**Satellites and Orbits: an Introduction**

**Glossary**

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  Apogee                   The point in an orbit farthest from the Earth.

  Argument of Perigee      The angular distance between the ascending
                           node and the point of perigee.

  Artificial               Made by humans; produced rather than natural.

  Closed Orbit             A circular or elliptical orbit. A body on a
                           closed orbit constantly travels around a
                           planet or a star.

  Eccentricity             The ratio between distance from the center of
                           the ellipse (which isn\'t the center of the
                           Earth) to the focus of the ellipse (which is
                           the center of the Earth) and the semi-major
                           axis. The value can be from 0 to 1, with 0
                           being a circle and the numbers up to one
                           defining increasingly elongated and flattened
                           orbits.

  Echo                     A type of passive communication satellite
                           launched by NASA in the 1960\'s.

  Epoch                    An arbitrary fixed instant of time or date
                           used as a chronological reference for orbital
                           motions.

  Equatorial Orbit         An orbit that lies at any altitude above the
                           Equator, i.e. has an inclination of 0 degrees.

  Geostationary Orbit      A Geosynchronous Orbit having zero inclination
                           so that the spacecraft hangs motionless with
                           respect to a point on the planet below.

  Geosynchronous Orbit     A direct, circular, low-inclination orbit
                           around Earth having a period of 23 hours 56
                           minutes 4 seconds and a corresponding altitude
                           of 35,784 km (22,240 miles, or 5.6 Earth
                           radii).

  GOES                     Geostationary Operational Environmental
                           Satellite

  Gravity                  The force of attraction between all mass in
                           the universe; particularly the force by which
                           objects are drawn towards each other and to
                           the Earth.

  Inclination              The orbit ellipse lies in a plane, and this
                           plane forms an angle with the plane of the
                           Equator. This can be visualized as the tilt
                           with respect to the Equator.

  IPO                      Integrated Program Office. The tri-agency
                           government organization that manages the
                           National Polar-orbiting Operational
                           Environmental Satellite System program. It
                           employs personnel from the Department of
                           Commerce, Department of Defense and the
                           National Aeronautics and Space Administration.

  ISS                      International Space Station.

  Longitude or Right       The node\'s celestial longitude - an angle,
  Ascension                measured from the center of the Earth, from
                           the vernal equinox to the ascending node.

  Low Earth Orbit          The lowest altitude a spacecraft must achieve
                           to orbit the Earth. Altitudes for Low Earth
                           Orbit range from around 100 km to 1,500 km.
                           Spacecraft orbiting in Low Earth Orbit can
                           circle the Earth once every ninety minutes or
                           so.

  LTAN                     Local time of the ascending node - the local
                           time of the satellite as it crosses the
                           Equatorial plane from the southern to northern
                           hemisphere.

  Motion                   A natural event that involves a change in the
                           position or location of something \[syn:
                           movement\].

  NASA                     National Aeronautics and Space Administration.

  Natural                  Existing in or produced by nature; not
                           artificial or imitation; \"a natural pearl\";
                           \"natural gas\"; \"natural silk\"; \"natural
                           fertilizers\" \[ant: artificial\].

  NOAA                     National Oceanic and Atmospheric
                           Administration.

  Nodes                    The points where an orbit crosses a plane,
                           such as a satellite crossing the Earth\'s
                           Equatorial plane. If the satellite crosses the
                           plane going from south to north, the node is
                           the ascending node; if moving from north to
                           south, it is the descending node.

  NPOESS                   National Polar-orbiting Operational
                           Environmental Satellite System.

  Open Orbit               An open orbit follows a mathematical shape:
                           either one known as a parabola or another
                           called a hyperbola. Both are sweeping curves
                           that never join up.

  Orbit                    The path in space along which an object moves
                           around a larger object, such as the Earth.

  Path                     The route or course along which something
                           travels or moves.

  Perigee                  The point in an orbit closest to the Earth.

  Period                   The length of time required for a satellite to
                           complete one orbit.

  Polar Orbit              An orbit with an inclination of 90 degrees. A
                           polar-orbiting satellite will pass over or
                           very nearly over, both the North Pole and the
                           South Pole each orbital period.

  Prograde                 Any orbit in which the spacecraft moves from
                           west to east is termed prograde. This is the
                           usual direction of rotation in our Solar
                           System. Only a handful of objects orbit or
                           rotate in the opposite direction.

  Projectile               A fired, thrown, or otherwise propelled
                           object. A projectile is an object upon which
                           the primary acting force is gravity.

  Retrograde               Any orbit in which the spacecraft moves from
                           east to west. This is the less usual direction
                           in the Solar System; however, it is not
                           impossible. Venus has retrograde spin and some
                           comets -- notably comet Halley has a
                           retrograde orbit.

  Satellite                A celestial body that orbits a planet; a moon.
                           An object launched to orbit Earth or another
                           celestial body.

  Sputnik                  The world\'s first artificial satellite.
                           Launched October 4, 1957 by the Soviet Union.

  Telstar                  An early communications satellite owned by a
                           consortium including NASA and American
                           Telephone and Telegraph. Telstar was launched
                           on July 10, 1962.

  TIROS                    Television Infrared Observation Satellite. The
                           first of a series of NASA meteorological
                           satellites to carry television cameras.

  Vernal Equinox           Reference point where the right ascension is
                           defined to be zero.
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