1 .
Directions: Fill in each blank with a proper word chosen from the box. Each word can be used only once. Note that there is one word more than you need.
A. emerge B. absorbing C. subject D. defining E. movement
F. originally G. course H. universally I. happens J. constant K. corresponds |
How Long Is a Second?
The length of a second depends on how you’re measuring it. There are 24 hours in a day, 60 minutes in an hour, and 60 seconds in a minute — so surely a second is 1/86400, of a day, right? Well, it turns out that 1 time isn’t that simple.
“The second was 2 based on the length of the day,” Peter Whibberley, a senior scientist at the National Physical Laboratory in the U.K., told Live Science. “People observed the sun passing overhead and started measuring its 3 using sundials (日晷). However, sundials have a few disadvantages. Aside from the obvious problem of not being able to read a sundial when the sun isn’t visible, relying on Earth’s daily turning is surprisingly inaccurate. “The turning is not precisely 4 ,” Whibberley said. “The Earth speeds up and slows down over time.” So how can we precisely measure time if using the length of a day is so unreliable?
In the 16th century, people turned to technological solutions to this problem, and the first recognizable mechanical clocks began to 5 . The earliest mechanical clocks, which were designed to click at a specific frequency, averaged over the 6 of a year.
By around 1940, quartz crystal clocks (石英钟) had become the new gold standard. However, problems arose, and this was where atomic clocks came in. “Atoms exist only in particular energy states and can only change from one state to another by 7 or giving out a fixed amount of energy,” Whibberley explained. “That energy 8 to a precise frequency, so you can use that frequency as a reference for time keeping.” The astronomical second continued to vary. Every few years, scientists must add a second to allow Earth’s slowing turning to keep up with atomic time.
In fact, scientists are discussing whether it’s time to redefine the second again. But while several important questions still need to be answered before this 9 , it’s clear that the strictly correct definition of a second is 10 to change.
