To measure time intervals, natural units of time associated with astronomical phenomena are used. Time itself is divided into large and small gaps. The latter include seconds, minutes, hours and days: they are associated with the rotation of our Blue planet around its axis. Long periods of time are associated with the rotation of the planet around the sun. The calculus of years is based on a tropical year - this is the time period during which the planet makes a complete revolution around the Sun from its position at the zenith above the tropics until the next such position. One tropical year is 365 days, 5 hours and 48 minutes.
Julius Caesar in 45 BC introduced the Julian calendar, which had 365.25 days, which was eleven minutes more than in the tropical year. However, over time, a large time difference has come up. To eliminate this, in the sixteenth century, Pope Gregory XIII introduced another calendar that takes into account a leap year. It excluded the extra days that came running on the Julian calendar. The Gregorian calendar is used to this day, in it time is divided into years, months, weeks and days.
Time reference points
In order to capture any events in history, people needed a calendar. However, he needed a starting point. In ancient Egypt, a unique calendar was created, counting the days of the year. All events occurring in the world that we date by any day and year, century, era, begin from the same date (event) in the distant past. If this were not the case, then there would be confusion on Earth and people would not understand at what point they should be counted: some could now have 7000 years, and other nations could have the 1000th year. Our era begins with the birth date of Christ. It is from this year that the countdown begins.
Tropical month
There are many concepts of time, and they are all different: there is a local daily time, universal time, a tropical year, a tropical month, and other calculation times.
The tropical month is determined by the moon. By definition, this is the time period during which the longitude of the moon increases by 360 degrees. The tropical month is 29.5 sunny Earth days.
Even in Ancient Babylon, astronomers watched the Moon, its seven-day cycle. They saw all the changes taking place with the satellite of the Earth. So there were weeks that became a real public historical heritage and are still used in the calculation of time (we also have seven days in one week).
Tropical year
A tropical year is used to measure longer periods of time. This is the length of time between two identical intersections of the Sun of the celestial equator when it moves in orbit. The tropical year lasts 365.2 sunny days. However, this indicator is not constant: over a millennium, the duration changes by a couple of seconds.
The speed of orbit is also variable. From March to September, the movement of the planet takes 186 days, and the second part of the orbital path takes 179 days. The repetition of the movement of our planet around the Sun was called the annual movement of the Earth, as a result of which there is a change in seasonality.
Other types of calculus
In addition to the tropical year, other systems have been developed for calculus, such as the earth year, consisting of 365 days, the lunar year - represented by twelve successive repetitions of the phases of the moon.
There is still such a thing as a sidereal year. This is the period of time the body fully rotates around its central body or around the Sun or any other star. Our sidereal year was called tropical.
In astronomy there is the concept of a galactic year, providing for the revolution of the Sun around the center of the Milky Way galaxy. This year takes 223-230 million years.
Earth year
The revolution of our planet around the luminary has several types of years: tropical year, draconic, stellar, anomalistic. Each of them has its own characteristics.
An anomalistic year is the time interval between a pair of successive points through which the sun passes, passing in geocentric orbits. The abnormal year is 365.23 sunny days.
There is still such a thing as a dragon year. It represents the time interval between two successive passage of the luminary through the same section of the lunar orbit. Draconic year is 346.62 sunny days.
A stellar year is a time corresponding to one revolution of the Sun across the horizon relative to fixed stars. This period takes 365.25 sunny days.
On Earth, people are accustomed to seeing a calendar year. It averages 365 days. It is based on the rounded values of the tropical year. Those minutes and hours that have been rounded up are included every four years in a leap year of 366 days. Such calculations help to avoid a strong divergence of time and astronomical observations.
Tropical Year Counts
The tropical year begins its countdown from the chosen point of ecliptic longitude to the end of the full cycle of the seasons and the return of the Sun to this very point. When performing calculations, the spring equinox is usually taken as the reference point. For exact calculations, two planes are taken: the plane of the celestial equator and the plane of the ecliptic. These two lines have a point of intersection. When the Sun passes twice at this point, it is believed that a year has passed.
If another point is taken as the reference point, then the tropical year will be different. This is due to the fact that there are many variations in the angular velocity of the body. In this form, those angular seconds that the star does not pass through the ecliptic during the tropical year will lead to time shifts, and gradually night and day will shift.
The average length of the tropical year
The duration of the solar year depends on the reference point. Scientists did not immediately come to a unified method of counting time, although more and more they took the equinox days as a reference point . This is due to the fact that it was from this reference point that the measurement error was minimal.
The beginning of the tropical year may shift slightly, since planetary disturbances can occur during the passage of the Sun through the horizon.
Throughout the history of observations, the tropical year has changed more than once. It has never been equal to 365 days, since it lasts longer: its duration is always 365 days and five hours, but minutes and seconds are always different.
Tropical Year Options
If the Earth moved in space smoothly, without disturbances, then the tropical year would always be a constant unit. However, this does not happen, and this period of time is always different: it is strongly influenced by disturbances in the orbital motion of the planet and nearby cosmic bodies.
Scientists were able to identify the cyclical occurrence of disturbances equal to six minutes. Periodically they disappear. These indicators are taken into account when determining the annual cycle of the Earth.
Calendar year
As already mentioned, there are many varieties of calculus of time. On Earth, the Gregorian calendar is used for time calculations. He has his own periodicity, equal to four hundred years. In each period, months, days, and dates are repeated. On average, this calendar has 365.25 days, which is largely close to the tropical year.
Ever since they began to use this calendar, the days of the equinox have always remained in their place, which helped in agriculture. Also, this type of calendar simplified the calculations of Easter and other church holidays.
According to scientists, the tropical year will be out of sync with the Gregorian calendar for three days, but this will not happen soon, but after eight thousand years.
So what is a tropical year and what is it equal to? We can say that this is a calculation of the time to which we are accustomed. Our calendar is based on a tropical year, with 365 days and five hours. To synchronize our calendar with a tropical year, once every four years one day is added to it. This is a necessary measure, without which the temporary discrepancies would be large: in ten leap years, the year would shift by as much as forty days. However, this does not happen due to the introduction of a leap year in the calendar.