2. Winds Change the Length of a Day Through the Year, and El Niño Adds a Bump
Over a year, the length of a day rises and falls by about 1,000 microseconds, which is one millisecond. A NASA-hosted research abstract (Dickey and colleagues, 1993) notes that earlier work had already accounted for at least 95 percent of this yearly swing by spin traded between the atmosphere and the solid Earth[2]. As winds around the planet strengthen, weaken or change direction through the seasons, the air holds more or less spin, and the ground under it takes the opposite share.
Some years stand out. El Niño is a phase of a climate pattern in which the surface of the central and eastern Pacific Ocean becomes warmer than usual. The Paris Observatory's Earth-rotation team reports that it strengthens the east-west winds high in the lower atmosphere, so the air gains spin and the solid Earth slows down slightly. In the winter of 2015 to 2016, a strong El Niño showed up directly in the day length, which differed by about 0.5 milliseconds from its usual seasonal pattern[3]. In other words, you can see a weather pattern in how long a day lasts.
3. The Moon's Position and Big Earthquakes Nudge the Length of a Day Too
The Moon matters, though not only through the tides. A report from the website timeanddate, relayed by EarthSky, says the Earth spins more quickly when the Moon is far north or far south of the equator. The source does not explain the mechanism, so this article does not either[5].
Earthquakes move rock, and moving rock shifts weight. NASA's Jet Propulsion Laboratory calculated that Japan's 2011 earthquake (magnitude 9.0) shortened the day by about 1.8 microseconds, which is millionths of a second, and moved the Earth's figure axis by about 17 centimeters (6.5 inches). The 2004 Sumatra earthquake would have shortened it by 6.8 microseconds[1]. These are tiny next to the yearly swing of 1,000 microseconds. They are best thought of as proof that even a rock shifting underground leaves a trace in the spin.
4. Melting Ice Slows the Earth, and the Liquid Core Has Been Speeding It Up
Slower changes build up over decades. Duncan Agnew of the Scripps Institution of Oceanography, writing in Nature in 2024, points to two of them, according to a University of California San Diego news story. The first is the melting of land ice at high latitudes. As that water runs into the oceans and spreads toward the equator, more weight sits farther from the axis, and, like a skater extending arms, the Earth turns more slowly. The story says that before 1990 the trend was toward a faster spin, but since 1990 it has reversed and made the Earth rotate more slowly[4].
The second is the liquid core, a mass of molten iron inside the solid planet. The same story says the core's rotation has been slowing, passing spin to the solid Earth. As a result, a day is now 0.0025 seconds shorter than it was 50 years ago (a day in 1972 was 2,500 microseconds longer)[4]. Ice and core pull in opposite directions, and the leap-second schedule depends on which one wins.
5. Since 2020 the Days Have Been Getting Shorter, and No One Can Fully Explain It
Until 2020, the shortest day atomic clocks had ever recorded was 1.05 milliseconds under 86,400 seconds. Then short days started to pile up, and July 5, 2024 broke the record at 1.66 milliseconds[5].
Why? The EarthSky and timeanddate report says that, as of now, no one knows for sure. It quotes Leonid Zotov of Moscow State University: "Nobody expected this. The cause of this acceleration is not explained." Most scientists suspect something inside the Earth, the report says, while ocean and atmospheric models do not account for such a large speed-up[5]. So the winds, the El Niño years, the Moon, the ice and the earthquakes listed above are real, but they do not seem to be the whole story of this trend.
Because the days are getting shorter, Agnew suggested that a negative leap second, which would make one minute one second shorter, might be needed some time in 2028. The melting ice has so far been putting this off[4].
Try It Yourself: Spin Sharing on a Swivel Chair, Then Look at the Data
- Spin on a chair. Sit on a swivel chair, hold your arms out, and have an adult turn you slowly. Pull your hands in to your chest and notice the change. Do not hold anything heavy. Go slowly, and stop at once if you feel dizzy.
- Look at real numbers. The Paris Observatory's page on El Niño, listed below, includes a plot of day length. Does the line look flat? Compare it with the 1-millisecond swing described here.
- Follow the open question. Search for "length of day" together with "2020 acceleration" and see how new papers try to explain it. The honest answer so far is "not settled."