Exact time and determination of geographic longitude. How to determine local time How to determine time by coordinates

25.08.2023 | Nature

Conversations on the topic of summer, winter, zone, local, astronomical time prompted me to write this post. I'll tell you and show you how to calculate local time. You will learn what standard time is. There will be a little theory and history here. And there will be no medicine and physiology, and I will also leave aside political and economic views on this issue. I am not a doctor, not an economist, and certainly not a politician, I am a navigator.
Therefore, one of the sciences necessary for navigation even in our time - nautical astronomy - is well known to me.
Nautical astronomy allows you to control the position of a ship using observations of celestial bodies. The celestial bodies are constantly moving, and in order to determine their place at a given moment, it is necessary to know the laws of their movement, studied in astronomy. The same applies to artificial satellites Earth. Nautical astronomy, in addition, provides information about the service of time and various phenomena occurring on Earth (sunrises/sunsets, illumination, eclipses, etc.) and in the Universe.
The main problems solved by nautical astronomy are:
- determination of a place in the sea based on observations of luminaries;
- determination of corrections for directional instruments (compasses);
- providing time service.
Auxiliary tasks:
- determination of illumination;
- culmination of luminaries, etc.

Now we come to the topic of time. It is clear that humanity has accepted one day as a unit of time - during this time the Earth makes one revolution around its axis, or the period of time from sunrise to sunrise. Then this period of time was divided by 24 and we got 1 hour. Since one revolution is 360 degrees, we get that 1 hour is 15 degrees of rotation of the Earth around its axis (apparent movement of the Sun), and one degree of movement of the Sun (rotation of the Earth) is 4 minutes of time accepted by earthlings.
And let’s not complicate the task with the knowledge that:
- the Earth's rotation is slowing down by 0.00023 seconds per century;
- random abrupt changes in the speed of rotation of the Earth (several of them were noted, one in 1920 at 0.000000045 s);
- the value of true solar days during the year varies on average by 59.14 s;
- a year is not 365 days, but 365.2422 days.

Therefore, to discuss the topic of sleep-rest-work and changing clocks, we will not talk about true solar and sidereal times (astronomical). We will only operate with the average solar time accepted for calculating time on Earth. When the length of a day is calculated not from one day, but on average over 1 year.
Local time - time of the observer on a given meridian, and since There are countless meridians, and there are countless local times. But all observers on the same meridian have the same local time.
Greenwich time is the local time of an observer standing on the Greenwich meridian.
Because The Greenwich meridian is taken as the origin of longitude on the globe, then local time differs from Greenwich exactly by the longitude of the place, converted from an angular measure to a time one at the rate of 360 degrees = 24 hours.
There is a saying:
"Longitude west, Greenwich time best.
Longitude east, Greenwich time least".
Which, loosely translated, means that with eastern longitude you have more time than Greenwich, and with western longitude you have less time than Greenwich.

Now we know that Greenwich time is taken as the time reference point, but local time is inconvenient for Everyday life.
Therefore, a single time was adopted for an entire region or country (local observatory, ruler’s palace, etc.). But...
With this approach, inconveniences arise - the difference with the time of another region or country could contain fractional parts of hours and even minutes. The development of civilization and communication between peoples required streamlining the calculation of time.

At the astronomical congress of 1884, a system of standard times was proposed and gradually adopted in almost all countries of the world. In the zone time system, time is kept on 24 central meridians of the Earth, separated from each other by 15 degrees of longitude, so that in neighboring zones the time differs by 1 hour. Standard time extends to 7.5 degrees of longitude on either side of the central meridians.
Standard time Zone time is the average local time of the central meridian of a given time zone, accepted throughout the zone.

But there is a small trick - on the 12th central meridian the longitude is 180 degrees, and one part of its belt is in the eastern hemisphere, and the second is in the western. The time on the clocks of residents of this zone is the same, but the number on the calendar for residents of the eastern and western hemispheres is different and differs by 1 day. Residents of the Western Hemisphere have only yesterday, compared to us - residents of the eastern part of the Earth. And this meridian is called the date line.

With such a structure of zones, standard time cannot differ from local time by more than 30 minutes. However, the theoretical boundaries of the belts are observed only in the seas, oceans and sparsely populated areas. In fact, the boundaries of the belts are determined by the governments of countries, taking into account administrative, geographical and economic characteristics.

Maternity time. In order to save electricity on evening lighting, clocks in the USSR were set 1 hour ahead of standard time. Initially, this time was introduced only in the summer (summer time), but by decree in 1931 it was left permanent. That is, maternity time is standard time 1 hour.

Summer time. In a number of countries, moving clocks forward by 1 hour (sometimes by 2 hours) is carried out only for the summer. In the USSR, summer time (1 hour to maternity time or 2 hours to standard time) was introduced in 1980. I won’t describe the further game with time and time zones in our country - everyone knows.
Today our country is divided into time zones like this.

We convert the resulting longitude (in order not to waste time on trifles, we take only an integer number of degrees) into hours and minutes at the rate of 15 degrees - 1 hour, 1 degree - 4 minutes. We make a small assumption that the Sun passes its upper culmination (noon) at about 12.00 local time (in fact - 12.00 plus or minus about 15 minutes).
Now from 12 hours 00 minutes we subtract (for the eastern hemisphere, and for the western hemisphere we add) the resulting longitude value in hours and minutes. We get the Greenwich time of noon at a given meridian (longitude). Next, we add the difference on your watch with Greenwich (UTC, universal) time for the eastern hemisphere (and subtract it if we count the western hemisphere).
Ask: “Where can I find Greenwich Time?” I’ll answer - this is today’s Moscow minus 4 hours (today is February 3, 2013, otherwise we don’t know what will happen with the time tomorrow).

Example: eastern longitude 33 degrees, Moscow time, i.e. Greenwich 4 hours

Convert longitude to hours:
- 33/15=2.2 means 2 hours
- 2,2-2=0,2
- 0.2*60=12 means 12 minutes
- longitude 33 degrees expressed in hours - 2 hours 12 m.

Determining the time of our local noon GMT:
12h00m - 2h12m = 9h48m

Add the clock difference (what is on your hand or next to it) with Greenwich:
9h48m + 4h = 13h 48m.
This is the time of noon according to our watch (which is on your hand or nearby) in a place with a longitude of 33 degrees of the eastern hemisphere (remember - with an accuracy of 30 minutes, because the Sun is not always at its highest culmination at 12.00). For accurate calculations, navigators use astronomical tables.

Now it's standard time. You need to convert the longitude of the place into hours and round to the nearest hour.
For example: longitude 142.9 degrees East.
142,9/15=9,526
This means the 10th Eastern time zone. Those. 10 hours more than Greenwich.

A few words about sunrise. On the Equator, the Sun always rises around 6 am local time and sets at around 6 pm. Further north or south, the time of sunrise and sunset depends on the latitude of the place and the time of year. But on the days of the spring and autumn equinox at all latitudes, the Sun rises and sets, as on the Equator - around 6 and 18 hours local time.
Using the example of St. Petersburg and all places at latitude 60 degrees north. Local time:
March 20 sunrise at 6 a.m. sunset at 6:15 p.m.
June 21 sunrise at 2:36 am sunset at 21:28
September 22 sunrise at 5:45 am sunset at 6:00 pm
December 21, sunrise at 8:50 am, sunset at 14:55

Used: “Nautical Astronomy” by B.I. Krasavtsev (Moscow “Transport” 1986), MAE 2012.

This dependence allows the longitude of a place to be expressed in time and, conversely, time to be expressed in angular quantities, which is necessary when solving problems related to calculating time.

Taking into account that the Earth makes a full rotation of 360° in 24 hours, we can establish the following relationship between longitude and time:

15° = 1 hour; 1° = 4 min; 15" = 1 min; 1¢ = 4 s; 15" = 1 s; 1" = 1/15 s.

Example. GMT Tgr= 4 hours 20 minutes; longitude of the point lв= 90°.

Solution: 1. Convert the longitude of the point into time units: lt= 90: 15 = 6 hours.

2. Determine local time: Tm = Tgr + lв = 4 hours 20 minutes + 6 hours = 10 hours 20 minutes.

Standard time(T n) - local mean solar time of the middle meridian of a given time zone.

In 1884, by international agreement, the standard time system was introduced. The essence of standard time is that the entire surface of the Earth is divided into 24 time zones, from zero to 23 inclusive. Each belt occupies 15° of longitude.

Greenwich is taken as the average meridian of the zero zone, from which longitudes are measured. The middle meridians of neighboring zones are separated by 15°, which corresponds to 1 hour of time. The belts are counted towards the east. Each time zone has a single time for the entire time zone, which corresponds to the local mean solar time of the middle meridian this belt.

The number of a time zone is equal to the longitude of its mean meridian, expressed in time, and shows how many hours the time of a given zone is ahead of Greenwich Mean Time. On all the middle meridians of the zones, standard time coincides with local time, and on the boundaries of the zones, standard time and Greenwich time differ by 30 minutes. Standard time can be calculated using the following formula:

Тп = Tgr+N, where N is the time zone number.

The boundaries of time zones are drawn taking into account state and administrative boundaries in such a way that the population individual country, edges or regions maintained a single reckoning of time.

To determine the time zone of a particular locality, use time zone map , which is available in the Aviation Astronomical Yearbook (AAE) for the entire globe.

To determine what time zone a given location is in, you need to find it on a time zone map. If this point is not on the map, it is marked on the map according to its geographical coordinates, then by its position it is determined in which time zone it is located.

GMT (Universal Time) (Tgr) - mean solar time at the Greenwich meridian, starting at midnight.

Greenwich Time is based on the rotation of the Earth around its axis.

The time of the Earth's rotation can be determined using astronomical observations or calculated from sidereal time. However, Greenwich Time, determined from astronomical observations over time, will not correspond to the value of Universal Time, which is calculated from sidereal time. For this reason, the International Time Bureau (ITI) introduced a new term as the international time standard

Coordinated Universal Time (UTC)- atomic time, adjusted to be as close as possible to the mean solar time of the Greenwich meridian.

Atomic time is uniform, the beginning of its counting is combined with the universal time scale. According to the recommendations of the BIE, atomic time is adjusted so that the discrepancy between UTC and mean solar Greenwich time does not exceed 0.5 s.

International means of transport and communications, including civil aviation, coordinate their work according to UTC.

In practice, solving problems related to time calculation, it is necessary to determine Greenwich time using the local time of a given point and vice versa:

Tgr= Tm ± l ,

Where Tm _ local time; l - longitude of the point, eastern or western.

Example. Tm = 10 hours 20 minutes; longitude of the point l V= 90°. Determine Greenwich Time.

Solution: 1. Convert the longitude of the point to time: l t = 6 hours

2. Determine the time: Tgr = Tm - l V= 10 hours 20 minutes - 6 hours = 4 hours 20 minutes.

Maternity time (TD)- time zone time, changed relative to standard time by decision of the authorized body of the state:

T d = T n ± n h ac.

Maternity time is used for the purpose of more complete use of daylight by the population for reasons of saving electricity used for lighting enterprises and residential premises.

Summer time(T l) – changed, by decision of the plenipotentiary government agency maternity time, according to which every year, during the summer period, the clock hands move forward, and with the beginning of the winter period - back.

Summer time is used in many countries, for example, in England, France, and the USA.

Moscow time(T Moscow time) - Moscow maternity time, or standard time of the third time zone.

Hence, Moscow time During maternity time, it is 3 hours ahead of Greenwich Time.

In practice, it is necessary to determine standard and maternity time at a given point using Moscow time:

Dependence between times.

The transition from one time measurement system to another is performed using the formulas:

T m = T gr ± l; T d = T n + n h ;

T gr = T m ± l; T n = T d – n h ;

T gr = T n - N;

T n = T gr + N

T m = T n – N ± l; T gr = T Moscow time – 3h;

T n = T m ± l +N;

where N is the number of the time zone in which the location is located.

Exact time and determination of geographic longitude.

The sun always illuminates only half of the globe: on one hemisphere it is day, and on the other at this time it is night, respectively, there are always points where it is noon at the moment, and the Sun is at its highest culmination. As the Earth rotates around its axis, noon occurs in those places that lie to the west. The position of the Sun (or stars) in the sky determines local time for any point on the globe. The local time at two points (T1 and T2) differs exactly as much as their geographic longitude differs:

T1-T2 = L1- L2.

It is clear that noon occurs at a given point on the Earth later than at another, exactly as much time as it takes for the planet to rotate through the angle corresponding to the difference in their longitudes. So, for example, in St. Petersburg, which is located 8°45" west of Moscow, noon occurs 35 minutes later. By determining from observations the local time at a given point and comparing it with the local time of another, whose geographic longitude is known, you can calculate the geographic longitude of the observation point. It was agreed to count longitude from the prime (zero) meridian passing through the Greenwich Observatory. The local time of this meridian is called universal time (UT).

in other words, the local time of any point is equal to the universal time at that moment plus the longitude of this point from the prime meridian, expressed in hourly units. Accurate calculation of time is complicated by the fact that its previous standard - the Earth's rotation period - turned out to be not entirely reliable. The solar day has long been chosen as one of the basic units of time - the period of time that passes from one upper culmination of the Sun to another. But as the accuracy of astronomical observations increased, it became obvious that the length of the day did not remain constant.

The speed of rotation of our planet changes throughout the year, and in addition, its rotation slows down, albeit very slowly. Therefore, it is clear that the definition of a second as a unit of time constituting 1/86,400 of a day required clarification. You know the modern definition of a second from a physics course. The use of atomic clocks, which are available to precise time services and the state standard of time and frequency, provides an extremely small error in time counting (about 5 10-9 s per day). Precise time signals broadcast over the radio are transmitted precisely from atomic clocks. By receiving these signals and determining local time from observations of the moments of stellar culmination, it is possible to calculate the exact coordinates of any point on the earth's surface. These points serve as reference points when drawing up maps, laying gas pipeline routes, automobile and railways, construction of large facilities and a number of other works.

Precise time signals, along with other means (radio beacons, navigation satellites, etc.) are necessary in aviation and maritime navigation. If we used local time in our daily lives, then as we moved west or east we would have to continuously move the clock hands. The inconveniences arising from this are so obvious that currently almost the entire population of the globe uses standard time. The belt time keeping system was proposed in 1884. According to this system, the entire Earth was divided by longitude into 24 time zones (based on the number of hours in a day), each of which occupies approximately 15°. In fact, time is counted according to this system only on 24 main meridians, spaced 15° from each other in longitude. The time on these meridians, which are located approximately in the middle of each time zone, differs by exactly one hour. The local time of the main meridian of a given zone is called standard time. It is used to keep track of time throughout the entire territory belonging to this time zone. Standard time, which is accepted in a particular location, differs from universal time by a number of hours equal to the number of its time zone:

where UT is universal time, and n is the time zone number.

The boundaries of time zones are lines that run from the North Pole of the Earth to the South Pole and are approximately 7.5° from the main meridians. These boundaries do not always run strictly along the meridians, but are drawn along the administrative boundaries of regions or other regions so that the same time applies throughout their entire territory. Naturally, for example, Moscow lives according to the time of one (second) time zone. If we formally follow the accepted rule of dividing into time zones, then it would be necessary to draw the border of the zone in such a way that the city would be divided into two unequal parts.

In our country, standard time was introduced on July 1, 1919. Since then, the boundaries of time zones have been repeatedly reviewed and changed. Since January 1992, when clocks in Russia were moved forward one hour, we live according to the so-called maternity time, which was introduced in the USSR back in 1930. At the end of March, the country switches to summer time, the clock hands are moved one more an hour ahead. Daylight saving time is canceled at the end of September, the hands are set back one hour. The days when summer time is introduced and canceled are established annually by government decree. Moscow maternity time, which is shown by clocks not only in Moscow, but also in St. Petersburg and the central regions of Russia, differs from universal time by 3 hours in winter and 4 hours in summer.

When solving many problems in aviation astronomy, it is necessary to know local time, which underlies all astronomical observations.

Local time is the time on a given geographic meridian (the meridian of the observer). Each meri dian has its own local time. It can be stellar, true solar and average solar. All these times have some common features. Let us consider them in relation to local mean solar time, which is counted from the meridian of mean midnight.

In Fig. 3.9 point O represents the North Pole of the Earth, straight line OA is the meridian of average midnight, and straight lines OB and OS are the geographic meridians of points B and C of the earth’s surface, having geographic longitudes and Local mean solar time on the indicated meridians at the same moment is indicated by . Directly from the figure in question, one can establish the features of local time:

Rice. 3.9. Local mean solar time

on the entire geographical meridian, local time at the same moment is the same;

To the east of any meridian, local time increases, and to the west it decreases;

the difference in local times on two meridians at the same moment is always equal to the difference in the longitudes of these meridians, expressed in time units, i.e. This relationship is widely used in solving practical problems in aviation astronomy. It allows you to determine the local time at a given point by the known time of another point. It is inconvenient to use local mean solar time in everyday life, since even in different areas of the same big city it differs by a certain amount, and therefore it is very difficult to reconcile it with everyday life, transport and communications.

The relationship between time and longitude of place.

It was established above that local time is closely related to the longitude of a place. Consequently, there is a certain relationship between time and longitude of place, which can be established on the basis daily rotation Earth. In one day, the Earth makes a full rotation of 360° relative to that point celestial sphere, by which time is determined. Based on this, we can derive the following relationship between time and longitude of place: .

This dependence is valid for both solar and sidereal time, i.e., for any time measurement system. It allows the longitude of a place to be expressed in time and, conversely, time to be expressed in units of arc and greatly simplifies the solution of many practical problems of aviation astronomy.

Example 1. Convert longitude to time.

Solution. Knowing that 15° corresponds to , we determine the integer number of hours. and the remainder;

convert the resulting remainder of degrees into time: min; convert minutes of arc to time: . Finally we get: .

Example 2. Convert longitude in time to arc units.

Solution. Knowing that corresponds to min corresponds to correspond, we translate:

whole hours to arc units: minutes of time to arc units. seconds of time to arc units. .

Finally we get: .

Determining local time at a given point.

In the practice of aviation astronomy, the method of determining local time at a given point by the known time of another point is widely used. Local time at a given point is determined by the formula

where is the known local time at one point; - the desired local time at a given point; - the difference in longitude of these points, expressed in time.

When traveling around Russia and the World, you must remember that time may change depending on where you are. Since local time is generally used everywhere, it is necessary to be able to determine it anywhere in the world. The basis modern system Time zones are based on Coordinated Universal Time (UTC), which determines the time of all time zones. The UTC scale was introduced in 1964 and is established by atomic clocks. Coordinated Universal Time (UTC) always remains an independent reference for the whole world and from which, knowing the difference between your standard time, you can always calculate your local time.

The standard time system makes it easy to determine the standard time at any location. There is a certain relationship between standard time and time zones. The difference between the time zones of two points is equal to the difference in the time zone numbers. This relationship allows you to determine the time at a given point by the known standard time of another time point.

The time at a given point will be equal to the time of the point where it is known, plus or minus the difference in time zone numbers. The specified difference is added to the known standard time if the point whose time is determined is located east of the point whose time is known, and if it is to the west, it is subtracted.

In Russia:

According to the federal law “On Amendments to the Federal Law “On the Calculation of Time””, adopted on July 1, 2014, from October 26, 2014, Moscow time corresponds to the third time zone in the national time scale Russian Federation UTC+3. There are 11 time zones, corresponding in international numbering to time zones from 2 to 12 inclusive.

1st time zone (MSK-1, Moscow time minus 1 hour, UTC+2): Kaliningrad region;

2nd time zone (MSK, Moscow time, UTC+3): Republic of Adygea (Adygea), Republic of Dagestan, Republic of Ingushetia, Kabardino-Balkarian Republic, Republic of Kalmykia, Karachay-Cherkess Republic, Republic of Karelia, Komi Republic, Republic of Crimea, Republic of Mari El, Republic of Mordovia, Republic of North Ossetia - Alania, Republic of Tatarstan, Chechen Republic, Chuvash Republic - Chuvashia, Krasnodar Territory, Stavropol region, Arkhangelsk region, Astrakhan region, Belgorod region, Bryansk region, Vladimir region, Volgograd region, Vologda region, Voronezh region, Ivanovo region, Kaluga region, Kirov and Kostroma regions, Kursk region, Leningrad region, Lipetsk region, Moscow region, Murmansk region, Nizhny Novgorod region, Novgorod region, Oryol region, Penza region, Pskov region, Rostov region, Ryazan region , Saratov region, Smolensk region, Tambov region, Tver region, Tula region, Ulyanovsk region, Yaroslavl region, Nenets Autonomous Okrug; cities of federal significance - Moscow, St. Petersburg, Sevastopol;

3rd time zone (MSK+1, UTC+4): Udmurt republic, Samara Region;

4th time zone (MSK+2, UTC+5): Republic of Bashkortostan, Perm Territory, Kurgan Region, Orenburg Region, Sverdlovsk Region, Tyumen Region, Chelyabinsk Region, Khanty-Mansiysk and Yamalo-Nenets Autonomous Okrugs;

5th time zone (MSK+3, UTC+6): Altai Republic, Altai region, Novosibirsk, Omsk and Tomsk regions;

6th time zone (MSK+4, UTC+7): Republic of Tyva, Rep. Khakassia, Krasnoyarsk Territory and Kemerovo Region;

7th time zone (MSK+5, UTC+8): Republic of Buryatia, Trans-Baikal Territory, Irkutsk region;

8th time zone (MSK+6, UTC+9): Republic of Sakha (Yakutia) (Aldansky, Amginsky, Anabarsky, Bulunsky, Verkhnevilyuysky, Vilyuisky, Gorny, Zhigansky national Evenki, Kobyaisky, Lensky, Megino-Kangalassky, Mirny, Namsky , Neryungrinsky, Nyurbinsky, Olekminsky, Oleneksky Evenki national, Suntarsky, Tattinsky, Tomponsky, Ust-Aldansky, Ust-Maysky, Khangalassky, Churapchinsky and Eveno-Bytantaysky districts), the city of Yakutsk, Amur region;

9th time zone (MSK+7, UTC+10): Republic of Sakha (Yakutia) (Verkhoyansk, Oymyakonsky and Ust-Yansky districts), Primorsky and Khabarovsk territories, Magadan region, Sakhalin region (Alexandrovsk-Sakhalinsky, Anivsky, Dolinsky , Korsakovsky, Kurilsky, Makarovsky, Nevelsky, Nogliki, Okha, Poronaysky, Smirnykhovsky, Tomarinsky, Tymovsky, Uglegorsky, Kholmsky, Yuzhno-Kurilsky districts), Jewish Autonomous Region, the city of Yuzhno-Sakhalinsk;

10th time zone (MSK+8, UTC+11): Republic of Sakha (Yakutia) (Abyisky, Allaikhovsky, Verkhnekolymsky, Momsky, Nizhnekolymsky and Srednekolymsky districts), Sakhalin region (North Kuril region);

11th time zone (MSK+9, UTC+12): Kamchatka Territory, Chukotka Autonomous Okrug.