The 1 second leap: Cause and Effect

Many of u may have heard bout the news that from Jan 1st 2009 onwards, 1 second will be added to our clock. And even more will think "what's so big deal?" Well, let me start by introducing some history to you fellow earthlings...let u ponder this question first. How long (in seconds) does the Earth takes to complete 1 rotation (earth completing 1 rotation; as in complete 1 day & 1 night)? Well, to fellow physicist, this must be a very easy questions; where u take 60 sec multiplied by 60 min & multiplies again by 24 hours in 1 day which = 86400 seconds. However, to be exact, in this present day, our Earth takes 86400.002 sec to complete 1 rotation. Well, that's still not much to you right? Add this all up for 1 whole year...which will give you approx 0.7 sec. Does this starts to make more sense where does the 1 sec comes from? According to our 'official Earth timekeepers', for approx every 18 months, the earth motion is slowed down by 1 sec. Let me intro with more; heard of earth quakes, tsunami, dinosours etc? i doubt you haven't. But do you know what's the connection between earth rotation speed and all these phenomena? Well there is. Now let the history begin....

Once upon a time, a beautiful babygirl, Earth was born. At that time, she was 'spinning' (or more to rotating) at 6.5 hours per day/night cycle which takes 63000 sec shorter than our present time. Earth, later at the age of about 1 billion years, she spins at 20.11 hours per day/night cycle. 100 000 000 years later again, she slows down to 23.6 hours. (FYI: Earth now is about 4.5 billion years old. Pretty old yea)

When the Earth was very young, its faster 'spinnings' caused it to have a much larger equatorial circumference than it has now. It may have been spinning fast enough to have a "Saturn type ring" around it. It is well known that the rotation of planet Earth is gradually slowing. For its entire lifetime, its rate of rotation has been gradually slowing. As the Earth loses its kinetic energy due to all forms of friction acting on it; tides, galactic space dust, solar wind, space weather, geomagnetic storms, etc, it will slow down (as what we learnt b4, friction will cause deceleration). The reason for adding a leap second is that the planet does not rotate exactly once every 24 hours (86,400 seconds). The rotation actually takes 86,400.002 seconds (as i mentioned above) so that each day this little difference builds up between the atomic clock and the earth's rotation. When the difference builds up enough, the time keepers must add another second (leap second) to keep the stars location, relative to the planet's rotation, in exact sync with the superaccurate atomic clocks.

The Earth's rotation is slowing but at a much slower rate than 1 leap second every so many years. The length of time it takes the Earth, at the present time, to rotate once is 86,400.002 seconds compared to 86,400 seconds back in 1820. The rotation has slowed roughly only by 2 milliseconds since 1820. That seems like an insignificant amount of time BUT over the course of Earth's entire lifetime, it has had very profound effects on the geophysics of her.

It has caused mountains to rise, earthquakes, etc. to occur. This article is about about, factoring in the tremendous geophysical activity that was caused, by the Earth's slowing rotation, in the interior of the planet, its crust, oceans and atmosphere over its entire lifetime. SOunds very technical n very 'physics'? this is an interesting fact...try to read on...

Tracing these tiny milliseconds back for 4.5 billion years , this rate of rotation (6.5 hours/cycle) would have exerted tremendous stress throughout the newly-borned Earth. If the rate of rotation has been slowing quite constantly, over the ages, the earth was rotating very fast originally, for it to slow down to the present 24 hour rate it is today. Whatever it was - its angular momentum was much faster and therefore its "centrifugal force" was tremendously strong when Earth was very young. This caused the young, fast spinning, Earth to have a much more oblate shape, possibly even having a "Saturn type ring" orbiting around it.

In the past, when the Earth was younger and spinning faster, the stronger angular momentum caused the Earth to assume a more pronounced oblate spheroid shape much greater than the twenty-seven miles it is today. A 15% increase in the equatorial circumference of the faster rotating young planet, relative to its present rotation, could produce approximately 3600 more miles of surface around the young planet's equatorial zone. Since the ratio of strength of gravity and angular momentum (centrifugal force) is always changing, the shape of the planet never reaches equalibrium - it is always changing.

The constant struggle between gravitational force and the weakening angular momentum caused by the slowing of the Earth's rotation, has had a profound effect on the Earth's geophysical activity throughout the ages. This constant struggle between these two gigantic forces has caused a continual adjusting of the Earth's shape as the ratio of the strengths of angular momentum and gravity have changed. FYI (revision): Gravity is a centripetal force, it exerts its tremendous force inward, toward the center of gravity, always trying to form the Earth into a perfect sphere while the angular momentum is an outward tangential inertia (centrifugal force) that causes the oblate shape of the Earth.

The angular momentum of the spinning planet tends to adjust the bulge to fit the changing conditions. A stronger angular momentum due to a faster rotation would increase the Earth's bulge while a weaker angular momentum due to a slowing rotation would allow gravity to form the planet into a more perfect sphere. The decrease in the size of the equitorial bulge may also had some effect on the Earth's angular momentum but the bulge does not decrease fast enough to actually increase the rotation of the Earth by any significant amount (imagine if it's 1 hour leap or 1 day leap or even more, how significant would the effect me?). Plus the fact all the leap seconds have been positive, indicating that the Earth is constantly slowing.

This weakening allows gravity to pull the planet's shape into a more perfect sphere. This change in shape causes tremendous stresses within the Earth and in its crust as it struggles to fit on the slowly changing mantle.

If the Earth's rotation was not slowing, there would be none of this stress, and its shape would have remained constant throughout the ages. Its rotation is gradually slowing however, (even taking the conservation of momentum into consideration), causing its angular momentum to become progressively weaker. Gravity remains just as strong as ever and continues to exert the same amount of inward pressure.

This on-going struggle between gravity's centripetal force and angular momentum's centrifugal force causes Earth to gradually change her shape from that an oblate spheroid with a bulging equator and flatter pole regions to that of a more perfect sphere to conform with the changing conditions. Throughout the ages, since birth, Earth has been going through this constant gradual change. The surface of the Earth's mantle, especially in the tropical and subtropical regions of the Earth is becoming smaller while the pole regions have uplifted and become rounder.

This slow & continual change in the Earth's interior has set up tremendous dynamic pressures and stress within the Earth's crust as it endeavors to conform to the ever changing mantle upon which it floats. It is the imbalance between the angular momentum and gravity that cause the dynamic (subduction) movements of the continental crustal plates as the crusts shifts and crunches to fit into the ever diminishing area of the mantle. It is this extremely slow, tremendously powerful, change that cause earthquakes to occur, volcanoes to erupt and the Earth's vast mountain ranges to rise. As the equatorial shape of the Earth shrinks, the immense resulting pressure within the mantle causes it to gradually ooze upward creating the Atlantic and Pacific ridges ("The Ring of Fire").

The Earth's slowing rotation also had a profound effect on its climate too over the ages. In the past, the faster rotation caused the Earth's ocean waters to concentrate more in the tropical and subtropical regions. This left the oceans in the pole regions relatively shallow compared to the equatorial region. Even today the Arctic Ocean's average depth is more shallow than the oceans in the equatorial regions.

Also the continent of Antarctica's plateau has greater average height above sea level than the other continents, even though burdened with heavy glaciers. This is due to the lower sea level in the pole regions rather than a higher continent. The concentration of ocean water over a much larger area, in the warmer regions of the Earth, must have had a profound effect on the Earth's climate in past geological times.

The faster rotation also had an enormous effect on the Earth's dynamic atmosphere creating more energetic global winds and perhaps, greater evaporation from the warm equatorial seas. The blanket of atmosphere may also have been thicker in the equatorial regions. The final result of all these effects created a vastly different climate than our present climate.

The planet's former magnificent ring had long ago settled back on the surface of Earth and all that remained was the moon still orbiting the Earth. Within this vast geologic time many mountain ranges had risen only to be eroded away and replaced by new ranges as the crust strives to conform with the ever changing mantle. The age of the dinosaurs had come and gone and with only a 27 mile bulge left there will probably be no significant new mountain ranges created in the future. The Earth has reached its "maturity" and is settling into a period of much less geophysical activity compared to the past.

Even at the Earth's present subdued rate of rotation, at the equator, the rotation is still fast enough to help propel rockets into space with up to 13 percent less fuel, which allows heavier payloads. Just imagine the heavy payloads that could have been launched in the past when the planet was spinning much faster.

Note: December 26th 2004 earthquake that caused the devastating tsunami in southeast Asia was just another "minor" adjustment that the planet had to make to adjust to its changing shape. It has made millions of these adjustments over its lifetime.

*Adapted from http://novan.com/earth.htm [Chapter 13, (The Earth's Slowing Rotation) "The MIND of MANKIND"]