Gravity is defined from Newton's law which states:
"Every body attracts every other body with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them"
From this it is obvious that if the Earth is increasing in size and mass then gravity (g) will also be increasing. The amount of change is extremely small, I calculate it as a change of only 3.6 microGal per year, this is only 0.0000000036 of the Earth's gravity.
As an example of how small this change is, consider how other variables affect the Earth's gravity.
* The Earth's rotation causes centrifugal acceleration at the equator where rotational velocity is 1,674 km/hr. This effect means gravity is 5.186 microGal less at the equator than at the poles.
But at least the variation in gravity due to the spin of the Earth is constant. Worse yet is that it is also continuously changing as a result of a number of phenomena affecting the whole Earth such as;
* earth tides
* the wobble of the Earth on its axis
* density variations in the atmosphere
* very small changes in the density of the material beneath the measuring station, perhaps simply due to rainwater
* small changes in the elevation of the measuring station. We might assume that solid rock doesn't move up or down but this is wrong, the weight of the continental glaciers of the last great ice age depressed the surface of the Earth. As the great ice sheets retreated, the Earth's crust began to rebound to its former level. This land uplift causes a typical decrease in gravity of a few microGal per year which is assumed to be due to a land uplift of 10's of millimetres per year. All these effects mean that the accuracy needed to measure the predicted variation in gravity is on the limit of current instruments' accuracy.
The absolute gravimeter measures the acceleration of a mass in free fall in a vacuum. The latest FG5 absolute gravimeter, for example, can now achieve an accuracy of 2 microGals (0.000000002 of gravity). This level of accuracy has only recently been achieved so it will still need several more years of measurements before any gravity increase (or lack of it) becomes apparent.
From this it is obvious that if the Earth is increasing in size and mass then gravity (g) will also be increasing. The amount of change is extremely small, I calculate it as a change of only 3.6 microGal per year, this is only 0.0000000036 of the Earth's gravity.
As an example of how small this change is, consider how other variables affect the Earth's gravity.
* The Earth's rotation causes centrifugal acceleration at the equator where rotational velocity is 1,674 km/hr. This effect means gravity is 5.186 microGal less at the equator than at the poles.
But at least the variation in gravity due to the spin of the Earth is constant. Worse yet is that it is also continuously changing as a result of a number of phenomena affecting the whole Earth such as;
* earth tides
* the wobble of the Earth on its axis
* density variations in the atmosphere
* very small changes in the density of the material beneath the measuring station, perhaps simply due to rainwater
* small changes in the elevation of the measuring station. We might assume that solid rock doesn't move up or down but this is wrong, the weight of the continental glaciers of the last great ice age depressed the surface of the Earth. As the great ice sheets retreated, the Earth's crust began to rebound to its former level. This land uplift causes a typical decrease in gravity of a few microGal per year which is assumed to be due to a land uplift of 10's of millimetres per year. All these effects mean that the accuracy needed to measure the predicted variation in gravity is on the limit of current instruments' accuracy.
The absolute gravimeter measures the acceleration of a mass in free fall in a vacuum. The latest FG5 absolute gravimeter, for example, can now achieve an accuracy of 2 microGals (0.000000002 of gravity). This level of accuracy has only recently been achieved so it will still need several more years of measurements before any gravity increase (or lack of it) becomes apparent.
Further Reading
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NASA's press release "Earth's girth grows" here
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Absolute Gravimeter Measures here
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Updated 1Mar12
