See Calendar... Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron
Husband. continuation of time, in which the sun, with its imaginary flow, its course, returns to the same point; the time the earth flows around the sun, 12 months, or 52 weeks with one or two days. Tropical, true, solar or astronomical... ... Dahl's Explanatory Dictionary
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This article is related to the calendar. There is also a music group called “Leap Year”. Leap year (lat. bis sextus “second sixth”) year in the Julian and Gregorian calendars, the duration of which is 366 days, one day more ... ... Wikipedia
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- (JD) astronomical method of measuring time, the number of days since noon on January 1, 4713 BC. e. (4712 BC according to the astronomical count of years) according to the Julian calendar. The Julian day was first proposed by the English astronomer John Herschel... ... Wikipedia
ISO 8601 is an international standard issued by ISO (International Organization for Standardization) that describes the date and time format and provides guidelines for its use in an international context. Name of the norm ... ... Wikipedia
CALENDAR- [lat. Kalendarium, from Kalendae in ancient Rome. K. name of the 1st day of the month], a time reckoning system based on the periodic recurrence of certain natural phenomena. Astronomical basis and typology K. appeared in ancient times as ... Orthodox Encyclopedia
Chronology: an auxiliary historical discipline that establishes the dates of historical events and documents; sequence of historical events in time; a list of any events in their time sequence Astronomical ... ... Wikipedia
1. Day as a unit of time
First of all, let us recall that the unit of time in astronomy, as in other sciences, is the second of the international system of SI units - the atomic second. Here is the definition of the second as given by the 13th General Conference of Weights and Measures in 1967:
A second is the duration of 9,192,631,770 periods of radiation from a cesium 133 atom, emitted by it during the transition between two hyperfine levels of the ground state (see the page of the International Bureau of Weights and Measures, some clarifications are also given there).
If the word "day" is used to denote a unit of time, it should be understood as 86400 atomic seconds. In astronomy, larger units of time are also used: the Julian year is 365.25 days exactly, the Julian century is 36525 days exactly. The International Astronomical Union (a public organization of astronomers) in 1976 recommended that astronomers use just such units of time. The main time scale, Time Atomic International (TAI), is based on the readings of many atomic clocks in different countries. Consequently, from a formal point of view, the basis for measuring time has left astronomy. The old units "mean solar second", "sidereal second" should not be used.
2. A day as the period of rotation of the Earth around its axis
Defining this use of the word “day” is somewhat more difficult. There are many reasons for this.
Firstly, the Earth's rotation axis, or, scientifically speaking, its angular velocity vector, does not maintain a constant direction in space. This phenomenon is called precession and nutation. Secondly, the Earth itself does not maintain a constant orientation relative to the vector of its angular velocity. This phenomenon is called pole movement. Therefore, the radius vector (a segment from the center of the Earth to a point on the surface) of an observer on the Earth’s surface will not return after one revolution (and never at all) to its previous direction. Thirdly, the speed of rotation of the Earth, i.e. The absolute value of the angular velocity vector also does not remain constant. So, strictly speaking, there is no specific period of rotation of the Earth. But with a certain degree of accuracy, a few milliseconds, we can talk about the period of rotation of the Earth around its axis.
In addition, we must indicate the direction relative to which we will count the Earth's revolutions. There are currently three such directions in astronomy. This is the direction to the vernal equinox, to the Sun and the celestial ephemeris.
The period of rotation of the Earth relative to the vernal equinox is called the sidereal day. It is equal to 23h 56m 04.0905308s. Please note that the sidereal day is a period relative to the spring point, not the stars.
The vernal equinox point itself undergoes a complex movement on the celestial sphere, so this number should be understood as an average value. Instead of this point, the International Astronomical Union proposed using the "celestial ephemeris origin". We will not give its definition (it is quite complicated). It was chosen so that the period of rotation of the Earth relative to it was close to the period relative to the inertial reference frame, i.e. relative to stars, or more precisely, extragalactic objects. The angle of rotation of the Earth relative to this direction is called the sidereal angle. It is equal to 23h 56m 04.0989036s, slightly more than a sidereal day by the amount by which the spring point shifts in the sky due to precession per day.
Finally, consider the rotation of the Earth relative to the Sun. This is the most difficult case, since the Sun moves in the sky not along the equator, but along the ecliptic, and, moreover, unevenly. But these sunny days are obviously the most important for people. Historically, the atomic second was adjusted to the period of rotation of the Earth relative to the Sun, with averaging done around the 19th century. This period is equal to 86,400 units of time, which were called mean solar seconds. The adjustment occurred in two steps: first, “ephemeris time” and “ephemeris second” were introduced, and then the atomic second was set equal to the ephemeris second. Thus, the atomic second still “comes from the Sun,” but atomic clocks are a million times more accurate than “earthly clocks.”
The rotation period of the Earth does not remain constant. There are many reasons for this. These include seasonal changes in the distribution of temperature and air pressure around the globe, internal processes, and external influences. There are secular slowdowns, decadal (over decades) unevenness, seasonal and sudden. In Fig. 1 and 2 show graphs showing the change in the length of the day in 1700-2000. and in 2000-2006. In Fig. 1 there is a tendency for the day to increase, and in Fig. 2 - seasonal unevenness. Graphs based on materials from the International Earth Rotation and Reference Systems Service (IERS).
Is it possible to return the basis of time measurement to astronomy and is it worth doing? This possibility exists. These are pulsars whose rotation periods are preserved with great accuracy. In addition, many of them are known. It is possible that over long periods of time, for example, decades, observations of pulsars will serve to clarify atomic time and a “pulsar time” scale will be created.
The study of the uneven rotation of the Earth is very important for practice and interesting from a scientific point of view. For example, satellite navigation is impossible without knowledge of the Earth's rotation. And its features carry information about the internal structure of the Earth. This complex problem awaits its researchers.
People began to use astronomical phenomena to measure time very early on. Much later, they realized that the basic units of such measurement cannot be established arbitrarily, since they depend on certain astronomical patterns.
One of the first units of time measurement, naturally, was the day, i.e., the time during which the Sun, having appeared in the sky, “goes around” the Earth and reappears at its original point. Dividing the day into two parts - day and night - made it easier to fix this period of time. Different peoples associated the time of day with the change of day and night. The Russian word “day” comes from the ancient “sutikat”, i.e. to connect two parts into a whole, in this case to connect night and day, light and darkness. In ancient times, the beginning of the day was often considered to be sunrise (cult of the Sun), among Muslims it was sunset (cult of the Moon); in our time, the most common boundary between days is midnight, i.e., the time conventionally corresponding to the lower culmination of the Sun in a given territory.
The rotation of the Earth around its axis occurs evenly, but a number of reasons make it difficult to choose a criterion for accurately determining the day. Therefore, there are concepts: sidereal day, true solar and average solar days.
The sidereal day is determined by the time interval between two successive upper culminations of one star. Their value serves as a standard for measuring the so-called sidereal time; there are, respectively, derivatives of the sidereal day (hours, minutes, seconds) and special sidereal clocks, without which not a single observatory in the world can do. Astronomy needs to take sidereal time into account.
The usual routine of life is closely connected with other solar days, with solar time. A solar day is measured by the length of time between successive upper culminations of the Sun. The duration of a solar day exceeds a sidereal day by an average of 4 minutes. In addition, the solar day, due to the unevenness of the Earth's motion in its elliptical orbit around the Sun, has a variable value. It is inconvenient to use them at home. Therefore, the abstract average solar day, determined by the calculated uniform movement of an imaginary point (the “average Sun”) along the celestial equator around the Earth with the average speed of movement of the true Sun along the ecliptic, is taken as the standard.
The time interval between two successive culminations of such an “average Sun” is called the average solar day.
All clocks in everyday life are adjusted to mean time, and mean time is the basis of modern calendars. Mean solar time, measured from midnight, is called civil time.
As a result of the inclination of the ecliptic relative to the plane of the celestial equator and the inclination of the Earth's rotation axis relative to the plane of the Earth's orbit, the length of day and night changes throughout the year. Only during the spring and autumn equinoxes throughout the globe is day equal to night. The rest of the time, the height of the solar climax changes daily, reaching a maximum for the northern hemisphere during the summer solstice and a minimum during the winter solstice.
The average solar day, like the sidereal day, is divided into 24 hours, each of which has 60 minutes, each of which has 60 seconds.
A more fractional division of the day first appeared in Ancient Babylon and is based on the sexagesimal counting systemVolodomonov N. Calendar: past, present, future. Page 88.
Since a day is a relatively short period of time, larger units of its measurement were gradually developed. At first, counting was done using fingers. As a result of this, such units of time as ten days (decades) and twenty days appeared. Later, an account based on astronomical phenomena was established. The unit of measurement of time was the interval between two identical phases of the Moon. Since it was easiest to notice the appearance of a narrow crescent moon after moonless nights, this moment was considered to be the beginning of a new month. The Greeks called it neomenia, that is, the new moon. The day during which the first setting of the young Moon was observed was considered the beginning of the calendar month among peoples who count according to the lunar calendar. For chronological calculations, the time interval separating the true new moon from neomenia is important. On average it is 36 hours.
The average length of a synodic month is 29 days, 12 hours, 44 minutes and 3 seconds. In the practice of constructing calendars, a duration of 29.5 days was used, and the accumulated difference was eliminated by the special introduction of additional days.
The months of the solar calendar are not related to the phases of the Moon, so their duration was arbitrary (from 22 to 40 days), but on average it was close (30-31 days) to the duration of the synodic month. This circumstance to some extent contributed to maintaining the count of the day for weeks. The seven-day period of time (week) arose not only because of the veneration of the seven gods, corresponding to the seven wandering celestial bodies, but also because seven days constituted approximately a quarter of the lunar month.
The number of months in a year accepted in most calendars (twelve) is associated with the twelve zodiacal constellations of the ecliptic. The names of the months often show their connection with certain seasons of the year, with larger units of time - seasons.
The third basic unit of time (the year) was less noticeable, especially in lands closer to the equator where there was little difference between the seasons. The size of the solar year, i.e. the period of time during which the Earth makes a revolution around the Sun, was calculated with sufficient accuracy in Ancient Egypt, where seasonal changes in nature were of exceptional importance in the economic life of the country. "The need to calculate the rise and fall of the Nile created Egyptian astronomy."
Gradually, the value of the so-called tropical year was determined, i.e., the time interval between two successive passages of the center of the Sun through the vernal equinox. For modern calculations, the length of the year is 365 days, 5 hours, 48 minutes and 46 seconds.
In some calendars, years are counted according to lunar years, associated with a certain number of lunar months and not related to the tropical year.
In modern practice, the division of the year not only into months, but also into half-years (6 months) and quarters (3 months) is widely used.
Tropical year(also known as solar year) in a general sense is the period of time during which the Sun completes one cycle of changing seasons, as seen from the Earth, for example, the time from one spring equinox to the next, or from one day of the summer solstice to the next. Since antiquity, astronomers have gradually refined the definition of a tropical year and currently define it as the time required for the Sun's mean tropical longitude (longitudinal position along the ecliptic relative to the position at the vernal equinox) to increase by 360 degrees (that is, for one complete seasonal cycle).
Length of the tropical year
By definition, a tropical year is the time required for the Sun, having begun its movement from a chosen ecliptic longitude, to complete one complete cycle of seasons and return to the same ecliptic longitude. Before considering the example, the concept of equinox should be clarified. When performing calculations in the solar system, two important planes are used: the ecliptic plane (the Earth's orbit around the Sun), and the celestial equator plane (the projection of the Earth's equator in space). These planes have an intersection line. Direction along this line of intersection from the Earth towards the constellation Pisces is the March equinox, which is indicated by the symbol ♈ (the symbol is similar to the horns of a ram and is a symbol of the constellation Aries, where the equinox point was located in the distant past). Opposite direction along a line towards the constellation Virgo is the September equinox and is represented by the symbol ♎ (again, the symbol refers to the constellation Libra, which had its equinox point in ancient times). Due to the precession and nutation of the Earth's axis, these directions change compared to the direction to distant stars and galaxies, the directions to which do not have a noticeable shift due to the great distance to these objects (see International Celestial Reference System).
The ecliptic longitude of the Sun is the angle between ♈ and the Sun, measured in an easterly direction along the ecliptic. Its measurement is fraught with certain difficulties, since the Sun moves, and the direction relative to which the angle is measured also moves. For such a measurement it is convenient to have a fixed (relative to distant stars) direction. The direction ♈ at noon on January 1, 2000 was chosen as such a direction; it is denoted by the symbol ♈ 0.
Using this definition, the vernal equinox was recorded on March 20, 2009 at 11:44:43.6. The next equinox was March 20, 2010 at 17:33:18.1, giving a tropical year of 365 days, 5 hours, 48 minutes, 34.5 seconds. The sun and ♈ are moving in opposite directions. When the Sun and ♈ met at the equinox in March 2010, the Sun moved east 359° 59" 09", and ♈ moved west 51", for a total of 360° (all relative to ♈ 0).
If we choose a different ecliptic longitude of the Sun as a reference point, the length of the tropical year will already be different. This is due to the fact that, although the change in ♈ occurs at an almost constant rate, there are significant variations in the angular velocity of the Sun. Thus, the 50 arcseconds or so that the Sun does not travel across the ecliptic in a full tropical year "store" different amounts of time depending on its orbital position.
Average length of the tropical year according to the spring equinox
As mentioned above, the length of the tropical year depends on the choice of reference point. Astronomers did not immediately come to a unified method, but most often they chose one of the equinoxes as a starting point, because the error during these periods is minimal. When comparing measurements of the tropical year over several consecutive years, differences were found associated with nutation and planetary disturbances acting on the Sun. Mees and Savoy give the following examples of intervals between the spring equinoxes:
| Days | Watch | Min. | Sec. | |
|---|---|---|---|---|
| 1985-1986 | 365 | 5 | 48 | 58 |
| 1986-1987 | 365 | 5 | 49 | 15 |
| 1987-1988 | 365 | 5 | 46 | 38 |
| 1988-1989 | 365 | 5 | 49 | 42 |
| 1989-1990 | 365 | 5 | 51 | 06 |
Until the beginning of the 19th century, the length of the tropical year was determined by comparing the dates of the equinoxes over a long period of time. This approach made it possible to calculate the average length of the tropical year.
A comparison of the average time intervals between the equinoxes and solstices for astronomical year 0 (1 year BC according to the traditional account) and 2000 is presented in the table:
Current value of the average length of the tropical year
The average length of the tropical year since January 1, 2000 is 365.2421897 days or 365 days 5 hours 48 minutes 45.19 seconds. This value changes quite slowly. An expression suitable for calculating the length of a tropical year in the distant past:
365.242 189 669 8 − 6.153 59 ⋅ 10 − 6 ⋅ T − 7 , 29 ⋅ 10 − 10 ⋅ T 2 + 2.64 ⋅ 10 − 10 ⋅ T 3 (\displaystyle 365(,)242\ 189\ 669\ ( ,)153\ 59\cdot 10^(-6)\cdot T-7(,)29\cdot 10^(-10)\cdot T^(2)+2.64\cdot 10^(-10)\cdot T ^(3))Where T- time in Julian centuries (1 Julian century is exactly 36,525 days), measured from noon on January 1, 2000
Variations in the length of the tropical year
With an undisturbed (Keplerian) motion of the Earth, the duration of the tropical year would be constant in time. However, the actual orbital motion of the Earth is perturbed. A consequence of the disturbed motion of the Earth is interannual variations in the length of the tropical year. As studies show, these variations are periodic, as they are associated with periodic disturbances of the Earth’s orbital motion by nearby celestial bodies. The main period in the variations is a three-year cycle with an average amplitude of 0.006659 days (9 minutes 35 seconds). This cycle, as a rule, alternates every 8 or 11 years with a two-year cycle, the average amplitude of which is 0.004676 days (6 minutes 44 seconds). The two- and three-year periodicity is explained by the commensurability in the orbital motion of the Earth and the nearest planets - Mars (orbital resonance 2:1) and Venus (3:5). In their alternation, two- and three-year cycles form series lasting 8 (2+3+3) and 11 (2+3+3+3) years, which correspond to the phases of the 19-year nutation cycle.
I am sure that absolutely each of us has periodically and repeatedly drawn our attention to how quickly time flies. I’ve been thinking and realized that just recently I thought that it was soon the end of summer and we needed to close the beach season, but now I’m writing this message to you and I understand that it’s already mid-autumn. I see time moving year after year. But what is a year? Now I want to write my thoughts on this matter.
Year - how much is it
If we talk about what a year is, then we need to remember - this is the time that our planet needs to make one revolution around the Sun. Typically it takes 365 days, but once every four years there is a leap year. In short, its difference lies in its duration - 366 days. People whose birthday falls on February 29 are very unlucky, because their holiday is celebrated only once in four years.
The Importance of Time Tracking
I very often notice the habit of wasting a huge amount of free time. What does this have to do with? Perhaps out of sheer laziness, social networks have also become an integral part of our daily schedule. Some things put off until later, due to untimeliness, take much more effort and, of course, time. I, like you, am not immortal and I have one life, and time, as we have already discussed, flies very quickly. Isn’t this the best reason to figure out together how to spend it as efficiently as possible? Perhaps I will write a few, in my opinion, main points on this matter.
- Limit wasting time on social networks.
- Keep a diary in which you detail your plans and tasks for each day.
- Learn time management techniques.
- Try to communicate less with people who waste a huge amount of your time.
Take your time seriously
What is a year? Is this a lot or a little? In my opinion, it all depends on how rationally you spend the time of your life. One year is enough for one person to learn a new profession, find a hobby and make many new friends, while another will waste it lying on the couch watching TV. I hope you make the right choice and live your life well. The world is full of everything beautiful and interesting, so let's not pass by it all!
