An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A plot displaying the amplitude of → cosmic microwave background anisotropy as a function of angular size or → multipole index. Same as → angular fluctuation spectrum. The plot, based the on WMAP and other data, shows a plateau at large angular or length scales (→ Sachs-Wolfe plateau), then a series of peaks at progressively smaller scales. These features arise from the gravity-driven acoustic oscillations of the coupled photon-baryon fluid in the early Universe (→ baryon acoustic oscillation). In particular, a strong peak is seen on an angular scale (at l ~220), corresponding to the physical length of the → sound horizon at the → recombination era. It depends on the curvature of space. If space is positively curved, then this sound horizon scale will appear larger on the sky than in a flat Universe (the first peak will move to the left). The second peak (l ~ 550), which is the first harmonic of the main peak, relates to the baryon/photon ratio. The third peak can be used to help constrain the total matter density.

→ angular; → fluctuation; → spectrum.

The angular velocity of a point in a circular orbit around a central mass. It is given by: Ω_K = (GM/r³)^1/2, where G is the → gravitational constant, M is the mass of the gravitating object, and r is the radius of the orbit of the point around the object.

→ Keplerian; → angular; → velocity.

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.

Having the base or section in the form of a rectangle. Shaped like a rectangle.

Adj. of → rectangle.

A → window function that is constant inside a specified interval.

→ rectangular; → window.

The → angular momentum of a body rotating about an axis. The rotational angular momentum of a solid homogeneous sphere of mass M and radius R rotating about an axis passing through its center with a period of T is given by: L = 4πMR²/5T.

→ rotational; → angular; → momentum.

→ Angular momentum per unit mass.

→ specific; → angular; → momentum.

An intrinsic quantum mechanical characteristic of a particle that has no classical counterpart but may loosely be likened to the classical → angular momentum of a particle arising from rotation about its own axis. The magnitude of spin angular momentum is given by the expression S = ħ √ s(s + 1), where s is the → spin quantum number. As an example, the spin of an electron is s = 1/2; this means that its spin angular momentum is (ħ /2) √ 3 or 0.91 x 10^-34 J.s. In addition, the projection of an angular momentum onto some defined axis is also quantized, with a z-component S_z = m_sħ. The only values of m_s (magnetic quantum number) are ± 1/2. See also → Stern-Gerlach experiment.

→ spin; → angular; → momentum.

Of a rotating body, a vector of magnitude ω (→ angular velocity) pointing in the direction of advance of a right-hand screw which is turned in the direction of rotation.

→ vector; → angular; → velocity.