1. Why Noon Happens at Different Times in Different Places Because the Earth Spins
The Earth rotates 360 degrees in about 24 hours. Since 360 divided by 24 is 15, the Earth turns 15 degrees every hour. This means that the time when the sun is directly overhead happens earlier for places in the east and later for places in the west. If the longitude differs by 1 degree, the time of the sun's position shifts by 4 minutes. To find the difference in hours, you divide the difference in longitude by 15.[1] These times, which match the sun in a specific location, are called local time.
You can imagine this by sitting on a swivel chair and looking at light from a window. Depending on which way you face, the moment you see the window changes. The Earth works the same way, but it is the Earth that is spinning, not the sun that is moving.
2. How Railway Schedules Led to Standard Time
If every town used its own clock based on the sun, small differences were not a problem for people who walked from place to place. It became a problem when railways started running. If clocks at each station were different, it was impossible to create a timetable. According to records, railways in the UK adopted Greenwich Mean Time in the 1840s to coordinate train operations. In North America, railway companies introduced four standard time zones on November 18, 1883.[4]
Standard time was not first decided by governments. It spread because of the practical needs of the railways. Because the records vary on the exact year the UK made this law, no specific year is given here.
3. What the 1884 International Meridian Conference Decided About the 0-Degree Line
As trains and ships began to cross borders, a common reference line for the world became necessary. In October 1884, the International Meridian Conference was held in Washington, USA. It is said that 26 countries and 41 representatives participated.[3] The conference decided that the meridian passing through the Greenwich Observatory would be 0 degrees longitude. The vote was 22 in favor, 1 against, and 2 abstentions.[3]
However, this conference did not decide the world's time zones. The conference suggested a common idea for the day but noted that it did not prevent the use of local or standard time. Whether to adopt standard time was left to the judgment of each country.[3]
4. Why Japan Chose 135 Degrees East for Japan Standard Time
In July 1886, Japan set the mean solar time at 135 degrees East longitude as Japan Standard Time by imperial order. It actually began to be used on January 1, 1888.[2] Since 135 divided by 15 is 9, Japan Standard Time is 9 hours ahead of Coordinated Universal Time (UTC).[2] The number 135 is a multiple of 15 and passes near the center of the country. The candidate lines that are multiples of 15 degrees around Japan were 120, 135, and 150 degrees East.[5]
The line at 135 degrees passes through Akashi City in Hyogo Prefecture. However, Akashi was not chosen to make the line 135 degrees. Rather, Akashi just happened to be on the line. This is how a curator at the Akashi City Museum of Science and Technology explained it in reports.[5]
5. Why Some Countries Use Half-Hour Time Zones or a Single Time for the Whole Country
If 15 degrees equals 1 hour, standard times around the world should be in 1-hour steps. However, some countries are different. India is 5 hours and 30 minutes ahead of UTC, and Nepal is 5 hours and 45 minutes ahead.[4]
Conversely, there are examples of wide countries using only one time. China is about 5,000 km (about 3,100 miles) wide from east to west, but it uses only one standard time: Beijing Time (UTC+8).[4] The shape of standard time lines is changed not just by calculations of longitude, but also by what each country values.
6. Why the International Date Line Bends for Political Reasons
Because the Earth is round, a date-change line is needed somewhere to switch the date. Otherwise, the dates would not match after one full circle. The International Date Line runs roughly along 180 degrees longitude over the Pacific Ocean. When crossing from east to west, the date advances by 1 day. When crossing from west to east, it goes back by 1 day.[6]
The line bends to accommodate countries’ choices. For Kiribati, the line used to cut through the middle of the country. So, they changed the date for the eastern half and skipped December 31, 1994.[6] As a result, the line bends far eastward around Kiribati.
Samoa skipped December 30, 2011 and moved from the east side of the line to the west side. One reason given was that they only had 4 weekdays in common for business with Australia and New Zealand.[6] Thus, standard times now range from UTC−12 to UTC+14. The total span is 14 minus (−12), which equals 26 hours. It is not 24 hours because of the bends in the line and how each country chooses its standard time.[4]
7. How to Calculate Time Differences Between Cities Using an Atlas or Globe
Using an atlas or globe, find the longitude of two cities you like and divide the difference in longitude by 15. You can calculate the time difference. You can check your answer against the explanations of "Local Time" and "Standard Time" on the National Astronomical Observatory of Japan's Calendar Wiki.[1][2]
Make the room dark and shine a flashlight on a ball. If you turn the ball slowly, the place where the light hits moves. This can recreate the change of day and night on Earth on a small scale. Be careful not to shine the light into people's eyes.