What time is it?

Hopefully you guys have sprung your clocks forward by an hour to account for Daylight Savings Time. Bryan over at Why Now says the invention of the clock was the first step towards fascism. Well, maybe so, but not for exactly the reason you might think. Rather: The clock is what enables navigators to know where they are. And if fascists didn't know where they were, they couldn't try to conquer the world.

Wait, you say, what does a clock have to do with knowing where you are? Well, mankind has known that the world is round for quite a number of centuries, since long before Columbus (that thing about people thinking the world was flat in Columbus's time? Not true). The Greek mathematician Eratosthenes even used trigonometry to measure its circumference within 10% back in 240BC. Navigators could figure their latitude -- how many degrees north of the equator -- they were by using a sextant to shoot an angle to the North Star at night, could use that and the circumference of the Earth to measure how far they needed to sail north or south to get to the same latitude as some port and then go east or west to reach the port. But how far east or west?

To know their longitude, navigators need to know what time is it?. Wait, you say, how can that be so? Well, let's say you're leaving from Lisbon, Portugal, and sailing west. You know the longitude at Lisbon. You set your clock to 12 noon at the exact same time your sundial says 12 noon. Then you start sailing west with your clock and your portable sundial. Look at your clock when your portable sundial says 12 noon. The difference between what your clock says and what your sundial says is how many minutes of longitude you've sailed. And, since we know the circumference of the Earth, yay, we know how far east or west we need to sail to reach the port we're aiming for! So the invention of accurate clocks is what allowed navigators to accurately pinpoint where they were on the featureless oceans and how far they needed to sail to get where they were going.

Now I hear you saying, "what use is that to me today, when we have GPS?" But GPS is all about using clocks to navigate! Each GPS satellite has an extremely accurate atomic clock within it, synchronized to the master atomic clock in Colorado before it is launched. Each GPS satellite is constantly transmitting its position in its orbit and its time. The GPS receiver then receives that information a hundred milliseconds or so later, and can use the time delay between when the satellite transmitted the information, and when the receiver received the information, to know how far it was from the satellite (using the speed of light delay to compute that), which in turn puts the receiver somewhere on a sphere N miles from the satellite. Do this for two satellites, and you know you're on a circle where the two spheres intersect. Do this for three satellites, and then the three spheres intersect at a point, and you know exactly where you are -- you are at that point where the three spheres intersect. But this assumes that GPS receivers have an accurate atomic clock of their own inside. They don't. That's why they need a fourth satellite to determine their position -- the fourth satellite is what allows them to compute the actual Colorado atomic clock time based on the differences between the four satellites.

So: It seems rather odd that it's clocks that allow you to know where you are, but true. The invention of highly accurate clocks may have been the first step towards fascism -- but it was also an important step towards civilization as we know it. And now you know what time it is, so I'll let you go spring your clocks forward :).

-- Badtux the Timely Penguin