Perhaps the greatest， mind-bending quirk of our universe is the inherent trouble with timekeeping： Seconds tick by ever so slightly faster atop a mountain than they do in the valleys of Earth. For practical purposes， most people don’t have to worry about those differences.
也许我们宇宙中最令人困惑的奇异现象就是关于计时的固有难题：地球上，时间在山顶会比在山谷过得略快。从实际用途来看，大多数人无需担心这些差异。
On the lunar surface， a single Earth day would be roughly 56 microseconds shorter than on our home planet—a tiny number that can lead to significant inconsistencies over time. NASA and its international partners are looking to create an entirely new “time scale，” or system of measurement that accounts for that fact that seconds tick by faster on the moon.
在月球表面，一个地球日会比在地球上短大约56微秒，虽然这个数字很小，但随着时间的推移会导致显著的不一致。美国国家航空航天局（NASA）及其国际合作伙伴正在寻求创建一个全新的“时间尺度”或测量体系，以应对月球上时间走得更快这一事实。
Such a framework will be crucial for humans visiting our closest celestial neighbor. Astronauts on the moon， for example， are going to leave their habitats to explore the surface and carry out science investigations. They’re also going to be communicating with one another or driving their moon buggies while on the lunar surface. “When they’re navigating relative to the moon，” said Cheryl Gramling， the lunar position， navigation， and timing and standards lead at NASA’s Goddard Space Flight Center in Maryland， “time needs to be relative to the moon.”
这样的框架对人类探访宇宙中我们最近的邻居至关重要。例如，月球上的宇航员将离开他们的驻地，去探索月球表面并进行科学调查。他们还需要在月球表面相互通讯或驾驶月球车。“当他们在相对于月球的位置进行导航时，时间也需要以月球的为准。”美国马里兰州戈达德航天飞行中心的月球定位、导航与授时系统标准化项目负责人谢丽尔·格拉姆林说。
A brief history of Earth time
地球时间简史
Simple sundials or stone formations， which track shadows as the sun passes overhead， mark a day’s progression just as the shifting phases of the moon can log the passing of a month on Earth. Those natural timekeepers have kept humans on schedule for millennia. But perhaps since mechanical clocks gained traction in the early 14th century， clockmakers have grown ever more persnickety about precision.
简单的日晷或石堆通过追踪太阳在头顶移动所产生的影子来标记一天的进程，正像月亮的盈亏变化可以记录地球上一个月的流逝。这些天然计时器已让人类遵循着时间的轨迹度过了数千年。但可能从14世纪初机械时钟开始普及以来，钟表制造者们在精准度方面就变得越来越挑剔。
Exacting the measurement of seconds also grew more complicated in the early 1900s， thanks to Albert Einstein， the German-born physicist who rocked the scientific community with his theories of special and general relativity.
20世纪初，德国出生的物理学家阿尔伯特·爱因斯坦提出狭义和广义相对论，震撼了整个科学界，对秒的测量也变得更为复杂。
General relativity is complicated， but in broad terms， it’s a framework that explains how gravity affects space and time. Imagine that our solar system is a piece of fabric suspended in the air. That fabric is space and time itself， which， under Einstein’s theories， are inextricably linked. And every celestial body within the solar system， from the sun to the planets， is like a heavy ball sitting atop the fabric. The heavier the ball， the deeper the divot it creates， warping space and time. Even the idea of an earthly “second” is a humanmade concept that’s tricky to measure. And it was Einstein’s theory of general relativity that explained why time passes slightly more slowly at lower elevations—because gravity has a stronger effect closer to a massive object （such as our home planet）.
广义相对论很复杂，但大体上说，它是一个解释引力如何影响空间和时间的框架。
