Scientists measure local gravity with instruments called gravimeters, and they map larger-scale variations from space by precisely tracking satellites. Earth’s gravitational acceleration is approximately 9.8 meters per second squared near the surface, but its exact value is slightly different from place to place.
What does a gravimeter actually measure?
A gravimeter detects very small changes in the local acceleration due to gravity. Some instruments track how a carefully supported test mass responds; others observe an object in free fall. The instrument is calibrated and surveyed carefully, because vibrations, instrument drift, elevation and surrounding terrain can influence the reading.
Why isn’t gravity exactly the same everywhere?
Earth rotates and is slightly wider at the equator than at the poles. Gravity also changes with height above sea level and with the mass distribution under your feet. Dense rocks, mountains and changing underground water can all contribute. Scientists apply corrections before interpreting an observed difference as a geophysical signal.
How can satellites measure Earth’s gravity?
Twin satellites flying one behind the other experience slightly different accelerations as they pass over regions with different amounts of mass. Extremely precise ranging instruments detect changes in their separation. Researchers combine these measurements with satellite-position and other instrument data to make maps of the gravity field.
What do gravity maps tell us?
Maps of gravity anomalies help scientists investigate subsurface geology and Earth’s shape. Measurements repeated through time can also reveal large shifts in the distribution of water—for example, losses from ice sheets or changes in regional groundwater storage. A colorful gravity map exaggerates tiny differences for clarity; Earth is not actually shaped like its brightly colored lobes.
Frequently asked questions
Would you notice weighing less in a different city?
Usually not with an ordinary bathroom scale. Local differences are generally much smaller than the changes you’d experience by traveling to another planet.
Does a gravimeter measure the same thing as a bathroom scale?
No. A bathroom scale estimates the force of your weight; a gravimeter aims to resolve very small variations in gravitational acceleration.
Sources: NASA GRACE Fact Sheet and NASA JPL: Measuring Gravity from Space.
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