Time in astronomy. Time - sidereal, Greenwich Mean Time, standard Time Use of time in astronomy
The time it takes for the Earth to rotate around its axis can be measured by observing the daily rotation of the celestial sphere.
The duration of a complete revolution of the celestial sphere can be determined with a high degree of accuracy as the time interval between two successive culminations of the same name (for example, upper) of a star or a certain point on the celestial sphere. The point of the vernal equinox (T) is chosen as such a point.
Etc The period of time between two successive upper culminations of the vernal equinox is called the sidereal day.
The moment of the upper culmination of point T is taken as the beginning of the sidereal day.
A sidereal day is divided into 24 sidereal hours, an hour into 60 minutes, a minute into 60 seconds. It is easy to see that the position of point T relative to the meridian, characterized by the arc of the celestial equator, enclosed between the meridian and point T and counted in the direction of the daily rotation of the celestial sphere (marked with a green arrow), determines the fraction of the day that has passed from the beginning of the given day to the moment in question. In other words, the indicated arc of the equator is a measure of time in this moment. Since this arc is equal in degrees to the spherical angle formed by the meridian and the great circle drawn through the pole and point T (displayed by the red arrow) and called hour angle, then we come to the following definition: sidereal time S is currently equal to the hour angle of the vernal equinox. Since the day is divided into 24 hours, and the circle contains 360°, we get the following ratios:
1 hour = 15°, 1 minute - 15", 1 second - 15".
Since the hour, minute and second represent units of measurement of the hour angle, the designations of these units are placed, like the designations of degree units, at the top right of the corresponding figure. Therefore, the record of the moment in time will look like this: S = 14h06m27s.
Sidereal time is used in astronomical observations. For everyday purposes it is inconvenient, since our life is consistent with the Sun.
Sunny time
By analogy with the sidereal day, the concept of true solar day is introduced, which is the time interval between two successive upper culminations of the center of the solar disk.
True solar time is the hour angle (/0) of the center of the Sun. Since the Sun as a result annual movement moves along the ecliptic in the direction opposite to the daily movement by approximately 1° per day, then the true solar day is longer than the sidereal day on average by approximately 4 minutes.
Uneven flow of true solar time
True solar time is inconvenient in that it is very difficult to construct a clock that runs according to this time, since the hour angle of the Sun varies unevenly. This occurs, firstly, as a result of the uneven movement of the Sun along the ecliptic and, secondly, as a result of the inclination of the ecliptic to the equator. The movements of the Sun along the ecliptic near perihelion and aphelion over equal periods of time will be unequal, and equal movements of the Sun along the ecliptic near the equinox and solstices will correspond to unequal changes in the hour angle (Fig. 38).
Mean ecliptic and mean equatorial Sun
To eliminate the unevenness of true solar time, the concept of the “average Sun” is introduced, meaning by this term some auxiliary moving point. The “mean ecliptic sun” is a point that moves uniformly along the ecliptic and passes through perihelion and aphelion simultaneously with the center of the true solar disk. Replacing the true Sun with the “mean ecliptic” eliminates the unevenness of solar time caused by the inconstancy of the speed of the Sun’s movement along the ecliptic. To eliminate the influence of the inclination of the ecliptic to the equator, the concept of the “mean equatorial sun” is introduced, which is a point moving uniformly along the equator and passing through the points of the spring and autumn equinoxes simultaneously with the “mean ecliptic sun”.
Mean solar time
The imaginary “mean equatorial sun” participates in the daily rotation of the celestial sphere in the same way as the true Sun. The period of time between two successive identical culminations of the “mean equatorial sun” is called the mean day. The beginning of the average day is taken to be the moment of the culmination of the “average equatorial sun”. The hour angle of the "mean equatorial sun" determines the average time at a given moment. An average day is divided into 24 average hours, an hour into 60 minutes and a minute into 60 seconds.
Standard time
Each point on the surface of the Earth has its own local time, which differs (depending on longitude) from the time of another point by any number of hours, minutes and seconds. In practical life, using local time is very inconvenient, especially for transport and communications. This circumstance posed the task of streamlining the calculation of time throughout the Earth. Currently, this problem is resolved by the introduction of a standard time system.
The entire globe is divided into 24 zones along the meridians every 15°. Middle "initial or zero"zone passes through the Greenwich meridian and throughout this zone the local time of the Greenwich meridian is adopted. In the next eastern zone, the local time of the middle meridian of this zone is adopted, differing from world time by an hour, etc. This time is designated Ta and is called zone time, and the zone are called sentinels.
In any point on Earth, standard time differs from local time by about half an hour (maximum). The introduction of standard time leads to the fact that in a number of settlements located in close proximity to each other, the time differs by an hour. However, this is redeemed by the fact that minutes and seconds on everything globe when using standard time, they are the same and the time of different points differs from each other only by an integer number of hours.
The boundaries of time zones are drawn, in some cases departing from the meridians, along state, administrative or natural (rivers, mountain ranges) boundaries
Date line
Local or standard time, counting east of prime meridian(passing through Greenwich) will increase in proportion to longitude. If we consider local time, counting west of the prime meridian, then local time will decrease. In this regard, consider the following fact.
Let three observers, being at the same place of mid-latitude, begin simultaneously counting the days, marking them by sunrise, and the first remains in place, the second goes to trip around the world along the parallel to the east, and the third - on a trip around the world along the parallel to the west. When all three observers have gathered in one place again, the observer will tell the remaining observers that the time between meetings has passed. N days, and a person traveling in an easterly direction will say that it has passed (N+1) days. This is due to the fact that the second observer, when moving to the east, will observe the culmination of the Sun each time a little earlier than the stationary observer.
An observer traveling west will say that it has passed (N - 1) days, since, moving in the direction opposite to the rotation of the Earth, he will observe the culmination of the Sun each time with some delay compared to a stationary observer.
To harmonize the counting of days, for stationary observers and travelers, by international agreement, "date line" " It is all located on the surface of the ocean and runs approximately along the 180th meridian, counting from Greenwich. When crossing this line in a westerly direction, one day is eliminated from the count of days (for example, the second number when recording is immediately followed by the fourth number). When crossing the international date line in an easterly direction, an extra day is added when counting days (for example, when recording a number, a number is repeated twice).
Counting meridians from Greenwich is convenient, because in this case the date line falls on an easy-to-remember figure (180°) of longitude, which will not take place if the meridians are counted from some other observatory.
| time, sidereal, GMT, offset, zoneLECTURE NOTES
from discipline "Seafaring Astronomy"
direct training 070104 “Sea and river transport”
(code and name of preparation)
specialty 6.070104 “Sea and river transport”
(code and name of specialty)
specialization.
(name of specialization)
branch "Shipwater" .
(department name)
Looked at the meetings of the cycle committee
branch "Ship watering on sea routes"
Protocol No. "" 2015
Head of the cyclical commission
M. A. Kotolup
PLAN – SUMMARY OF TOPIC No. 1
"Time and its measurement"
1. The concept of time and methods of measuring it.
2. Sidereal time.
3. Solar and mean solar time.
4.Time used in daily activities.
The concept of time and methods of measuring it.
To measure any physical quantity First of all, you need to choose units of measurement that are convenient for practical application and definitely permanent.
Since ancient times, the period of one revolution of the Earth around its axis or reflecting its revolution of the celestial sphere, i.e., was adopted as the basic unit of time. day. This period is practically constant (minor changes in the Earth’s rotation period, discovered relatively recently, are not taken into account in nautical astronomy).
Having established the unit of measurement of time, it is necessary to select the initial (zero) moment of measurement and some point on the sphere, by the movement of which it would be possible to count time intervals. To do this, astronomy uses the daily movement of the vernal equinox or the Sun. The movement of the Aries point is measured sidereal time, according to the movement of the Sun - sunny.
To begin counting the unit of time of day, it is convenient to select the moment when the point of Aries or the Sun intersects the plane of the observer’s meridian, since this plane coincides with the geographic meridian, the position of which on Earth is determined by the longitude of the observer. Therefore, the time in each system also depends on which meridian is chosen as the initial one: Greenwich, local or some other.
Sidereal time.
One revolution of the Earth around its axis or one revolution of the celestial sphere around the axis of the World can be noted by the completed daily movement of a star. It is more convenient in astronomy to use the vernal equinox point for this purpose. Υ , which occupies a very definite position on the sphere and participates in the daily movement, like all the luminaries.
Sidereal day - this is the period of time between two successive upper culminations of the vernal equinox on a given meridian of the observer.
The sidereal day is divided into more small units: sidereal hours, minutes And seconds.
Sidereal time (S) They call the number of stellar units that have passed from the moment of the upper culmination of the vernal equinox to this moment.
Sidereal time can be measured in time or arc units.
Sidereal time is not used to measure large periods of time in everyday life, because does not have a calendar date.
Due to the uniform rotation of the celestial sphere, the period of time that has passed since the moment of the upper culmination of the Aries point and expressed by the value S, is numerically equal to the Wth hour corner of Aries in degree units.
Therefore, there is a dependence
S=t Υ w
This makes it possible to express time intervals, both in hours and in degrees. To move from degrees to hours and back, use the following ratios:
24 hours =360°; 1h=15°; 1 m =15"; 1 s =15" or 0.25";
360° = 24 hours; 1° = 4 M.
A similar transition from one measure to another is necessary when solving astronomical problems. Therefore, in MAE and MT - 75 there are tables to facilitate this translation with an accuracy of tenths of an arc minute (0.1 1) or up to one time second (1 s),
At the same moment, sidereal time S is equal to the Wth hour angle of any star plus its right ascension α and is called the basic formula of time.
S=t w +α
It connects the coordinates of luminaries with time, allows you to move from stellar time to solar time and solve other important problems. In nautical astronomy, this formula is often used to calculate the hour angles of stars:
t w * =S-α *
To simplify the calculations, we replace subtraction with a more convenient addition, adding 360° to the right side of the equation, which is equivalent to 0°:
t w * =S+360°-α *
Designating 360°- - α*=τ*, we finally get:
t w * =S+τ *
When solving problems on the basic formula of time, you can freely add or subtract 360° (24 hours) to any part of the equation, since this is equivalent to 0° (0 H). In the process of solving such problems, quite often it is necessary to switch from degree units to hourly units and back.
Sunny and mean solar time.
Everyday life people of our planet is organized by the Sun depending on the light and dark periods of the day. For this reason alone, sidereal time is inconvenient. In addition, due to the annual movement of the Sun, which lags behind the point every day Υ by 1° or 4 m, the beginning of the sidereal day throughout the year occurs at different moments of the day and night. So, on March 21, the beginning of the sidereal day will be in the middle of the day, on June 22 - in the morning, on September 23 - at night, on December 22 - in the evening. This time measurement system cannot be used in everyday life. Therefore, sidereal time is used only in theoretical conclusions and in computational problems of nautical astronomy.
It is more appropriate to take as a unit of time the interval between two successive culminations of the center of the Sun, which is called solar (true) days. These days are approximately 4 m longer than the sidereal days. However, the change in the right ascension of the Sun is not the same throughout the year, i.e., the duration of the solar day is also not the same. The difference between the longest and shortest solar days reaches 51 s or almost 1 m. It is impossible to use a variable value for the unit of calculation of exact time, therefore solar (true) days are not used, and there is no system for measuring time based on the movement of the true Sun. This is due high requirements to the accuracy of timing at modern development science, technology and economics. It is very difficult to create devices that would change their course depending on changes in the length of the solar day.
The true Sun cannot be “forced” to move along the ecliptic at a constant speed. To obtain a constant unit of time, it is necessary to replace the Sun with a point on a sphere that has uniform annual motion. For this purpose, a special fictitious point of the celestial sphere was established - average Sun, which replaces the true Sun in measuring time.
Let us imagine that the Sun moves along the ecliptic at a speed equal to the annual average speed of the true Sun. As calculations have shown, such a point will not move far from the true Sun. However, due to the inclination of the ecliptic to the equator at an angle of 23.5°, the daily change Δα will still be unequal, i.e. even then the solar day will turn out to be variable in size. Therefore it was established that own movement The average Sun does not move along the ecliptic, but along the equator in the same direction as the movement of the true Sun . Thus, the average Sun has the following features:
Participates in daily movement along with the celestial sphere;
It has its own annual movement along the equator, directed against the daily one;
Its daily movement along the equator is constant and equal to the annual average movement of the projection of the true Sun onto the equator; this value is equal to 3 m 56 s, i.e. about 1 °;
The meridians of the average and true Sun are located not far from each other, therefore the culminations of the true and average Sun differ practically little in time.
Taking into account these features, we can define the initial constant unit of this system.
Average day - this is the period of time between two successive lower culminations of the average Sun. Since the beginning of the average day is taken to be the moment of the lower culmination of the average Sun, the date change occurs at night, which is more convenient in everyday life.
Average, or civil time T they call the number of average hours, minutes and seconds that have passed from the moment of the lower culmination of the average Sun to this moment.
Mean time is necessarily assigned a calendar date, in contrast to sidereal time, which has no date.
The ± signs are chosen so that the result is obtained in no more than 24 hours (360°).
The day is traditionally divided into 24 hours, an hour into 60 minutes, and a minute into 60 seconds. Since we measure right ascension in hours, minutes and seconds, the moment in time on a sidereal clock is determined by the right ascension of the star that is currently culminating. It follows that sidereal time is measured by the hour angle of the vernal equinox (Fig. 19) in the same way as we determine time by the angles of rotation of the hour and minute hands. Indeed, by definition, the hour angle of the vernal equinox point is zero at the moment when sidereal time is zero. The hour angle changes evenly, since the celestial sphere also rotates evenly, i.e., by measuring the hour angle in hourly measure, we immediately obtain the time during which the celestial sphere returned to this angle.
Sidereal time is extremely convenient for astronomers. Knowing it, you can immediately figure out which stars are observed at this moment in time. It’s easy to define it. Of course, it can be installed accurately (down to tenths or hundredths of a second) only with the help of special tools. But with an accuracy of up to several minutes, the astronomer determines it with one glance.
Sidereal day- this is the period of time between two successive upper culminations of any star. It is customary to consider the moment of the culmination of the vernal equinox as the beginning of the sidereal day.
Pictures (photos, drawings)
On this page there is material on the following topics:
Sidereal time
Sidereal time is the time elapsed from the upper point of the vernal equinox or the point of Aries to any other position, or, more simply, the hour angle of the vernal equinox. Used primarily by astronomers to determine where to point a telescope in order to see the desired object. Denoted by the letter S.
When determining the point of the vernal equinox, you can take into account or not take into account nutation in different ways - the weak irregular movement of a rotating solid undergoing precession. Depending on this, sidereal time is: true, quasi-true and average.
With true sidereal time, the true point of the vernal equinox is considered, which has precessional and nutational motion, which shifts in the ecliptic plane at a speed of 50.25" per year due to general precession in longitude and at the same time periodically fluctuates due to nutation.
When determining the quasi-true, its short-period part is excluded from nutation.
And finally, when determining the average sidereal time, nutation is not taken into account at all.
Sidereal time varies at different longitudes of the Earth: with a change in longitude by 15° east, it increases by about 1 hour.
Depending on the place, they distinguish: local true sidereal time - the hour angle of the true point of the vernal equinox for a given place (for the local meridian); local mean sidereal time - hour angle of the midpoint of the vernal equinox; Greenwich true sidereal time - the hour angle of the true point of the vernal equinox on the Greenwich meridian; Greenwich mean sidereal time is the hour angle of the midpoint of the vernal equinox on the Greenwich meridian.
The time interval between two successive upper culminations of a star on the same geographic meridian, or in other words, the period of rotation of a celestial body relative to the stars around its axis, is called sidereal days. Sometimes a definition is used in which the sidereal day is the time period of a complete revolution of the Earth relative to the point of Aries.
To measure sidereal days, you first need to measure the hour angle (t) of a star for which right ascension (α) is known.
For the Aries point, the hour angle at the moment of its upper culmination is 0°. Since the beginning of the sidereal day coincides with the beginning of counting the hour angles of the luminaries, then, consequently, sidereal time at a given moment is the hour angle of the point of the vernal equinox, i.e. S = t. Let us transfer the projection of the celestial sphere to the plane of the celestial equator. Let point C represent the position of any star on the sphere at a given time; ♈ - position of the vernal equinox point (Aries point). From the figure it can be seen that sidereal time at a given moment is equal to the sum of the right ascension and the hour angle of the star at the same moment, i.e. S = t + α. This formula
also called the basic time formula.
At the moment of the upper culmination of the light, its hourly angle t = 0°, and then s = α.
At the moment of the lower culmination of the light, its hour angle t = 12h, and sidereal time s = α + 12h.
The sidereal day is divided into smaller periods: sidereal hours, minutes and seconds.
Sidereal hour is equal to 1/24 sidereal day and is 0 hours 59 minutes.
50.1704387847 sec. The duration of a sidereal minute is 0 hours 0 minutes. 59.8361739797451 sec. Sidereal second - 0.9972695663290856 sec. The sun and the vernal equinox are constantly changing, i.e. The upper culmination of the Sun on different days of the year occurs at different moments of the sidereal day. Only once a year, on the day of the vernal equinox at noon, the location of the Sun and the points of the vernal equinox coincide. After one sidereal day, the point of the vernal equinox will again be at the upper culmination, and the Sun will arrive at the meridian in only about 4 minutes, since in one sidereal day it will shift east relative to the point of the vernal equinox by almost 1° towards its visible movement. Those. 24 hours of sidereal time corresponds to 23 hours 56 minutes. 4.091 sec. mean solar time. In a year, there are exactly one more sidereal days than average solar days.
So on March 21, the Sun is located at the point of Aries, while the sidereal day begins at noon.
In a day, the Sun will move along the ecliptic by about 1° and will culminate 4 minutes after the Aries point. Three months later - on June 22nd - the culmination of the Aries point will occur at 6 o'clock in the morning. On September 23, the sidereal day will begin at midnight. On December 22, the sidereal day will begin at 18:00 pm. An average value based on measurements at a number of locations. Mean solar time
prime meridian. The prime meridian is conventionally taken to be the meridian of the Greenwich Observatory (Great Britain).
Coordinated Universal Time (UTC)
Replaces Greenwich Mean Time as the generally accepted international time standard. It is the basis for civil time in many countries and is used to broadcast the universal time signal used in aviation.
Local time
Time on the observer's meridian. It can be true solar, average solar and stellar.
True solar time (Ti)
Hour angle (angular distance measured along the celestial equator to the west of the celestial meridian, expressed in hourly units at the rate of 24 hours = 360o (1 hour = 15o, 1 minute = 15") of the Sun, increased by 12 hours (measured west of celestial meridian). The moment the Sun crosses the meridian is called true noon. True solar time is shown by a simple sundial. To convert time to angular value and back, you can use the table:
Converting time to angular value and back
Time measured by the hourly angle of some imaginary point, called the mean sun, moving uniformly along the equator, the position of which coincides with the center of the true Sun at the moments of the autumn and spring equinoxes. It differs from solar time due to the ellipticity of the earth's orbit and its inclination towards the equator. The difference between the mean solar time and the true solar time is equal to a correction called the equation of time (the current difference between the true and mean solar time), not exceeding 16 minutes, calculated theoretically and given in astronomical calendars. Average solar noon is 12 noon local time. The difference between the mean and true solar time or the difference between the right ascensions of the true and mean Sun is called "equation of time".
Sidereal time (S)
Local sidereal time at a given moment is numerically equal to the hour angle of the vernal equinox, also called Aries point. The time interval between two successive culminations of the same name on the vernal equinox on the same geographic meridian is called a sidereal day. A full rotation of the vernal equinox, like any other point on the celestial sphere, occurs in 23 hours 56 minutes 04 seconds of mean solar time, since the Sun, moving along the ecliptic, lags somewhat behind the daily rotation of the celestial sphere. A year contains exactly one more sidereal days than average solar days. A sidereal day is divided into sidereal hours, minutes and seconds. The sidereal day is 3 minutes 56 seconds shorter than the average solar day, the sidereal hour is 9.86 seconds shorter than the generally accepted one. Sidereal time is used in aviation astronomy when determining the lines of position and heading of an aircraft by the stars or the position of the aircraft (MS) using astronomical methods.
Local civil time
Mean solar time, measured from the moment of the lower culmination of the mean Sun.
Standard time (Tp)
A time equal to the local civil time of the middle meridian of a given time zone. It is established by international agreement in regions and countries so that throughout the planet the difference between local time and universal time is an integer number of hours. To do this, the entire surface of the Earth is divided approximately along the meridians into 24 time zones. The middle meridians of time zones run at longitudes 15, 30, 45, ... degrees west of Greenwich along the points earth's surface, in which the mean solar time (MT), respectively, is 1, 2, 3, ... hours behind Greenwich. Typically, cities and their surrounding areas live according to the time of the nearest middle meridian. The lines dividing zones with different official times are called time zone boundaries. Usually they do not follow strictly along the meridians, but coincide with administrative boundaries.
Time ratios
So, for each point on the Earth located at longitude X, you can indicate the local true solar time Ti; local mean solar time MT; standard time Tp; seasonal winter time Tz; seasonal summer time Tl; local sidereal time S. Here are the formulas for those who need to convert one time to another (due to maternity time, the last two formulas are correct for Russia):
- MT = Ti + t,
- MT = UTC + X,
- Tn = UTC + n,
- Tz = UTC + n + 1 h,
- T = UTC + n + 2 h,
- S = s + MT (approximately),
Where t is the equation of time; n - time zone number; s - sidereal time at Greenwich midnight (the sidereal time table is given in astronomical calendars).
Example: the longitude of Moscow X is 2 hours 30 minutes. Average solar noon is 12:00 local time (MT). In world time it corresponds to UT = 12 hours - 2 hours 30 minutes = 9 hours 30 minutes, in Moscow winter time - 12 hours 30 minutes, in Moscow summer time - 13 hours 30 minutes.
Thus, if you are a resident of Moscow, then your time is 3 hours ahead of the world time in winter and 4 hours ahead in summer. But all of these, except for true solar time, are conditional points not directly related to real ones. astronomical events. Only the time of sunrise, sunset and the moment of true noon, established with the help of a sundial directly at the right moment at the right point on the Earth, have a real connection with cosmic processes. (although, to be completely accurate, the true sunrise occurs 5 minutes later than observed, and the true sunset occurs 5 minutes earlier due to the phenomenon of atmospheric refraction).
Time in FS2004
Time in FS2004 is calculated using GMT in full accordance with astronomy. Time zones change every 15 degrees of longitude. Accordingly, the calculation of zone (winter, summer) time must be done independently according to location and GMT time. Additional utilities or scripts are used to set the simulator time to standard time (see Links). But it must be remembered that, in some cases, due to such utilities, the operation of some devices, traffic and other time-related applications will look different than without them.