An Etymological Dictionary of Astronomy and Astrophysics
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The orbit of a spherical object of a finite mass around another spherical object, also of finite mass, governed by their mutual → gravitational forces only.

→ Keplerian; → orbit.

The velocity of an object orbiting another object according to → Kepler's laws.

→ Keplerian; → orbital; → velocity.

A quasi-periodic path resembling a → Lissajous figure around the L1 or L2 → Lagrangian points of a two-body system. Lissajous orbits, resulting from a combination of planar and vertical components, are used by certain space telescopes (such as → WMAP, → Planck Satellite, and → Herschel Satellite) that are required to be in a stable position relative to the Earth and Sun while making long-term observations.

→ Lissajous figure; → orbit.

Same as → lunar node.

→ lunar; → orbit; → node.

Same as → sidereal month.

→ lunar; → sidereal; → orbital; → period.

The minimum distance between the paths of two orbiting objects around a → primary. Such distance between an object and Earth is called Earth MOID.

→ minimum; → orbit; → intersection; → distance.

The path followed by a body moving in a gravitational field. For bodies moving under the influence of a centrally directed force, without significant perturbation, the shape of the orbit must be one of the conic section family of curves (circle, ellipse, parabola, or hyperbola).

L. orbita "wheel track, course, circuit."

Madâr, from Ar.

A gradual change in the orbit of a spacecraft caused by aerodynamic drag of a planet's outer atmosphere and other forces.

→ orbit; → decay.

1) Of or relating to an orbit.
2) A wave function that describes the state of an electron with a given energy (n, l, and m_l quantum numbers) in an atom (atomic orbital) or in a molecule (molecular orbital).

Orbital, from → orbit + → -al.

1) Mechanics: The → angular momentum associated with the motion of a particle about an origin, equal to the cross product of the position vector (r) with the linear momentum (p = mv): L = r x p. Although r and p are constantly changing direction, L is a constant in the absence of any external force on the system. Also known as orbital momentum.
2) Quantum mechanics: The → angular momentum operator associated with the motion of a particle about an origin, equal to the cross product of the position vector with the linear momentum, as opposed to the → spin angular momentum. In quantum mechanics the orbital angular momentum is quantized. Its magnitude is confined to discrete values given by the expression: ħ &radic l(l + 1), where l is the orbital angular momentum quantum number, or azimuthal quantum number, and is limited to positive integral values (l = 0, 1, 2, ...). Moreover, the orientation of the direction of rotation is quantized, as determined by the → magnetic quantum number. Since the electron carries an electric charge, the circulation of electron constitutes a current loop which generates a magnetic moment associated to the orbital angular momentum.

→ orbital; → angular; → momentum.

The → perpendicular to the → orbital plane.

→ orbital; → axis.

Objects in orbit around Earth created by humans, that no longer serve any useful purpose. They consist of everything from entire spent rocket stages and defunct satellites to explosion fragments, paint flakes, dust, and slag from solid rocket motors, and other small particles. Also called space junk and space waste.

→ orbital; → debris .

An electron contained within an atom which may be thought of as orbiting around the nucleus, in a manner analogous to the orbit of a planet around the Sun.

→ orbital; → electron .

Any of the six parameters needed to specify the → orbit of an object around a → primary body (such as a planet around the Sun or a satellite around the Earth) and give its position at any instant. Two of them define the size and the form of the orbit: → semi-major axis (a) and → eccentricity (e). Three angular values determine the orbit position in space: the → inclination (i) of the object's → orbital plane to the reference plane (such as the → ecliptic), the → longitude of ascending node (Ω), and the → argument of periapsis (ω). And finally the sixth element is the → time of periapsis passage which allows calculating the body's position along the orbit at any instant.

→ orbital; → element.

The → sum of the → potential energy and the → kinetic energy of an object in → orbit.

→ orbital; → energy.

An → orbital element that defines the angle between the orbital plane of a solar system body (planet, comet, asteroid) and the plane of the ecliptic. The orbital inclination of the Earth's orbit is 0°; those of Mercury, Venus, and Mars are 7.01°, 3.39°, and 1.85° respectively.

→ orbital; → inclination .

The moving of a spacecraft between two different orbits resulting from a change in its velocity (acceleration). Generally, manoeuvres are caused by → thrust from the spacecraft's motors.

→ orbital; → maneuver.

→ orbital maneuver.

→ orbital; → manoeuvre.

Theoretical prediction according to which a → giant planet, formed in the outer regions of a → protoplanetary disk, could migrate inward by losing → energy and → angular momentum as the result of → gravitational interactions with the remnants of the disk. This orbital migration could explain the presence of giant gaseous Jupiter-like planets (→ hot Jupiters) very close to their host stars.

→ orbital; → migration.

One of the two points of intersection of the orbit of a secondary body with the plane of reference through the primary.

→ orbital; → node.