Even though the Earth recently had the shortest day/night period, on average the Earth’s days have recently become longer, but the mechanism remains somewhat puzzling.
The pace at which the Earth spins is not entirely constant, as it is influenced by the mass distribution of the Moon and planet. Days can therefore be microseconds longer or shorter than 24 hours. The shortening of our days has just reversed, with no evident cause.
The fact that days are becoming longer appears to contradict the news that they are getting longer, since June 29 this year was the shortest day since atomic clocks and pulsars given us the ability to properly calculate their length.
However, until 2020, our days were becoming shorter on average, and have been increasing longer since – June 29 was an outlier.
Planetary scientists are perplexed since the change is the quickest in the 50 years since we have been able to properly monitor Earth’s spin, and we don’t know why.
Some of the factors influencing the duration of the day are well established. The interaction between Earth and the Moon that drives the tides steadily drains energy from the system, slowing Earth. Days were half an hour shorter when dinosaurs dominated the Earth, and this tendency will likely continue in the long run. Our days will eventually be longer than Mars’ (24 hours, 37 minutes, and 22 seconds).
Shorter-term influences are also known. Professor Matt King and Dr Christopher Watson of the University of Tasmania explain in The Conversation that the Earth works like an ice skater who spins faster when they bring their hands into their chest. It’s the only way they can keep their angular momentum going. Anyone who wants to see this process in action without the use of ice may use weights and a spinning chair – but don’t blame us if you go too fast.
Glacial melting has lowered pressure at the poles since the end of the last Ice Age. This has created not just an isostatic rebound, as seen in the rising of continents that are no longer carrying as much weight, but it has also led the mantle to transfer itself from the equator to the poles. This acts as a counterforce to the Moon, forcing the planet’s rotation to accelerate. The average day lost around 3 milliseconds between 1972 and 2020.
The distribution of planetary mass can be more unpredictable when earthquakes transfer mass towards or away from the poles, causing days to lengthen or shorten proportionately. According to King and Watson, even the weather has an impact. Storms that dump a lot of rain near the equator halt rotation. Snowfall has the opposite effect in higher latitudes until the water returns to the sea.
“We can find tidal fluctuations in length-of-day data going back 18.6 years,” King and Watson explain.
However, when all of the known effects – those that speed up the Earth and those that slow it down – are tallied together, they do not add up to the recent measurements. Something else is going on, but we have no idea what it is.
Accelerated polar melting, the consequences of the massive Tonga eruption, and longer La Nia occurrences have all been hypothesized, but King and Watson believe they are all implausible.
Whatever the source, the slowdown should be welcomed by major technology firms, who have become more vociferous opponents of leap seconds, which disturb their timing systems. So far, the world has not required a negative leap second, jumping from 23:59:58 to midnight, but this may become necessary if we have a streak of sufficiently short days. These are projected to be far more disruptive, and the current downturn should put off the need for one.
Leave a Reply