The term sounder is sometimes used for instruments that take measurements, either up in the atmosphere from the surface, or down into the atmosphere or near an object from above. The action is called sounding, (i.e., the verb, to sound). The term is borrowed from nautical terminology for the action of determining the depth of the water underneath a ship, something once done by dropping something to touch the bottom, and later using devices such as sonar. The term is used in meteorology in making measurements above ground, e.g., using balloons or rockets (sounding rockets), being used later for the study of the extended upper atmosphere, often used for sub-orbital space probes. The term is also used in planetary science for instruments observing the atmosphere or surface features of solar system planets from orbit, or being sent closer to the planet, even impacting. The term gravity sounding is also used, for probes aiming to study a body's gravity, e.g., by simply orbiting close to it while its trajectory is monitored. Similar experiments have investigated planets' magnetic fields. The term sounding is sometimes also used for studies at a distance of atmospheres, etc.