This is because Earth is not a perfect sphere, and its interior mass is not evenly distributed. There are slight variations of gravitational pull depending on where on Earth’s surface you are.
In fact, you can now check to see which points on the Earth’s surface have the strongest and weakest gravitational pulls with a “gravity map”, available at BBC.com. The map is a 3-dimensional model of the earth that represents the amount of gravitational pull on a specific point on the Earth as its altitude. It uses data compiled from the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE).
GOCE is a satellite that orbits at a low altitude (the lowest ever for a satellite, at 254.9 km or 158 miles) that uses a gradiometer to detect the gravitational pull at different parts of the Earth. Because of this, the data collected from GOCE is the most accurate data on the Earth’s gravity that we have ever been able to compile. It has even been able to collect data from hard-to-reach areas like Antarctica, the Himalayas, the Andes, and Southeast Asia.
19 Astonishing Facts