The phenomena on the basis of which man makes ways of determining time are astronomical phenomena. They recur periodically. The determination of time is therefore based on astronomical observations. Astronomical observatories determine time, store it with the help of accurate clocks and make it public. The conditions of life on Earth depend very much on the apparent movement of the Sun. The position of the Sun on the celestial sphere determines the length of the day, night, and the seasons. The commonly used time reckoning is based precisely on the apparent motion of the Sun.
Time is determined by the hour angle of the Sun, a value that determines the Sun’s position on the celestial sphere. The solar day is called the period between two sunrises. This time is divided into 24 hours. A day is divided into 24 hours, an hour into 60 minutes, while a minute is divided into 60 seconds. Thus, this is the same division that is used to measure the angles of the positions of objects on the celestial sphere. Therefore, it can be said that one hour is the time in which the hour angle of the Sun will change by 1h or 15°. Thus, an hour can mean 1/24th of a day and then it is a unit of time, or 1/24th of a full angle and then it is a unit of angle. Since it is the angle that is used to measure time, so one hour is the time in which the Sun will move by an angle of 15°.
According to the accepted way of measuring time, the Sun’s hourly angle reaches 0 at the time of its rising, which is noon. If the direct hourly angle of the Sun were adopted as a measure of time change, the date change would occur during the day. This would be very inconvenient for practical reasons. It was therefore accepted to call the solar time of a given locality the hourly angle of the sun determined for that locality plus 12 hours. The dependence of solar time on longitude is the same as that of hourly angles. Longitude is the dihedral angle contained between the half-plane of the zero meridian and the half-plane of the meridian passing through a certain point on the Earth’s surface.
All localities located on the same meridian have the same time, at the same time to the east one encounters times greater by the amount of the change in longitude, to the west the times are correspondingly smaller. At the time of the Sun’s rising, it is solar noon solar time. As the Earth rotates from west to east, the sub-solar point (the Sun’s high point) moves across the Earth’s surface in the opposite direction, that is, from east to west. This is why sunrises, highs, and sunsets in locations to the east of the observer are recorded earlier than in locations to the west. For this reason, there will always be a later time in the east where the phenomena in question have already occurred than in the west. This is important when calculating the local time of a given point on Earth. Moving to add the time difference between the meridians, moving to the west this difference should be subtracted.
The sun in its annual motion , which is a consequence of the rotation of the Earth around the sun, moves increasing its position each day by 4 minutes ( as the equivalent of 1°, since 360° corresponds to 24 hours). The sun’s diurnal motion is not uniform, but slower in summer and faster in winter. This movement is faster when the Earth is near perihelion, while it is slower for the Earth being near aphelion. The consequence of this is that the length of the day varies from season to season. In connection with the non-uniform passage of solar time, solar mean time was introduced.
This time is measured by the hourly angle of the so-called mean sun plus 12 hours. The mean sun is a fictitious point moving on the celestial sphere with a constant speed corresponding to the speed of the sun on the celestial sphere. The two solar times discussed above are local times that depend on the longitude of a given point. Thus, there is no single time for the entire Earth. But the times of individual Earth meridians. As mentioned earlier, the solar day lasts a full 24 hours. However, the Earth makes a full rotation around its axis in 23 hours 56 minutes. This period is called the stellar day.
The stellar day is, therefore, shorter than the solar day, and therefore the difference between solar and stellar time grows steadily throughout the year. Stellar time is the time determined by the position of the stars on the celestial sphere. Stellar time changes constantly throughout the year. At the time of the spring solstice, it differs from solar time by exactly 12 hours and increases daily by about 4 minutes. At present, for practical reasons, such a state is unacceptable. With the development of mass media and communications, it would be impossible to have a different time at each longitude.
On the other hand, it is also impossible to establish a single time for the entire globe due to the fact that the date change would then occur in some places in the middle of the day. Using local time would cause a lot of trouble, so each country introduces so-called official time. Official time, therefore, is the contractual time in effect in a given territory (country, state, administrative unit). Most countries have introduced official time that differs from universal time by a total number of hours. Universal time is the solar mean time determined on the zero meridian, which is taken to be the meridian passing through the astronomical observatory in Greenwich, now a district of London in the United Kingdom. (In English it is called Universal Time UT, or Greenwich Mean Time, GMT). It is the zonal time of the first time zone, from which the time of the other zones is reckoned. This time was adopted not only for the zero meridian, but also on both sides of this meridian in the 15° latitude zone, which is 1h .
This is known as Western European Time. To the east of this zone, a strip of latitude 15° in which Central European Time is one hour greater than Central European Time has been taken again. In Poland, it is this time corresponding to the time at longitude 15°E that applies. Further east, a zone of Eastern European Time greater by 2 h than Universal Time has been delineated. Similarly, west of Greenwich there are zones with times smaller than Universal Time by 1 h , 2 h , 3 h , etc. East American time is 4 h smaller than universal time. Thus, the globe is divided into 24 time zones. Each of these zones has a time that differs by total numbers of hours from universal time. In practice, times were adopted not exactly according to longitude, but according to the borders of the countries lying in the zones. In some countries, for purely practical reasons, changing time to daylight saving time in such a way that on a certain day all clocks must be moved 1 h forward.
Throughout the European Union, clocks are moved forward one hour on the last Sunday in March at 1:00 a.m. Universal Time, while clocks are moved back one hour on the last Sunday in October at 1:00 a.m. The time changes are intended to result in more efficient use of daylight. In summer, standard time is moved forward one hour so the timing of human activity is better aligned with the hours when there is the most sunlight. Another reason for the introduction of local time is the economic considerations of saving electricity. The introduction of official times around the world has solved the problem of determining the hours of a date, but has made it necessary to set such a boundary from which dates will be counted. This boundary was called the international date change line.
This line was carried out essentially along the 180° meridian of longitude. Crossing it, any aircraft or ship changes the date in the logbook. If one crosses this line from east to west one day is left off in the logbook. Monday is followed by Wednesday, May 1 by May 3, and so on. One day is lost in the rally. If a ship crosses the date change line from west to east, it repeats the same date, so it gains one additional day. Monday is followed by Monday again, after May 1, May 1 again. However, due to political boundaries, in order to prevent one country from having different dates, this boundary does not coincide with the 180° meridian. It was shifted eastward as far as the Bering Strait so that all of Russia’s territory lay on the western side, it was shifted westward so that the Aleutian archipelago, which belongs to the United States, would be left entirely on the eastern side of the line. The third shift was made in the southern hemisphere to leave the Fiji Islands, Tonga, Chatham and Karmadec on the western side of the date change boundary.
There is a well-known story that when on September 6, 1522, the participants of Ferdinand Magellan’s expedition under the command of Sebastian del Cano sailed into a harbor in Spain after their round-the-world voyage, the date in their ship’s log did not match the date of the port. According to the ship’s log, it was only September 5. The chronicler of the expedition was accused of extreme negligence. However, the charge was wrong. Still sailing west, the sailors changed the ship’s time and thus gained a full 24 hours.
It is not only the rotational motion of the Earth that can serve as the basis of time reckoning. Among other periodic phenomena, noteworthy is the movement of the Moon around the Earth underlying the definition of the month, as well as the movement of the Earth around the Sun underlying the definition of the year. Both the month and the year can be defined in different ways.
- Siderial (stellar)month – the period of the full circulation of the Moon around the Earth, the time after which the Moon will return to the same place against the background of the stars. . Star month lasts 27 days 7 hours 43 minutes and 11.5 seconds
- Synodic month – it lies at the heart of calendar time reckoning, the period of the full cycle of the moon’s phases, e.g., the interval of time separating successive new or full moons. It is assumed that the synodic month lasts 29 days 12 hours 44 minutes and 3 seconds. The synodic month is more than 2 days longer than the siderial month. This time is needed for the Moon to again be in the same position relative to the Sun-Earth straight line. During the synodic month, a whole cycle of astronomical phenomena occurs – for example, it begins with a new moon, followed by the first quarter full moon and finally the last quarter.
- Dragon month- the period between two consecutive transits of the Moon through the same node of its orbit. A dragon month lasts 27 days 5 hours 5 minutes and 36 seconds.
- Anomalistic month-The period between two consecutive transits of the Moon through the perigee of its orbit. An anomalistic month lasts 27 days 13 hours 18 minutes and 37 seconds.
- Tropical year- the interval of time between two consecutive passes of the Sun through the point of the vernal equinox (Aries point). It is a period of repeating seasons and lasts 365 days 5 hours 48 minutes and 46 seconds.
- The stellar year-is the time between two consecutive transits of the Sun against the background of the same stars, or more precisely, between transits of the Sun through the same point on the ecliptic. For Earth, it lasts about 365 days 6 hours 9 minutes 9.54 seconds.
The solar average day, the synodic month, and the tropical year underlie the calendar reckoning of time. However, neither the tropical year nor the synodic month are characterized by the total numbers of average solar days. Thus, in order for each month, and the year to have a total number of days some simplifications were made. A calendar year has 365 average solar days. However, the Earth makes about 365 ¼ revolutions around the Sun in a full circle, so after each year there are about 6 hours left. After four years, that remainder will be 24 hours, or a full 24 hours. In 46 BC. Julius Caesar introduced a calendar in Rome in which three ordinary years were followed by a leap year having 366 days. The last 29th day of February was considered an additional leap day.
This is how the Julian calendar came into being. A leap year is expressed in this calendar by a number divisible by 4 without remainder. In the Julian calendar, a year lasts 365 days and 6 hours. The tropical year, as mentioned earlier, lasts 11 minutes less. Thus, during the time of this calendar, 11 minutes remained after each year, of which 1 day was accumulated after 130 years. As a rule of thumb, the nearest Sunday after the first spring full moon is Easter Sunday. However, during the use of the Julian calendar, the day of the vernal equinox shifted from March 21 to March 10. This became the cause of doubt about the date for determining Easter.
That is why in 1582 Pope Gregory XIII approved an amendment to the Julian calendar. In the Gregorian calendar, years expressed in full hundreds are not leap years unless they are denoted by a number that can be divided without remainder by 400. Thus, the years 1800 and 1900 had 365 days, while the year 2000 was leap years with 366 days. To this day, the Julian calendar is still used by some Orthodox Churches, and the Eastern Churches as a liturgical calendar, which is evident in the shifting of the dates of observance of religious holidays compared to holidays in the Roman Catholic Church
