Have you ever felt that your day has been very long? Has it ever happened that you wanted to just rush back home and crash on your bed but it never seemed that the night was coming anytime soon? Well, it may not be completely because of your tired mind playing tricks. You can put part of the blame on the Earth and part of it on our nearest neighbor in space, the Moon. Thanks to the interactions that these two have been having since their birth, the Earth's rotation is slowing down and our days have been growing increasingly long, albeit at a pace which is unnoticeable in a single lifetime. To get a hang of how slow this process is, let's have a look at some numbers. An average day in the Devonian Period, approximately 420 million years ago, was around 21.8 hours long. That's around 8000 seconds shorter than our present length of 24 hours. After all the mathematics, the Earth is losing approximately 20 seconds every million years. If that is not slow, I don't know what is.
Let's first answer this primary question. Why does the Earth rotate in the first place? We all take it for granted but we never really do ask why it happens. Well, simply put, the Earth's rotation is a remnant of the angular momentum of the original dust and debris that formed the Solar System. This dust and debris coalesced to form planets which gained this momentum and started rotating as a result. Other factors contributing to the rotation also include the hypothetical impact that formed the Moon and mantle-crust interactions as well as tidal action.
So why this downward trend in the speed of rotation? As I mentioned above, we do not have to look far for an answer. Our own Moon and its gravitational pull are culpable. We all have studied back in our school days that the attraction of the Moon and the Sun are the reason behind the tides that rock our shores four (two low and high tides respectively) or two (one low and high each) times a day. So far so good. We assume that the water of the oceans are pulled directly towards the Moon. This is where we go wrong.
The Earth's rotation drags this tidal bulge a little ahead of the position directly under the Moon. Now, this tidal bulge contains a sizable amount of mass of itself and this mass does not lie on the line that joins the gravitational centers of the Earth and the Moon. As a result, there exists a torque (in simpler terms, a rotational force) between the two bodies. This torque acts in the direction opposite to the Earth's rotation and decelerates it while at the same time, further increases the speed of the Moon in its orbit.
The Moon is not the lone culprit. The Earth's crust and its molten interiors contribute to this slowdown as well. Any interaction between the solid crust & mantle and the liquid core results in a drag which makes the Earth go that tiny bit slower. Major tectonic activities have also played their part, although not always to decrease the speed. For example, the Indian Ocean Tsunami on 26th December 2004 which was a result of the Sumatran Earthquake, cut 6.8 microseconds off from our day and no one (except some scientists) was the wiser.
You must be wondering how, apart from making your day even longer than it seems to be, does this affect your daily life? How does an increase in the day's length by two-thousandth of a second change anything? You would be surprised by how much.
Ever used a GPS to track your way through the maze that you call a city? Now, the GPS signals that help you pinpoint your location travel at the speed of light - 30,00,00,000 metres per second. This is roughly equal to 0.4 metres in one nanosecond. Imagine if you let the time go uncorrected and let the GPS satellite and the receiver on the ground go out of sync by that two-thousandth of a second each day, it will be a miracle if the GPS even shows you standing in your own city. Even as you are reading this blog, perfectly timed transmissions of data from all over the world come together and form this page without a glitch. A microsecond slip in this transmission and all you would be seeing right now would be a colourful mess.
Another peculiar phenomenon, predicted to occur billions of years into the future, is tidal locking. A tidally locked body rotates around itself and revolves around its neighbour at the same speed causing one side of the former to constantly face the central body. The best example of such a body is the Moon whose far side never is seen from the Earth. But, if this deceleration in Earth's rotation goes on, even the Earth will become locked and then only one side of the Earth will face the Moon. The Moon will be over a single place permanently in such a scenario.
To correct this and prevent this mess from happening (no one can prevent the tidal locking; I meant the GPS errors), there is a leap second, much like the leap year, which is occasionally added to keep the length of the day in check and not let it drift away from the atomic time (which is widely used in all the systems now). You can read more about it here :
http://en.wikipedia.org/wiki/Leap_second
Well, not to make your day any longer than it already has been, I'll finish this here and let you ponder over what you have read. Wishing you all the best of luck. Longer days lie ahead....
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| An artist's rendition of the protoplanetary disk and the early Solar System |
Let's first answer this primary question. Why does the Earth rotate in the first place? We all take it for granted but we never really do ask why it happens. Well, simply put, the Earth's rotation is a remnant of the angular momentum of the original dust and debris that formed the Solar System. This dust and debris coalesced to form planets which gained this momentum and started rotating as a result. Other factors contributing to the rotation also include the hypothetical impact that formed the Moon and mantle-crust interactions as well as tidal action.
So why this downward trend in the speed of rotation? As I mentioned above, we do not have to look far for an answer. Our own Moon and its gravitational pull are culpable. We all have studied back in our school days that the attraction of the Moon and the Sun are the reason behind the tides that rock our shores four (two low and high tides respectively) or two (one low and high each) times a day. So far so good. We assume that the water of the oceans are pulled directly towards the Moon. This is where we go wrong.
 |
| The tidal offset as a result of the Earth's rotation |
The Earth's rotation drags this tidal bulge a little ahead of the position directly under the Moon. Now, this tidal bulge contains a sizable amount of mass of itself and this mass does not lie on the line that joins the gravitational centers of the Earth and the Moon. As a result, there exists a torque (in simpler terms, a rotational force) between the two bodies. This torque acts in the direction opposite to the Earth's rotation and decelerates it while at the same time, further increases the speed of the Moon in its orbit.
The Moon is not the lone culprit. The Earth's crust and its molten interiors contribute to this slowdown as well. Any interaction between the solid crust & mantle and the liquid core results in a drag which makes the Earth go that tiny bit slower. Major tectonic activities have also played their part, although not always to decrease the speed. For example, the Indian Ocean Tsunami on 26th December 2004 which was a result of the Sumatran Earthquake, cut 6.8 microseconds off from our day and no one (except some scientists) was the wiser.
You must be wondering how, apart from making your day even longer than it seems to be, does this affect your daily life? How does an increase in the day's length by two-thousandth of a second change anything? You would be surprised by how much.
Ever used a GPS to track your way through the maze that you call a city? Now, the GPS signals that help you pinpoint your location travel at the speed of light - 30,00,00,000 metres per second. This is roughly equal to 0.4 metres in one nanosecond. Imagine if you let the time go uncorrected and let the GPS satellite and the receiver on the ground go out of sync by that two-thousandth of a second each day, it will be a miracle if the GPS even shows you standing in your own city. Even as you are reading this blog, perfectly timed transmissions of data from all over the world come together and form this page without a glitch. A microsecond slip in this transmission and all you would be seeing right now would be a colourful mess.
Another peculiar phenomenon, predicted to occur billions of years into the future, is tidal locking. A tidally locked body rotates around itself and revolves around its neighbour at the same speed causing one side of the former to constantly face the central body. The best example of such a body is the Moon whose far side never is seen from the Earth. But, if this deceleration in Earth's rotation goes on, even the Earth will become locked and then only one side of the Earth will face the Moon. The Moon will be over a single place permanently in such a scenario.
 |
| Only one side of the Moon faces us due to the equal length of its rotation and revolution time. |
To correct this and prevent this mess from happening (no one can prevent the tidal locking; I meant the GPS errors), there is a leap second, much like the leap year, which is occasionally added to keep the length of the day in check and not let it drift away from the atomic time (which is widely used in all the systems now). You can read more about it here :
http://en.wikipedia.org/wiki/Leap_second
Well, not to make your day any longer than it already has been, I'll finish this here and let you ponder over what you have read. Wishing you all the best of luck. Longer days lie ahead....
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