Navigate using the Stars

For centuries, travelers use the stars to guide them on their journey. How is it possible to know where you are just by looking up the sky? The sky is constantly in motion because we’re in motion. Every day we go spinning around the earth at 800 miles per hour like a giant roller coaster. Now you get the same relative motion if instead of the earth rotating, you have the sky rotating around the earth. This is what you’ll see if you track the night sky. The Earth’s axis points at the north star. So the sky rotates around it. When you’re at the North Pole the North Star is directly overhead. As you move south it moves closer and closer to the horizon until at the equator it’s right on the horizon. To find your latitude, just grab a sextant and measure the angle between the horizon and the North Star and that’s your latitude! Longitude is much harder to calculate because of how the sky changes over time. Longitude and time are closely linked as your longitude determines your time zone. Suppose you live in New York and you observe the sky at midnight. In two and a half hours, the earth will rotated such that Salt Lake is now where New York was. So the sky New York at midnight is almost identical to the sky in Salt Lake two and a half hours later. So you can only determine your longitude if you know what time it is. This is simple enough today but was actually quite difficult in the ancient world. Astronomers didn’t crack the problem until the 17th century. The telescope was invented in 1608 by a group of dutch spectacle makers. Just two years later, Galileo used this new device to discover four moons orbiting the planet Jupiter. The moons orbit at regular intervals like clockwork. In fact the moons provided an excellent way of measuring time which the astronomers then used to measure longitude. With this new knowledge, astronomers redrew the world maps. Here is a map made by Mercator before these innovations. If we shift to a modern map, notice how much it’s changing East to West, not North-South because of all the troubles computing longitude. Also notice how much smaller Europe is getting. Astronomers all tended to exaggerate the size of Europe before they knew any better. Louis XIV complained that he had lost more territory to his astronomers than to any of his enemies after they had taken about a fifth of his kingdom. In 1676, the danish astronomer Ole Romer discovered that the moons of jupiter appeared to be several minutes behind schedule when Jupiter was far from the earth he realized that he could explain this delay if it took light about 40 minutes to travel from Jupiter to the earth. This was the first evidence that light had a finite speed. While there were several methods for keeping time on land, They didn’t work well at sea. Pendulum clocks got thrown off by the motion of the waves. The metal parts would contract in the cold and expand in the heat. The problem of longitude plagued navigators for centuries. The British government offered a small fortune to anyone who could come up with a solution. The prize was won by English carpenter named John Harrison who spent over 30 years perfecting his chronometer an ingenious device that would revolutionize navigation. And we still use clocks to navigate. In the 1980s, the US military launched a fleet of satellites. Each satellite contains a very accurate atomic clock which it uses to constantly broadcast what time it is. The signal then travels to the ground. Let’s suppose it took a tenth of a second to reach the ground. Multiply that time by the speed of light and you find that the satellite is 19,000 miles away. So this tells us that we are now on a sphere 19,000 miles from the satellite. If we repeat this with several satellites, we can intersect the spheres and pinpoint a precise location. And that’s how your GPS works!