Look closely: Earth's shape and size aren't as constant as they seem.
At a given point in time, Earth usually seems stable. But over centuries, lands rise and sink, continents move and the balance of the ocean shifts. These changes are tracked through the science of geodesy, dedicated to measuring and precisely charting the size and shape of Earth. Geodesy began more than 2,000 years ago, when a clever human named Eratosthenes discovered that you could measure the circumference of the Earth by looking down a well. Over time, the field of geodesy has expanded and evolved dramatically. In modern geodesy, scientists use multiple techniques to map the world's ice sheets, watch sea level creep up and monitor the impact of earthquakes, droughts and floods. This animated video looks at the long history of geodesy and highlights some of the technologies NASA uses, such as radio telescopes and long-range lasers, to take the basic measure of our planet.