Altitude is the height of an object above Earth’s surface, sea level, or some other given level. For example, an airplane may cruise at an altitude of 30,000 feet (9,100 meters) above the ground. In geography, altitude is commonly called elevation. Elevation refers to the height of a physical feature or place above sea level. Mount Everest, the world’s highest mountain, has an elevation of 29,032 feet (8,849 meters) above sea level. The temperature usually drops about 3 or 4 °F per 1,000-foot (2 or 3 °C per 300-meter) increase in altitude, up to an altitude of 5 to 10 miles (8 to 16 kilometers).
In astronomy and in navigation by the stars, altitude refers to the size of the angle between imaginary lines from an observer to a heavenly body and to the horizon. Exact observations usually cannot be made at an altitude of less than 45°. At lower altitudes, there is too much absorption and bending of light rays as the rays travel through the earth’s atmosphere. In space navigation, altitude refers to the distance of a spacecraft from the surface of a reference body, such as the moon.
