International Date Line (IDL), Prime Meridian or Antemeridian ?????

So how much sleep did we get last night – 8 hours or 32 hours???

A rather strange question you may ask, 30 hours sleep in 1 night, no way, that’s impossible!. However, we retired at 22:00 on Sunday 19th January and awoke at 06:00 on Tuesday 21st January. With my rudimentary math skills, that makes 32 hours of sleep.

You may ask – did we awake completely refreshed – nope, no different than usual, so what happened? Well it’s really rather simple, provided you understand things like International Date Lines, Prime Meridian, Antemeridian, Time Zones, etc.

Read on for an explanation of how it works.

Being a predominantly westbound cruise, we gain an extra hour on many sea days, which is a great benefit when completing blog posts, working on photos, and still getting some sleep, but at some point, you have to pay for all those 25 hour days. Well, we paid for it by missing Monday 20th January. Therefore, on the Sunday evening we were some of the last in the world to see the sunset, but a mere 10 hours later we were the first to see it rise on Tuesday morning. Therefore, rather than being 11 hrs behind GMT (Z-11) we are now 13 hrs ahead, or Z+13. This aligns us with Tonga time.

This may raise some interesting questions:

• Is the IDL real or imaginary
• Is the IDL fixed or can it be moved
• What is the Prime & Ante Meridian
• What is GMT

Prime Meridian

The world has experienced a number of Prime Meridians, but the current Greenwich Meridian, located in the East end of London was introduced at the 1884 International Median Conference. A median is a Great Circle line from the North Pole to the South Pole and refers to lines of Longitude. The Prime Meridian or 0 degrees Longitude is located at the Greenwich Observatory, with areas to the East being East Longitude and West of Greenwich being West Longitude.

Antemeridian

The Antemeridian is opposite the Prime Meridian or 180 degrees East or West Longitude.

Greenwich Mean Time (GMT)

This is a system of time zones throughout the world, based on the Prime Meridian at Greenwich. The entire world is split into time zones either ahead of Greenwich (Z+) or behind Greenwich (Z-) depending on location to the East or West of the Prime Meridian. Our home on the West Coast of Canada lies to the West of Greenwich so we are in zone Z-8, or 8 hours behind UK.

How many time zones does the world have, most will probably immediately respond with 24, as that is how many hours are in 1 day. But, give it some thought and don’t forget that GMT or 0 is also a valid zone. Therefore, I come up with Z 0, Z+1 to 12 and Z-1 to 12 or 25 standard zones. But wait, we also have up to 15 or 16 non-standard zones – Newfoundland (Z-3.5), Tonga & Samoa are (Z+13) being a couple of examples.

International Date Line (IDL) – Does It Move or Cross Land

The IDL is an imaginary line that generally follows the Antemeridian or 180 degrees longitude. However, it does zigzag across the Antemeridian as it rounds various islands. On Dec 29^th 2011 the IDL was redrawn as Samoa changed time zones to be closer to important trading partners Australia and New Zealand. Samoa went from Z-12 to Z+13.

The answer to the amount of sleep is 8 hours, as the difference of 32 hours is reduced by zooming the cloxs ahead by 24 hours.