Why does rotation cause day and night?

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As the earth spins one side is facing toward the sun and the other side is facing away from the sun. The earth spins and the side of the earth that faced the sun now faces away. It takes 24 hours for the earth to spin around once.

JO3SH (Age 1234567890)




When the Earth was very young, its faster rotation caused it to have a much larger equatorial circumference than it has at the present time. 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 four and one half billion years, 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., like any flywheel, it will slow down. (The space surrounding the planet is far from empty).

While we know the Earth's rotation is slowing that is not the main reason why the extra "Leap Second" was added by our official time keepers this year. 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 so that each day this little difference builds up between the atomic clock and the earth's rotation.

When the difference builds up enough (.9 seconds), 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 the planet's entire lifetime, it has had very profound effects on the geophysics of the planet.

It has caused mountains to rise, earthquakes, etc. to occur as we will see. This article is 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.

The Sub-bureau for Rapid Service and Predictions of Earth Orientation Parameters of the International Earth Rotation Service (IERS), located at the US Naval Observatory, monitors the Earth's rotation. Part of its mission involves the determination of a time scale based on the current rate of the rotation of the Earth.

They estimate that the Earth's rotation is slowing at about 1.4 milliseconds per solar day per century which roughly agrees with the rate of rotation of the Earth has actually slowed down since 1820.

Tracing these tiny milliseconds back for 4.5 billion years adds up to a very significant amount of time for a solar day. I have determined that the day/night rotation was 63,000 seconds shorter than the present 86,400 seconds it is today. This would put the Earth's rotation at about 6.5 hours per day/night cycle, when it was created, 4.5 billion years ago. (This is a much faster rate of rotation than the Cassini-Huygens mission (2003 to 2004) determined Saturn's 10.5 hours rotation period to be.)

This rate of rotation would have exerted tremendous stress throughout the newly formed planet. If the rate of rotation has been slowing fairly constantly, over the ages, we can see that the earth was rotating very fast originally, for it to slow down to the present 24 hour rate it is today. What ever it was - its angular momentum was much faster and therefore its "centrifugal force" was tremendously strong when the planet was very young. This caused the young, fast spinning, Earth to have a much more pronounced 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.

This was a lot of surface to crunch into the ever changing shape of the planet as its rotation gradually slowed over the ages. 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 fast spinning star, Vega, makes a full rotation about its axis once every 12.5 hours, (only about 1/2 the Earth's early rate) which causes it to assume a 23% ellipical bulge at its equator, so a 15% greater bulge for the faster spinning Earth is not out of the question.)

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.

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. Evidence indicates the rotation is constantly slowing however so that the angular momentum is growing weaker.

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. 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 an ever more perfect sphere. This change in shape has set up 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 tremendous dynamic stress within the Earth as it adjusts its shape. It is gradually changing its shape from that of 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 its birth, the 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. The fact that the Earth is getting rounder, even today, has been confirmed by the satellite "Lageos" which is being monitored by scientists at the University of Texas Center for Space Research and at the Jet Propulsion Laboratory.

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 relentless, 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 oblate 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 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 relativly 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.

These effects where greatest in the Pre-Cambrian age of geological time. They have become more subdued, but still evident, in the later geological ages as our planet's rotation continues to gradually slow. (It would be interesting, if all this information were fed into a computer to see what the planet was like under these changing conditions, throughout its lifetime.)

After 3.5 billion years the planet's rotation had slowed to 20.11 hours per day/night cycle and at a 100,000,000 years ago its rotation period had slowed to 23.6 hours, not that much different than the present rate. Within these years a tremendous amount of geophysical activity had taken place as the earth's equatorial region had slowly disappeared, becoming ever more rounder until today the 3600 mile bulge had shrunk to a measely 27 miles.

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 strived to conform with the everchanging 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.
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