Precious Time: The Challenge of Building a Better Atomic Clock
Prior to the mid-18th century, it was tough to be a sailor — you couldn’t set out for a specific destination and have any real hope of finding it quickly if the trip required east-west travel.
At the time, sailors had no reliable method for measuring longitude, the coordinates that measure how far east and west one is from the international dateline. Longitude’s key was accurate timekeeping, as the English watchmaker John Harrison knew, and clocks just weren’t accurate yet.
By the 1700s, sailors had figured out they could measure latitude by studying the sun and its location at various times of the day, so north-south travel was not so problematic. However, the place where longitude equals zero, known as the International Date Line, does not have a basis in nature. As evidenced by several relocations of the prime meridian, located in Greenwich, England since 1884, its placement is arbitrary. After all, who’s to say whose daybreak starts the Earth’s next rotation? [Atomic Clock Is So Precise It Won’t Lose a Second for 15 Billion Years ]
“How you define time is pretty much arbitrary in the sense that in the past we defined a year by using how long it takes the earth to rotate around the sun,” Simien said. “So, basically, any periodic, consistent motion can be the basis for a clock. I used to joke with my relatives that I can say that time is how long it takes me to walk up and down five flights of stairs, while eating a bag of Doritos. But that wouldn’t be a good definition of time. Some days I might be tired, so I move slower. You wouldn’t want to base time on something that can vary so much.”
Sailors figured out that as they traveled east, time moved ahead — the sun set earlier than expected, for example. In fact, based on current parameters for time, for every 15 degrees of longitude a person moves east, the local time moves ahead an hour. That meant longitude could be grossly measured by contrasting the time of day from two places: a ship’s location and its departure port. But, like climbing stairs while eating chips, such measurements also require standards, which for those sailors meant building a clock from materials that didn’t rust and didn’t swell or contract with heat and cold, preserving a reference for the time “back home.”
Harrison, that English watchmaker, put together a clock of wooden wheels — replacing the prior state-of-the-art, a pendulum, with something called a grasshopper escapement, which on its first voyage in 1736 helped identify a 60-mile course divergence for his ship. As a result, he won the Longitude Prize for building the first compact marine chronometer.
The quest to improve timekeeping continues today, as scientists look at new materials that are even more rugged and precise, eliminating variables that might distort accurate timekeeping.
Atomic clocks in GPS satellites work with ground-based clocks so that positioning signals are synchronized as much as possible. Atmospheric distortions present challenges that can limit signal accuracy beyond the most precise atomic clock’s scope. So, while the U.S. Air Force operates the more than 30 GPS satellites in orbit, several government agencies, including NSF, the U.S. National Institute of Standards and Technology, the U.S. Department of Defense, and the U.S. Navy are invested in atomic clock research and technology.
Source : Livescience