The rotation speed of the Earth is not perfectly constant, being influenced by the mass distribution of the Moon and the planet, this means that the duration of the terrestrial days can vary, with a difference of longer or shorter microseconds than the classic 24 hours and recently, the shortening of our days has reversed, with no clear explanation of why.
The news that the length of Earth days is varying with the latter getting longer seems to be contradicted by the fact that June 29 this year was the shortest day ever since atomic clocks and pulsars have given us the ability to accurately measure their length.
On average, however, our days have shortened until 2020 and have lengthened since then, with June 29 being an aberration; this is puzzling to planetary scientists, because the change is the fastest in the last 50 years since we have had the ability to measure the rotation of the Earth so precisely and we don’t know why.
Some of the forces that alter the length of the day are well understood, for example the interaction between the Earth and the Moon driving the tides is slowly draining energy from the system and causing the Earth to slow down, whereas when dinosaurs ruled the Earth, the days they were half an hour shorter and, in the long run, this trend will continue, and eventually, our days will be longer than those of Mars (24 hours, 37 minutes and 22 seconds).
There are also known short-term factors. Professor Matt King and Dr Christopher Watson of the University of Tasmania explain in The Conversation the Earth works like an ice skater that spins faster when you put your hands in your chest. It’s the only way they can keep their angular momentum. Anyone wishing to experience this process in action without some ice on hand can use weights and a swivel chair, but don’t blame us for the nausea if you go too fast.
How the duration of earthly days in history has reversed
Since the end of the last ice age, melting glaciers have reduced the pressure at the poles. This not only caused an isostatic rebound, visible in the rise of continents that no longer carry as much weight, but also caused the mantle to redistribute from the equator to the poles. This provides a contrasting force to that of the Moon, causing the planet’s rotation to accelerate. Between 1972 and 2020 the average day lost about 3 milliseconds.
Planetary mass distribution can occur in more irregular ways as earthquakes move mass towards or away from the poles, lengthening or shortening the days accordingly. Time also has an effect, King and Watson note. Large storms that dump a lot of rain near the equator slow down the rotation. Snow events in higher latitudes have the opposite effect until the water returns to the sea.
“We can see tidal changes in day length records over periods up to 18.6 years”
add King and Watson.
However, when you add up all the known effects – those that speed up the Earth and those that slow it down – they don’t add up to recent observations. Something else is happening, but we don’t know what.
Accelerated polar melting, the effects of the giant Tonga eruption and the extended La Niña events have been proposed, but King and Watson consider them unlikely.
Whatever the cause, the slowdown should be welcomed by big tech companies, which have become increasingly vocal activists against leap seconds, disrupting their timekeeping systems.
So far the world hasn’t needed to add a negative leap second, jumping from 23:59:58 straight to midnight, but this may be necessary if we get a sufficiently short series of days. These are expected to be even more disruptive and the recent slowdown should delay the need for one.
If you are attracted to science or technology, keep following us, so you don’t miss the latest news and news from all over the world!
#Duration #Earth #days #years