# Question #dbd4a

Jun 9, 2016

The value of $9.8$ $\frac{m}{s} ^ 2$ is an approximation averaged on the Earth and valid now, not always.

The Earth is a geoid with an irregular shape (think about mountains or oceans) and the field that generates around follows its irregularities.
For example the acceleration on the poles is $9.823$ $\frac{m}{s} ^ 2$ while at the equator it is $9.789$ $\frac{m}{s} ^ 2$.

Of course the variations are not so big because you need a big mass to give some contribution to the gravity.

So the acceleration on Earth is not a constant, it changes from position to position and it changes in time because the Earth is continuously reshaping (that is redistributing the mass) under the influence of earthquakes, vulcans, wind, rain, etc.

The variations are small because the total mass of Earth is quite stable.