Weight is the measure of the force exerted by gravity on an object’s mass. Mass is the inherent amount of matter contained within an object, a value that remains constant regardless of location. You weigh less at the equator than you do at the poles. This measurable difference is caused by two distinct physical phenomena: the Earth’s non-uniform shape and its constant rotation.
How Earth’s Shape Affects Gravitational Pull
The gravitational force exerted on a person is directly tied to their distance from the Earth’s center of mass. According to the law of universal gravitation, the attractive force decreases rapidly as the distance separating centers increases.
The Earth is not a perfect sphere; it is classified as an oblate spheroid that bulges slightly around its middle (the equatorial bulge). This means the radius of the Earth is greater at the equator than it is at the poles. A person at the equator is approximately 21 kilometers farther away from the Earth’s center than a person at the poles. This increased distance weakens the true gravitational attraction, contributing to a measurable reduction in weight.
The Role of Rotational Force
The second and more significant reason for the reduction in weight at the equator is the Earth’s constant spin. This rotation generates an outward-acting centrifugal force that slightly opposes the inward pull of gravity. The magnitude of this rotational force depends on the speed of rotation, which varies significantly with latitude.
At the poles, the rotational speed is effectively zero. Conversely, the speed is maximized at the equator, where the circumference is largest. The maximum outward force occurs at the equator, working directly against gravity and reducing the net downward force felt by a person. This effect on apparent weight is negligible near the poles but becomes most pronounced in equatorial regions.
Calculating the Total Weight Difference
The total difference in a person’s weight between the poles and the equator is the combined result of the Earth’s oblate shape and the rotational force. The distance factor, caused by the equatorial bulge, accounts for about 30 percent of the total weight reduction. The effect of the Earth’s rotation contributes the remaining 70 percent, making it the dominant factor.
Overall, a person will weigh approximately 0.5 percent less at the equator than they would at the poles. For example, a 200-pound person at the North Pole would weigh about 199 pounds at the equator. This difference is scientifically measurable but is imperceptible in daily life. The slight variation in weight is often masked by other localized factors, such as changes in altitude or variations in the density of the Earth’s crust beneath the surface.