When the sun shines on the earth here, at this time, on the other side of the earth, we will see a scene of twinkling stars in the night sky. The Earth is constantly rotating, and the Sun can only illuminate half of the Earth. Therefore, the morning and dusk lines perpendicular to the direction of the sun's illumination divide the Earth into a day hemisphere and a night hemisphere. After 12 hours, day and night changed. As a result, this leads to different times of day when sunrise and sunset can be seen around the world. When the sun shines directly on the meridian where an area is located, the place in that area is 12 noon. Therefore, there is a time difference around the world, which is often referred to as time difference.
Time difference and longitude are very closely related, the earth is a sphere, a rotation is 360 degrees, here is divided into 360 longitudes. At the same time, it takes another 24 hours to rotate in one rotation, with an hourly rotation of 15 degrees and a rotation of 1 degree every four minutes. That is to say, for every 15 longitudes on Earth, the time difference is one hour, and for every longitude each time, there will be a four-minute time difference. Even if the longitude of the two places is only 0.25 degrees apart, there will be a 1-minute time difference between the two places. In short, as long as two places are not on the same meridian, the places of the two places will be different, and there will be a time difference. To this end, people divide the world into 24 time zones. The prime meridian is the middle time zone, the east to the east longitude 180 degrees east longitude is the eastern time zone, and the west to west longitude 180 degrees west longitude is the western time zone. Each time zone is 15 longitudes. However, the east and west twelve zones are 7.5 longitudes, combined into one time zone.
China's land area spans five time zones: The East Fifth District, the East Sixth District, the East Seventh District, the East Eighth District and the East Ninth District. That is to say, from the westernmost Xinjiang to the easternmost Heilongjiang, the time difference is more than four hours, and when the sun sets in the west of Heilongjiang, the sun in Xinjiang is still hanging in the sky. Despite this, China still uniformly uses the district of the East Eighth District, because Beijing is located in the East Eighth District, which is often referred to as "Beijing Time". However, Beijing time does not refer to the local time of the city of Beijing, but to the district time at 120 degrees east longitude. The longitude at which the longitude is located is 116 degrees east longitude, which is 16 minutes different from the time at 120 degrees east longitude. Moreover, Beijing time is not decided in Beijing, but from the National Timing Center of the Chinese Academy of Sciences in Xi'an. Before the founding of New China, our country also had time zones, but after 1949, the districts of the Eastern Eighth District were uniformly used. In addition to the fact that the population is mainly distributed in the central and eastern regions of our country, and the population in the western region can take care of more people by using this time less, there are also some factors of political convenience. In any case, the unified use of the District of the Eighth District of the East is also conducive to time management, and there will be no inconvenience of changing the time from one city to another.
Let's take a look at how time is calculated around the world. First, if region a and region b are in both the Eastern and Western time zones, the two regions only need to subtract the time difference. For example: I'm in the East 7th District, you're in the 5th East District, seven minus five, and there is a difference of two hours between us; you are in the West 6th District, I am in the West 2nd District, and there is a difference of four hours between us. Speaking of which, it is necessary to remind you of a crucial point: because the direction of the Earth's rotation is from west to east, the eastern region ushers in the sunrise before the western region, and at the same time, the eastern region also enters the night earlier than the western region. Therefore, the time in the east is faster than in the west. If the East 8th District is 6 a.m. at this time, then the East Ninth District is 7 a.m. at this time, and the time is one hour faster. Similarly, if the West Ninth District is 9 a.m. at this time, then the West 8 District is 10 a.m. at this time. Because the East Ninth District is in the east of the East Eighth District, and the West Eighth District is in the east of the West Ninth District, I think this is the simplest expression.
There is a second case, where one is in the Eastern Time Zone and the other in the Western Time Zone. I'm in the East Fourth District, you're in the West Sixth District, how long has it been since we were separated? This requires the addition of time zones, the East Four Districts (+4), the West Six Districts (-6), the sum is ten hours, and the difference between us is 10 hours. If the East Fourth District is at 6:00 a.m. on January 2, then the West Sixth District at this time is 8:00 p.m. on January 1 the day before. To sum up, it is subtracted from the same region and added by different regions.
Of particular wonder is the International Date Line, referred to as the Daily Line. It is located at 180 degrees east-west longitude in the Pacific Ocean and serves not to confuse global dates. Crossing this line, the date changes by one day. However, it does not completely coincide with the 180-degree meridian, but curves away from mainland countries and islands. Try to keep the entire line uninhabited so that there is no such thing as "two dates in the same urban area". If the line is crossed from west to east, the date is reduced by one day, for example, at 170 degrees east longitude is 3 pm on January 2, after crossing this line to the east, 175 degrees west longitude is 4 pm on January 1 the day before; if it is crossed from east to west, the date is added by one day. For example, if 175 degrees west longitude is 9 a.m. on January 4, and after crossing the sun line to the west, at this time, 170 degrees east longitude is 8 a.m. on January 5. This line is in the east and west twelve districts, if two people look at each other face to face, but there is a line between them, then there is a difference of 24 hours between them, one person is today, one person is yesterday. Doesn't it look like a time-spanning sight?