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Orbit

The path of a body in space, generally under the influence of gravity.

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Orbital elements

Variables which characterize the motion of an orbiting body. For a planet or satellite in an elliptic orbit, 6 orbital elements exist: the semi-major axis gives its size, eccentricity its shape and mean anomaly its position along the orbit, at the given time. The three other elements are three angles which give the orientation in space of its orbital plane, e.g. that plane's inclination (to the plane of the Earth's equator or the ecliptic,depending on choice of coordinates).

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Orbital period

The length of time required for a body to complete one full (closed) orbit.

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Ozone

A gas, an alternative form of oxygen in which 3 atoms of oxygen rather than 2 join to form a molecule. Even though ozone forms in the atmosphere only in very small quantities, it has an important impact. Ozone near the ground is produced as part of industrial air pollution, and since it is chemically very reactive, it is undesirable--it corrodes masonry, it causes paint to darken and is unhealthy to breathe. Ozone in the high atmosphere, around 25-30 km., is formed by solar ultra-violet light. It absorbs such light and prevents it from penetrating to the ground, where it could be harmful to the eyes and to skin.