Earth's gravity is the most overriding force in human history. This is so true that it feels silly to point out. It keeps us in our beds at night, pulls us back to the ground after every stride on a long run, stands up our homes and skyscrapers, and fixes the moon in the sky — it's the invisible bond that holds our entire world together.
And yet, precisely measuring Earth's gravity is surprisingly difficult.
We know how strong it is in general, but the force of our planet's pull varies significantly from one part of the planet to another. And detailed maps of that pull offer significant clues to what lies under Earth's surface, not to mention critical information for architects and engineers.
The gravitational field intensity of Earth, equivalent to g, is the net force per unit mass at the point due to gravitation (which arises due to mass distribution within Earth) and centrifugal force (from Earth's rotation about its own axis). The Earth is rotating and non spherically symmetric, it is slightly flatter at the poles and balging at Equator. Consequently, there are deviations in the magnitude of gravity across its surface.
The Earth's equatorial bulge causes objects at the Equator to be farther from planet's center than objects at the poles, so objects at the Equator experience a weaker gravitional pull. An object will weight 0.5% more at the poles than at the Equator. Beneath the Earth's surface the distribution of rocks and minerals is uneven creating packets of varying density therefore varying gravity.
Credit: NASA
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