Earth's Gravitational Field Is Getting Flatter

RJG2 writes "MSNBC has an article stating that Earth's gravitational field has changed, becoming stronger towards the equator, thus becoming flatter. The cause has yet to be determined, but it is assumed changes in ocean levels are responsible."

Comments

It's the SHAPE, see Net Scientist

[Great_Geek]

New Scientist also has an article on this, see
http://www.newscientist.com/news/news.jsp?id= ns999 92628

The MSNBC report is misleading - the measurement (by satellites)is of gravity, but the conclusion is about the shape of the planet. The prime suspect, currently, is ocean currents.

Nope, it flows to the equator

[sterno]

Since the earth is spinning, the water tends to be forced to the equator by centrifugal force (although I think more correctly it's centripedal force, but whatever). So no matter where the water comes from, it will tend to flow to the equator.

Actually one likely side-effect of long term global warming is, ironically, an ice age. The water moves to the equator, and this causes the earth to spin slightly slower. The side-effect being that this cools the earth. I forget exactly why this is because I learned it in high school physics which was just over a decade ago.

Re:Nope, it flows to the equator

[God! Awful]

Since the earth is spinning, the water tends to be forced to the equator by centrifugal force (although I think more correctly it's centripedal force, but whatever).

IMHO, high school physics teachers really dropped the ball in explaining this one. A whole generation of high school graduates is confused about centrifugal vs. centripedal.

A body that is spinning around an axis or orbiting around point must be under continual force. Otherwise, they would simply fly off in a straight line at a tangent to the curve. This is the centripedal force. The centrifugal force is a "pseudo force", which means that it only exists in a non-intertial frame of reference.

Basically, what happens is that when you accelerate (whether in a straight line or in a circle), your inertia feels like a force in your frame of reference. When you sit in a moving car, from your point of view you feel like you are sitting still and the car is moving. We know that when an object is at rest, the forces on it are balanced. Therefore, in your frame of reference you feel a pseudo force which balances out the force that is being applied on the car. The pseudo force is really just the effect of your inertia.

So how does this apply to the water? Well, everything on Earth has inertia, and this inertia wants to keep it going in a straight line, even though the Earth is rotating. Solid objects, such as humans are obviously kept in place by simple static friction and wind resistance. Water and air are more mobile and they are less subject to friction (although they are still very subject to air/water pressure). That is the main reason why wind and ocean currents are very obvious whereas continental drift takes centuries.

So in reality, it is the inertia of the water that makes it more buoyant at the equator. The water at the equator is spinning faster than the water at the poles, so it is slightly less subject to gravity. Therefore it bulges out, "making room" for some extra water from the poles to move towards the equator.

-a

A reference on leap seconds

[Adam J. Richter]

The earth's rotation is slowing anyway. This is the reason that they insert those "leap seconds" every few years to compensate for the lost time.

At first, I did not believe that such a small change could account for the leap seconds, but you're right :

Through the use of ancient observations of eclipses, it is possible to determine the average deceleration of the Earth to be roughly 1.4 milliseconds per day per century.

[...] Over the course of one year, the difference accumulates to almost one second, which is compensated by the insertion of a leap second into the scale of UTC with a current regularity of a little less than once per year. Other factors also affect the Earth, some in unpredictable ways, so that it is necessary to monitor the Earth's rotation continuously.

In order to keep the cumulative difference in UT1-UTC less than 0.9 seconds, a leap second is added to the atomic time to decrease the difference between the two. This leap second can be either positive or negative depending on the Earth's rotation. Since the first leap second in 1972, all leap seconds have been positive and there were 22 leap seconds in the 27 years to January, 1999. This pattern reflects the general slowing trend of the Earth due to tidal braking.