An official website of the United States government.

##### .cls-3{fill:#007faa;}.cls-4{fill:none;stroke:#046b99;stroke-miterlimit:10;}dot gov iconOfficial websites use .gov

A .gov website belongs to an official government organization in the United States.

##### .cls-1{fill:#549500;}.cls-2{fill:none;stroke:#458600;stroke-miterlimit:10;}https iconSecure websites use HTTPS

A small lock or https:// means you’ve safely connected to a .gov website. Share sensitive information only on official, secure websites.

# Gravity

## Global Positioning Tutorial

Gravity is the force that pulls all objects in the universe toward each other. On Earth, gravity pulls all objects "downward" toward the center of the planet. According to Sir Isaac Newton's Universal Law of Gravitation, the gravitational attraction between two bodies is stronger when the masses of the objects are greater and closer together. This rule applies to the Earth's gravitational field as well. Because the Earth rotates and its mass and density vary at different locations on the planet, gravity also varies.

One reason that geodesists measure variations in the Earth's gravity is because gravity plays a major role in determining mean sea level. Geodesists calculate the elevation of locations on the Earth's surface based on the mean sea level. So knowing how gravity changes sea level helps geodesists make more accurate measurements. In general, in areas of the planet where gravitational forces are stronger, the mean sea level will be higher. In areas where the Earth's gravitational forces are weaker, the mean sea level will be lower.

To measure the Earth's gravity field, geodesists use instruments in space and on land. In space, satellites gather data on gravitational changes as they pass over points on the Earth's surface. On land, devices called gravimeters measure the Earth's gravitational pull on a suspended mass. With this data, geodesists can create detailed maps of gravitational fields and adjust elevations on existing maps. Gravity measurements accurately reflect elevation changes on the Earth's surface.

##### Geodesy Lessons
Roadmap to Resources Subject Review Is the Earth round? (Ocean Facts)

Heard of geodesy? These videos from NOAA's National Geodetic Survey offer a deep dive into the science of knowing where you – and everything else is in the world!

The National Science Teaching Association (NSTA) includes this resource in its database. NSTA provides educators and students access to Web-based, educationally appropriate science content that has been formally evaluated by master teachers.

Author: NOAA