributed to the introduction of the concept of a week.
The division of time into years, months and days has arisen, as we have seen from the astronomical observations. However, further division of time into hours, minutes and seconds, on the contrary, is quite arbitrary, and also not very practical, because it does not correspond to our decimal system of measurement. If it is not difficult for us to convert rubles into kopecks, then to convert days into hours and minutes requires some mathematical work.
A day is the period of time between two moments of the highest position of the Sun in the sky. When the Sun occupies the highest point in the southern part of the sky, we say, "True local time is 12 hours. The Earth rotates around the axis evenly. The daily movement of the Sun in the sky would also look uniform if it wasn't for the Earth's one-year orbit around the Sun. This movement of the Earth is uneven, and the axis of orbit does not coincide with the axis of the Earth. As a result, the true solar day differs in duration, and this is inconvenient.
Twenty-four-hour sunny day lasts a little longer than the time in which the Earth has time to turn around its axis. To understand this, imagine that a bright star and the Sun would be both exactly in the south at the same time. The rotation of the Earth comes to an end when the star is back in the south. And the Sun has made little progress in the sky during this time. In other words, the Earth has to turn around a little more until the Sun is exactly in the south. The time between the two points of the highest star position in the south is called the starry day, and a slightly longer period of time between the two maximum points of the sun's position is called the sunny day. The average sunny day, referred to the invented middle sun, for 3 minutes. 56.55 sec. longer than the stellar day. Our time compares with the Sun, which gives the rhythm of our lives as a daytime luminary. Even more fantastic is the fact that our solar system rotates with the Galaxy, the Milky Way system. Just as the Moon revolves around the Earth and the Earth revolves around the Sun, our solar system revolves around the center of the galaxy, which takes 220 million years. This is the longest, most precisely defined time period. By the way, our Sun is so old that it has come this way twenty times already.