Earth’s rotation is notoriously wobbly. It could alter the fate of the leap second

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When the sun hits its highest point over the meridian line that travels north and south from the 351-year-old Royal Observatory Greenwich in London, it’s noon Greenwich Mean Time, a tangible and physical reference point that helped establish the world’s time zone system.

Timekeeping has long relied on Earth’s spin and location of the sun or moon in the sky. Before observatories such as Greenwich, humans used sundials — essentially a stick in the ground that casts shadows as the sun moves overhead. But this ancient link between Mother Earth and today’s clocks looks set to be severed.

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International timekeepers are on track to vote Thursday on a proposal to eliminate what’s known as a leap second — a tiny, but many say important, link between Earth’s rotation and the world’s time standard used in critical infrastructure such as telecommunications, power grids and computers.

Inserted into global clocks 27 times since 1972 , the leap second essentially allows two time systems to run in sync: astronomical time, long anchored by Earth’s rotation and measured by observatories, and Coordinated Universal Time, or UTC, the global standard that’s set by ultraprecise atomic clocks first developed in the 1950s. These cutting-edge, lab-based clocks rely on tracking the natural vibrations — or ticks — of atoms.

The two time-telling methods differ because Earth does not rotate at a steady pace but wobbles, essentially making it a “lousy” clock, said Jeff Sherman, a group leader of the National Institute of Standards and Technology’s Time and Frequency Division in Boulder, Colorado.

This wobbling stems from various factors, such as the moon’s gravitational pull, shifts deep in the planetary core, and the movement of water and ice on Earth’s surface — an effect that’s exacerbated by the melting of the polar ice caps due to the climate crisis.

“We have to make a choice on whether or not the most important thing is a sense of time given by objects in the sky directly above, say, the Greenwich longitude,” he said referring to the Greenwich meridian that marks 0 degrees longitude. “Or you decide to follow what you believe are more stable, laboratory, man-made clocks,” Sherman added.

Leap seconds have served as the compromise between the two time-telling systems. Most people never notice them — unlike a leap year , which adds an extra day to February every four years to account for the discrepancy between the Gregorian calendar used in most parts of the world and the time it takes Earth to complete one orbit around the sun.

But the addition of leap seconds, which happens at irregular intervals and is announced with only six months’ notice, creates problems for operators of data servers, high-frequency financial trading systems and other infrastructure that rely on precise timekeeping to work properly.

The extra second crashed Reddit and LinkedIn and knocked out Qantas’ booking system in 2012. And when the most recent leap second was added at the end of 2016, customers of cloud computing provider Cloudflare briefly experienced outages or delays after its servers failed to handle the introduction of a leap second properly. Big tech players such as Meta have been calling for its demise, describing the leap second as “a major source of pain.”

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The 69 member states of the International Bureau of Weights and Measures, an intergovernmental organization that determines measurement standards, will meet this week in Versailles near Paris. The group, known by its French acronym BIPM, will vote on a resolution to ditch the leap second, a move that would take effect in May 2027. A majority needs to support the proposal, said Patrizia Tavella, director of the time department at BIPM.

When atomic time was first introduced, Tavella said, it was important to keep it tethered to astronomical time because navigational tools still relied on traditional celestial methods and timekeeping. With the advent of GPS, however, that was no longer the case. “This correction in time has become a serious issue for modern applications,” she added.

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Members of BIPM have discussed the switch to what Tavella called Continuous Coordinated Universal Time for more than 20 years, she noted. At the group’s last meeting in 2022, member countries voted in favor of a resolution that called for a plan to phase out the leap second on or before 2035.

In the past, some countries, such as Russia and the United Kingdom, have opposed ditching the leap second, she noted. Russia’s 2022 opposition stemmed from the fact that its Global Navigation Satellite System, or GLONASS, incorporates leap seconds, unlike other satellite navigation systems, such as the United States’ GPS and the European Union’s Galileo, that do not. The United Kingdom was once opposed because of its cultural attachment to the foundational role that Greenwich played in the history of time, but its stance has since changed, she added.

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Since the world’s timekeepers last met in 2022, the debate over whether to abolish the leap second has taken on an urgency because of the prospect of something unprecedented: It will soon be necessary to subtract rather than add one second, known as a negative leap second, according to experts.

The long-term trend of Earth’s rotation is slowing, but in the near term it appears to be speeding up, potentially requiring a subtraction of a second from Coordinated Universal Time to keep astronomical and atomic time matching.

A 2025 report by the International Earth Rotation and Reference Systems Service, a consortium headquartered in Frankfurt, Germany, tasked with tracking Earth’s rotation, estimated that the likelihood of a negative leap second would rise sharply in the coming years — hitting a probability of 30% by 2035, according to BIPM.

Deleting a second has never previously been attempted and could create havoc in today’s digitally time-stamped world. BIPM said the operators of critical infrastructure are “largely unprepared” for such a move. “The preparation would create, for the industries that rely on time synchronization, a need for financial investment, whose amount is estimated to be similar to their preparations for the millennium bug,” the organization said, referring to Y2K, a computer flaw that programmers thought might cause severe systemic problems after December 31, 1999.

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However, not everyone agrees that it’s necessary to ditch the leap second.

The technical challenges of incorporating leap seconds, whether negative or positive, into digital systems were overstated, according to Marek Ziebart, a professor of space geodesy at University College London who said there was value in keeping the world’s fundamental timescale aligned with Earth’s rotation.

“We have a system in place that uses leap seconds. It works. It maintains the timescale that all of us use on a day-to-day basis,” Ziebart said. “If you abolish the leap second, a lot of things will have to change. People will have to check their software, redesign it, fix it.”

He added that it was also “intellectually comforting” to maintain a timescale tied to Earth’s rotation. “There’s an intrinsic relationship for human beings between rotation and time,” he said. “It relates to us as animals. We’re driven by these periodicities of the sun rising, the sun setting.”

Instead of juggling positive and negative leap seconds, the BIPM resolution essentially proposes that atomic time would be allowed to drift gradually away from astronomical time. Doing so means official timekeeping would become independent of Earth’s rotation, and another leap second would never be added to clocks.

However, the proposal does recommend that the maximum difference between atomic and astronomical time should not exceed 3,600 seconds or one hour. It would take more than 3,000 years for such a difference to occur, BIPM’s Tavella said.

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The move essentially punts the problem to future generations, and they would have to decide whether or not to take a “leap hour,” said Louise Devoy, senior curator of the Royal Observatory Greenwich . Timekeeping could look dramatically different then. Perhaps today’s atomic clocks, tuned to the steady vibrations of cesium atoms, could one day seem as quaint as sundials.

“I can’t imagine us changing it so drastically that it then becomes noticeable in our daily lives. We still have our daily routines, our natural circadian rhythm, our need to be in daylight,” Devoy said.

For most, herself included, ditching the leap second would be a behind-the-scenes shift that would make little material difference. “It’s just the way that our lives have changed,” she said. “We’re so dependent on technology now.”

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当太阳到达从伦敦格林尼治皇家天文台（已有 351 年历史）向南北延伸的子午线上的最高点时，就是格林尼治标准时间中午 12 点，这是一个切实存在的物理参考点，它帮助建立了世界时区系统。

长期以来，计时一直依赖于地球自转以及太阳或月亮在天空中的位置。在格林尼治天文台等天文台出现之前，人类使用日晷——本质上就是一根插在地上的棍子，随着太阳的移动投射出阴影。但地球母亲与现代时钟之间的这种古老联系似乎即将断裂。

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考古学家称，巨石阵是古代的一种计时系统。

国际计时机构将于周四就一项提案进行投票，该提案旨在取消闰秒——闰秒是地球自转与世界时间标准之间的一个微小但许多人认为重要的联系，而世界时间标准被用于电信、电网和计算机等关键基础设施。

自 1972 年以来，闰秒已被 27 次插入全球时钟，其本质是使两个时间系统能够同步运行：天文时间，长期以来以地球自转为基准，由天文台测量；以及协调世界时 (UTC)，这是由 20 世纪 50 年代首次开发的超精密原子钟设定的全球标准。这些尖端的实验室时钟依靠追踪原子的自然振动——或称“滴答”声。

科罗拉多州博尔德市国家标准与技术研究院时间与频率部门的小组负责人杰夫·谢尔曼表示，这两种计时方法之所以不同，是因为地球的自转速度并不恒定，而是在摆动，这本质上使地球成为一个“糟糕的”时钟。

这种摆动源于多种因素，例如月球的引力、行星核心深处的位移以及地球表面水和冰的运动——而气候危机导致极地冰盖融化，加剧了这种影响。

“我们必须做出选择，究竟是选择格林尼治经线正上方天空中的天体所提供的时间感，还是选择遵循你认为更稳定、实验室里制造的人造时钟，”他指的是标志着0度经度的格林尼治子午线。

闰秒是两种计时系统之间的一种折衷方案。大多数人从未注意到过闰秒的存在——这与闰年不同，闰年每四年在二月增加一天，以弥补世界大部分地区使用的公历与地球绕太阳公转一周所需时间之间的差异。

但是，闰秒的增加（其间隔不规律，且仅提前六个月通知）会给数据服务器运营商、高频金融交易系统和其他依赖精确计时才能正常运行的基础设施带来问题。

2012年，闰秒的出现导致Reddit和LinkedIn崩溃，澳航的订票系统也一度瘫痪。而最近一次闰秒是在2016年底，云计算服务提供商Cloudflare的服务器未能正确处理闰秒的引入，导致其客户短暂遭遇服务中断或延迟。Meta等大型科技公司一直呼吁取消闰秒，称其为“一大痛点”。

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国际计量局（一个负责制定计量标准的政府间组织）的69个成员国本周将在巴黎附近的凡尔赛宫举行会议。该组织（简称BIPM）将就一项取消闰秒的决议进行投票，该决议将于2027年5月生效。BIPM时间部门主管帕特里齐亚·塔韦拉表示，该提案需要获得多数成员国的支持才能通过。

塔维拉说，原子时最初提出时，必须将其与天文时保持一致，因为当时的导航工具仍然依赖传统的天文方法和计时方式。然而，随着GPS的出现，情况就发生了变化。“这种时间校正已成为现代应用中的一个重要问题，”她补充道。

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她指出，国际计量局（BIPM）成员讨论向塔维拉所称的“连续协调世界时”（CCUT）过渡已有20余年。在2022年该机构的上次会议上，成员国投票通过了一项决议，呼吁制定一项计划，在2035年或之前逐步取消闰秒。

她指出，过去一些国家，例如俄罗斯和英国，曾反对取消闰秒。俄罗斯反对取消2022年冬奥会闰秒的原因在于，其全球导航卫星系统（GLONASS）包含闰秒，而其他卫星导航系统，例如美国的GPS和欧盟的伽利略系统，则不包含闰秒。她补充说，英国曾经反对取消闰秒，是因为其文化上对格林尼治在时间史上的奠基性作用有着深厚的感情，但其立场后来发生了改变。

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自世界计时机构上次于 2022 年举行会议以来，关于是否取消闰秒的辩论变得紧迫起来，因为出现了一种前所未有的情况：据专家称，很快就需要减去一秒而不是增加一秒，这被称为负闰秒。

地球自转的长期趋势正在减缓，但短期内似乎正在加快，可能需要从协调世界时中减去一秒，以保持天文时间和原子时间一致。

总部位于德国法兰克福的国际地球自转和参考系统服务联盟（负责跟踪地球自转）在 2025 年发布的一份报告中估计，未来几年出现负闰秒的可能性将急剧上升——据国际计量局 (BIPM) 称，到 2035 年，负闰秒的概率将达到 30%。

删除一秒时间从未有人尝试过，这在如今这个数字化时间戳时代可能会造成严重混乱。国际计量局（BIPM）表示，关键基础设施的运营者“基本上没有为此做好准备”。该机构指出，“对于依赖时间同步的行业而言，准备工作将需要大量的资金投入，预计其数额与他们当年应对千年虫问题的准备工作相当。”这里提到的千年虫问题，是指程序员们认为可能在1999年12月31日之后引发严重系统性问题的计算机漏洞。

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然而，并非所有人都认为有必要取消闰秒。

伦敦大学学院空间大地测量学教授马雷克·齐巴特表示，将闰秒（无论是负闰秒还是正闰秒）纳入数字系统的技术挑战被夸大了，他认为保持世界基本时间尺度与地球自转一致是有价值的。

“我们目前采用闰秒制度，而且这套制度运行良好，能够维持我们日常使用的时间尺度，”齐巴特说。“如果取消闰秒，很多事情都必须改变。人们需要检查他们的软件，重新设计，并进行修复。”

他还补充说，保持与地球自转相关的时间尺度也“令人感到理智上的慰藉”。他说：“对人类而言，自转和时间之间存在着内在联系。这与我们作为动物的身份有关。我们受日出日落这些周期性规律的驱使。”

国际计量局的决议并非简单地调整正负闰秒，而是提议允许原子时逐渐偏离天文时。这样做意味着官方计时将不再受地球自转的影响，时钟也将不再添加新的闰秒。

然而，该提案建议原子时间和天文时间之间的最大差异不应超过3600秒或1小时。国际计量局的塔韦拉表示，出现这样的差异需要3000多年。

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格林尼治皇家天文台高级馆长路易丝·德沃伊表示，此举实际上是将问题推给了后代，他们将不得不决定是否采用“闰小时”。届时，计时方式可能会截然不同。或许，如今利用铯原子稳定振动来计时的原子钟，有一天会像日晷一样显得古老而简陋。

“我无法想象我们会做出如此巨大的改变，以至于在我们的日常生活中产生明显的影响。我们仍然有日常作息、自然的昼夜节律，仍然需要接触日光，”德沃伊说。

对大多数人来说，包括她自己，取消闰秒只是幕后发生的改变，不会带来实质性的影响。“这就是我们生活方式的改变，”她说，“我们现在太依赖科技了。”