## time in the world

The ancients relied to calculate the time before the invention of the clock on the length and direction of the shadows formed by the sun, so they used a sundial that works as a sundial that determines the length of the day through the shadows, and in later periods the hourglass was adopted to determine certain periods, and oil lamps and candles were used to determine the time. It was believed in the past that the earth is fixed and does not move until scientists proved in the sixteenth century that the earth revolves around the sun, resulting from its movement of the four seasons, and it revolves around its axis, and night and day occur.

And when the earth rotates around its axis to form the day cycle, part of the earth is illuminated by sunlight, and the other part is dark. So the time cannot be the same in every place on earth, rather the timing differs from one region to another. Based on that, the earth was divided into 24 time zones with 15 degrees in each, a region that shares the same timing, because the earth rotates 15 degrees every hour. The time in the world is based on a single point of reference centered at the Greenwich Observatory in England; So the time is called Greenwich Mean Time or Coordinated Universal Time.

### Greenwich Mean Time

Greenwich Mean Time (GMT) is known as the reference time for time zones in the world, and it is a time based on the Greenwich meridian that passes through the Greenwich region in London, and it is considered a zero longitude of the globe, and the latitude of this region is 38 ".28'.51° north of the equator. When the sun is at its highest point above zero longitudes, i.e. the Greenwich meridian, the time is exactly 12 noon in the Greenwich region. And because the Greenwich meridian is the zero point, it is a reference for all meridians, as The longitude lines were divided accordingly, and each region that passes through 15 meridians has a unified time, as the time increases by an hour every 15 meridians, and this time is not affected by the seasons, as it is fixed, and although scientists replaced Greenwich Mean Time with atomic time, Greenwich Mean Time is still the same. user in the world.

#### Greenwich Mean Time Throughout History

In the year 1670 AD, the scientist John Flamsteed thought about the process of converting solar time into an average time and began making conversion tables and publishing them that year. In the year 1767 AD, the astronomer Royal Neville Maskelyne began determining the lunar distance based on Greenwich as a time standard, and his studies were applied by navigators who were able to determine their location in the sea based on Maskeln's data. Greenwich Mean Time did not begin to be widely adopted until the fifties and sixties of the nineteenth century when moving between countries became wider by rail; Hence the need for an international time standard, and in 1880 AD all of Britain became under Greenwich Mean Time. And when the International Meridian Conference was held in 1884 AD, they issued a recommendation to adopt Greenwich Mean Time and consider it the reference line for longitudes.

#### Greenwich Mean Time calculation method

It is possible to know Greenwich Mean Time for a country relative to another country whose time is known, depending on the longitude coordinates of their geographical location, and this is done by following the following steps:

- We have the timing of one of the two countries known, let it be Q.
- We determine the location of the two countries, and if they are in the same direction, we subtract the number of their longitudes and let the result be x.
- If the two countries are in opposite directions, we add the sum of their longitudes and let the result be r.
- We multiply the result of addition or subtraction by four minutes, i.e. (4 x h), or (4 x r), and let the result be p.
- It gives us the result of the difference between their times in minutes, so we convert it to hours, i.e. we divide p by 60 minutes (p/60), and let the result be d.
- We subtract the result d from the time of the given country, i.e.: d - s, and it gives us the time of the country that we want to know.
- Example: If the time in the city of Dubai = is 12 noon, and its location is 45 degrees east, and we wanted to know the time for the Moroccan city of Fez, and its location is 5 degrees west.

**the solution:**

The two cities are in different directions, so we add 45 + 5 = 50.

We multiply the result by 4: 50 x 4 minutes = 200 minutes.

We convert the result into hours 6/200 = 3.3 hours, which is approximately 3:20 hours.

We subtract Dubai time from the difference between the time of the two countries: 12 - 3:20 = 8:40 in the morning, which is the time in the city of Fez.

#### The difference between Greenwich Mean Time and Universal Time

Greenwich Mean Time has not remained constant over time; It is now known as Coordinated Universal Time / UTC, with a difference between the two times, as Greenwich Mean Time starts at 12 noon; Because this makes it easier for scientists to record the observational data they monitor at night to track the same day they started working. As for the rest of the world, their time starts at 12 midnight, and this change occurred in the 1920s and 1930s when the new time was called Universal Time. The other difference between Greenwich Mean Time and Universal Time lies in the fact that Universal Time is the same in all seasons, while Greenwich Mean Time is subject to the daylight saving time that some countries adopt. Universal Time has another name, Zulu Time, based on the foreign word Zones, which means regions.

There is a time based on the resonance frequency of the cesium atom called atomic time, and it is calculated by 400 atomic clocks in 70 laboratories around the world. Scientists have suggested the adoption of atomic timing; Because solar time adds a second to it every year, which causes a difference in the work of navigation communications networks and others, as some of them add this second and others do not add it, which leads to the creation of some problems. Scientists decided to dispense with this second, so the new time becomes different from Greenwich Mean Time by a minute after 60 and 90 years, and by an hour after 600 years.