Dan Heim taught physics and mathematics for 30 years — more if you count his grade-school science club. Since , he's been a freelance writer and creates educational computer graphics and animations. His weekly blog Sky Lights covers topics including astronomy, meteorology, and earth science, and questions from readers are encouraged.
We all heard about the International Date Line IDL in geography class — it was this special line on the globe where the day and date change. But beyond that, our teachers didn't say much more. That's probably why I keep getting questions like these on my blog: What is the IDL, why do we need it, and who invented it?
When you cross the IDL, what exactly happens? If you stood on the IDL with one foot on each side, what day would it be? Is the IDL a legal thing, or just some kind of scientific idea? Could Superman repeatedly fly around the world and go back in time? Does the IDL apply in outer space? Frequent international travelers are comfortable with the IDL. It's part of their routine. They're used to the idea of changing the day and date, yet even they would struggle with most of those questions.
So here's a definitive explanation of the IDL. The lyrics from that classic Chicago song come to mind: Does anybody really care? It's a story that bridges geography, history and astronomy — stick with it to the end, and you will understand the IDL.
Plus, you'll be able to answer those six questions posed above. Before there were clocks Back in the days before mechanical clocks, time was measured mostly using sundials. People relied on the definition that "noon" was when the Sun was highest in the sky, and due south. One "day" was simply the amount of time between two consecutive "noons. The Sun at apparent true noon. The problem was, each city experienced noon at their own apparent Depending on longitude, adjacent cities could have a time of, say, Near the equator, traveling westward by about 1, miles 1, kilometer delays the arrival of noon by one hour.
In the nineteenth century, the emergence of transcontinental railroads further complicated matters. That century also saw accurate mechanical timepieces becoming widely available. Travelers found themselves resetting their watches by several minutes at every station to the east or west.
This was inconvenient at best. Also in that century, the emergence of telegraphy created time-keeping issues for commercial and military entities — the early adopters.
The telegraph, invented in by Pavel Schilling, was the first true "instant messaging" system. It allowed communication over great distances using electricity, which moves nearly at the speed of light. And of course, to use either system effectively, it's helpful to know the clock times at both the sender's and receiver's locations. Latitude and longitude Before we explain how time zones solved these clock problems, let's do a quick review of latitude and longitude.
Somewhere around BCE, Hipparchus of Nicea, a Greek mathematician and astronomer, proposed a global grid of longitude and latitude lines to measure position. It was a coordinate system for locating points on the surface of a sphere. The vertical axis measured "latitude," and the horizontal axis "longitude. During the Age of Discovery, beginning in the 15th century, cartographers saw the need for standardized latitude and longitude measurement.
If your intent is to map or claim a geographic location, you need to describe its position unambiguously. Britain "ruled the waves" at the time, and took the early lead in this endeavor. Portugal and Spain, the other major seafaring nations, were using their own systems, but eventually deferred to England.
However, the selection of a starting point for longitude measurement the 0-degree meridian was arbitrary. It was based more on national pride and convenience. In , England designated the Prime Meridian 0 degrees longitude as the meridian running through Greenwich Observatory. They were the dominant seafaring nation in that era, had colonies around the globe, were using state-of-the-art mechanical clocks, and were scientifically qualified to establish a standard.
You've heard the saying "The sun never sets on the British Empire. England had colonies all around the globe, so it was always "daytime" somewhere in the British Empire. Time zones By the latter part of the nineteenth century, scientists, railroads and other emerging industries felt the need for a global standard of time.
The first such system, using 24 standard time zones, was proposed by Sir Sandford Fleming in Sandford was a Scottish engineer, who helped design the Canadian railway network. His system wasn't officially sanctioned by any global entity, but by it spawned the adoption of the time zone system in use today. Nation by nation, the world bought into Fleming's idea. Within each time zone, all clocks would be set to an average time that best represented where the Sun was located in the sky.
That time is called mean solar time. Sundials, by comparison, measure apparent solar time, sometimes called true solar time.
The time-zone process began in for the United States, when the nation was divided into four standard time zones. Each zone was centered on a meridian of longitude: By , the global system of time zones we use today was fairly well established.
Increasing global connectivity demanded some universal system of time measurement, and standard time zones were the answer. Most time zones do not precisely follow meridians of longitude. They zig and zag as needed to keep islands, smaller countries and large metropolitan areas on the same clock time — an obvious concession to convenience.
The numbering system makes it easy to find the time in other zones. When atomic clocks were invented in the s, it became possible to measure time with an accuracy better than that provided by the rotating Earth. GMT was an "average time" system based on telescopic observations from Greenwich Observatory. On December 31st, when you hear that a "leap second" has been added to or subtracted from the world's clock, that's a correction between GMT and UTC. Earth's rotation period can vary from exactly 24 hours by a fraction of a second either way, depending on geologic perturbations.
For example, as glaciers melt, there's a transfer of mass from higher latitudes toward the equator. As with a figure skater who slows his spin rate by extending an arm or leg, the law of conservation of angular momentum requires a reduction of the spin rate to compensate for this redistribution of mass. Scientists estimate the recent magnitude 9. Astronomers must also consider the difference between apparent and mean time.
That difference will depend on how far east or west one is located within a time zone, and also on the equation of time , which depends on the date. And then there's that confusing correction called daylight saving time DST.
But again, to understand the IDL, we can ignore these complications. What is the IDL? We all know the day and date change at midnight, regardless of your location on the planet. But to use a global time zone system with an IDL, the day and date have to be separated at two locations — you can't split a circle into two parts with a single "cut.
They the IMC selected the degree meridian as the other "cut," not because it was directly opposite the Prime Meridian any meridian could have been that other "cut". So the choice of degrees was arbitrary, but it established the IDL in use today. The bottom line is, this accommodation keeps the island nations of Oceania each on their own clock and calendar. But there are exceptions. Fractional time zones are used in 16 locations around the globe. Countries simply choose what works best for them.
Watch the IDL work Study that first paused frame before you hit "play. For the sake of labels, let's say the green wedge represents the first hour of Saturday. The blue part of the Earth is still on Friday. The red part which will appear later will be Sunday. That green wedge is the first time zone west of the IDL. West is clockwise as seen in this view from above the North Pole. Of note, this green time zone: All locations in a given time zone must be on the same clock time.
There are some exceptions: But we can ignore that for now. The model in my animation is idealized in many ways. Most importantly, all time zones are exactly 15 degreeswide, and centered on 24 evenly spaced meridians of longitude. This is not quite the way things are in the real world, but it greatly simplifies my model. Now feel free to hit "play. Watch what happens when the IDL returns to midnight and the next day and date begins.