Why Have Earth Days Been Randomly Changing in Length?
- ( ¿Por qué los días terrestres cambian aleatoriamente de duración?)
We all know that a single Earth day technically contains 86400 seconds. This has been known since
the year 1000 when Muslim scholars subdivided the mean solar day into 24 hours and the corresponding
minutes and seconds. But, as soon as our instruments improved, we discovered the Earth’s
rotation period varied irregularly, at something like a fraction of a second each year. We will
see how that is possible later in this video, but for now…imagine if our clocks were off by
just a few seconds. Over time, those few seconds can add up and cause significant discrepancies
in timekeeping. That's why scientists needed to define the concept of a "leap second," to
ensure that our clocks are as precise as possible. To put it into perspective, a leap second is like
a leap year, but on a much smaller scale. A leap year is added every four years to
account for the fact that the Earth's orbit around the sun isn't exactly 365 days long.
So just like how we add an extra day in February every four years, we also add an
extra second to our clocks as needed to account for the minute changes in the Earth's rotation.
Some people might think that being born on a leap year is unlucky
since they only get to celebrate their actual birthday once every four years.
But let's be honest, they get to age four times slower than the rest of us,
so maybe it's not so bad after all! But all jokes aside, the gradual slowdown of
the Earth's rotation is a real phenomenon that has important implications for our planet's climate,
geography, and even timekeeping. So let's dive into the science behind it all,
and explore what's causing our planet to change its rotation velocity over time.
Let’s go!
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Leap second The definition of a leap second is crucial
for accurate timekeeping on Earth, as it helps to align atomic time with the Earth's rotation.
All of this is really important because, hey, you don’t want to
be 1 second late to your job interview at NASA! The Earth's rotation speed changes irregularly,
and randomly, and this unpredictability is why leap seconds are announced only six months in
advance. Physicists Stephenson and Morrison developed a mathematical model that shows a
steady increase in the mean solar day by 1.70 ms per century, while the rate of lengthening of the
mean solar day has been about 1.4 ms per century over the last few centuries, which is the sum
of the periodic component and the overall rate. One thing is clear from the model: the Earth is
slowing down. But why? How is that even possible? The primary reason for the slowing down of the
Earth's rotation is tidal friction, which would lengthen the day by 2.3 ms (milliseconds) per
century. Other contributing factors are the Earth's crust movement relative to its core,
changes in mantle convection, and any other events or processes that cause a significant
redistribution of mass. These processes alter the Earth's moment of inertia, which affects
the rate of rotation due to the conservation of angular momentum. Some redistributions increase
the Earth's rotational speed, shorten the solar day, and oppose tidal friction.
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How the Moon slows down the Earth? Let’s focus on tidal friction.
What is it? And how does it exactly slow down the Earth by 2.3 ms per century?
Tidal friction occurs when there is an interaction between two objects, such as the Earth and the
Moon. The gravitational forces from the Moon create tides in the oceans, causing them to rise
and fall twice a day. As the tides move across the Earth's surface, they create friction which
generates heat. This heat causes the Earth's rotation to slow down gradually over time.
As the tides move across the Earth's surface, they rub against the land and create heat.
This heat then causes the Earth's rotation to slow down. It can be hard to imagine how tidal
friction does it, but let me simplify it down to a very familiar example.
If you've ever turned a bicycle upside down and spun one of its wheels,
you could slow down how fast the tire spins by putting your hand on the tire. Tidal friction
is like a break applied to Earth's spin, and it lengthens our days as a result.
Tidal friction is not magic, but rather a natural process that is governed by the laws of physics.
It is important to note that the amount of friction generated by the tides depends
on a variety of factors, such as the size and distance of the moon, and the shape of the Earth.
This phenomenon is not only important for understanding the Earth's rotation but also
for the presence of life on our planet. Without the tides created by the Moon,
the oceans on Earth would be stagnant and life as we know it might not have evolved. The tides
provide essential nutrients and oxygen to marine life and help regulate the Earth's climate.
The Earth-Moon system is not the only one generating tidal friction. Other famous
examples are the moons and planets in our solar system. For example, we know the moons of Jupiter
and Saturn experience strong tidal friction, which causes them to have active volcanoes and geysers.
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How does the ice change the Earth's rotation? But this is not the end. Sometimes,
the Earth likes to speed up its rotation too! For instance, let me tell you what happened 20
thousand years ago, with the end of the ice age. The melting of ice from glaciers and
polar ice caps leads to a redistribution of mass on the Earth's surface, which affected
the planet's rotation. As the ice melted, the water was flowing toward the oceans, causing
a shift in the Earth's mass distribution, thus, changing the moment of inertia of the Earth. This
change in moment of inertia affected the overall rotation of our planet, causing it to speed up.
According to a study published in the journal Science Advances, the loss of ice mass in
Greenland and Antarctica has caused an increase in the Earth's rotational speed. The researchers
found that the melting of ice from these regions has led to a shift in the position of the Earth's
axis, causing it to tilt more towards the east. This shift has increased the Earth's
rotational speed, shortening the length of a day by about 1.8 milliseconds over the last century.
What’s really interesting is that…we, as humans… can change the rotation rate of the Earth.
And it’s not necessarily a good way of doing that. Think about the melting of
ice due to climate change. It can have a similar effect on the Earth's rotation,
although the impact will be relatively small. A study published in the journal Science Advances
found that the melting of glaciers and ice caps due to climate change could cause the Earth's
rotation to speed up by about 12 microseconds per year. While this is a small effect,
it could add up over time and have a significant impact on the length of a day in the future.
--- Is there anything else you want to know about
climate change and its effects on Earth’s physical parameters? Let us know in the comments below!
--- How do
earthquakes influence the rotation of the Earth? Earthquakes can also influence the duration of
the day on Earth but in a different way. When an earthquake occurs, it can cause a redistribution
of mass on the Earth's surface. This change in mass distribution affects the Earth's
moment of inertia, which can either speed up or slow down the rotation of the Earth.
For example, the magnitude 9.0 earthquake that struck Japan in 2011 caused enough mass
redistribution to shorten the length of a day by about 1.8 microseconds,
or about 1.8 millionths of a second. This is because the earthquake shifted a significant
amount of mass from the Earth's equator to its poles, which made the Earth spin slightly faster.
However, not all earthquakes have this effect. The size, location, and type of earthquake all
play a role in how much mass is redistributed and how it affects the Earth's rotation.
Scientists are still studying this phenomenon to
better understand how earthquakes can influence the rotation of our planet.
--- The shortest day on Earth
At this point, you might have one big question left: if the duration of days on Earth changes
randomly, what’s the shortest day ever recorded? Well, this is indeed a really good question.
And the answer is: probably, it happened on June 29, 2022.
The duration of that day was 1.6 milliseconds under 24 hours, making
it the shortest day we know of in history. This really points out something crucial:
the Earth’s changing in rotation depends on a variety of factors,
and the random combination of some of them can have different outcomes on the length of a day:
it could make it at times shorter, at times longer. Will the shorter day add
up over a long period of time, we might need to use a “negative” leap second.
--- Negative leap second
In fact, it is vital to note that leap seconds are not indicators of a slowing of the Earth's
rotation rate. Instead, they are indicators of the accumulated difference between atomic time
and time measured by Earth's rotation. The average length of the day in 1972
was approximately 86400.003 seconds, while in 2016, it was approximately 86400.001 seconds,
indicating an overall increase in Earth's rotation rate over that time period. Positive leap seconds
were added during that time because the annual average length of the day remained greater
than 86400 International system seconds, not because of any slowing of Earth's rotation rate.
Recent reports show that Earth was spinning faster in 2020 and experienced
the 28 shortest days since 1960, each lasting less than 86399.999 seconds. That’s what led
engineers worldwide to discuss a negative leap second and other possible timekeeping
measures that could eliminate leap seconds. This, in turn, highlights the importance of
accurately defining a leap second to ensure the precision of timekeeping on Earth.
--- Hey!
This video ends here! What do you think about the change in the Earth’s rotation?
Let us know in the comments below! I’ll see you next time on the channel.