And here, I thought everyone welcomed the New Year at the same time.
Oh! no!
You are forgetting that the earth rotates to produce day and night. The sun is shining on only one side of the earth at any time.
There are two lines that determine where it is a certain date:
- The line that moves around the earth to delineate midnight. All of the time zones bordering one side of it have times after 12 midnight, and all of the time zones bordering the other side of it have times before midnight.
- The International Date Line determines where any calendar date starts and where it ends.
The International Date Line is nominally 180 degrees longitude (opposite the Prime Meridian of 0 degrees longitude). But has bends in it so entire countries that cross the 180 degrees meridian have the same date throughout their borders.
This simplified image is from
International Date Line - Wikipedia - each color is one date.
The red line is the simplified date line and rotates with the earth. The midnight point is the point at the bottom away from the sun.
There are 27 time zones from +14 to -12. Three of them (+12 and -12, +13 and -11, and +14 and -10) have the same clock time, but different dates.
My favorite way of demonstrating the progression of a date change is to follow the Santa Claus delivery route:
Santa follows 12 midnight around the world to match the point on the earth. directly away from the sun.
- Santa's first stop is Kiritimati (Christmas) Island in Kiribati. This happens when it is 10 AM 12/24 in London and 5 AM 12/24 in New York.
- Santa arrives in London at 12 M 12/25. It is 2 PM 12/25 in Kiritimati and 7 PM 12/24 in New York.
- Santa arrives in New York at 12 M 12/25. It is 7 PM 12/25 in Kiritimati and 5 AM 12/25 in London.
- Santa finishes at Baker Island at 12 M 12/25. It is 2 AM 12/26 in Kiritimati, 12 N 12/25 in London 7 AM 12/27 in New York.
It takes 27 hours to complete the route. Imagine Santa being near the 24:00 / 0:00 point on the diagram, starting and ending at the date line..
New Years follows the same sequence.
Note that for 3 hours every day, three dates exist on the earth at the same time. When it is 11 am in London on Dec 24, it is 1 am on Dec 25 on Kiritimati, and 11 pm on Dec 23 on Baker Island.