An Etymological Dictionary of Astronomy and Astrophysics
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A phenomenon in which the observed and predicted phases of Venus do not coincide. At eastern elongation, when the planet is visible in the evening sky, dichotomy (half-phase) usually comes a day or two earlier than predicted, while at western elongation dichotomy occurs a day or two later.

Named after Johan Schröter (1745-1816), German astronomer, who first described the effect in 1793; → effect.

A phenomenon in which the observed and predicted phases of Venus do not coincide. At eastern elongation, when the planet is visible in the evening sky, dichotomy (half-phase) usually comes a day or two earlier than predicted, while at western elongation dichotomy occurs a day or two later.

Named after Johan Schröter (1745-1816), German astronomer, who first described the effect in 1793; → effect.

Same as → shielding effect.

→ screen; → effect.

An → electromotive force produced in a closed electric circuit formed by connecting conductors of different metals in series when the two junctions junctions are maintained at different temperatures. The circuit constitutes a → thermocouple.

Named for the German physicist Thomas Seebeck (1770-1831), who discovered the effect; → effect.

The decrease in attraction between an electron and the nucleus in any atom with more than one → electron shell. The repulsion forces from other electrons in shells cause the net force on electrons in outer shells to be significantly smaller in magnitude. Also known as → screening effect.

→ shield; → effect;

Same as → Schottky noise; → shot noise.

Translation of Ger. Schroteffekt, from Schrot "small shot, buckshot" + Effekt; → effect.

The tendency of an → alternating current to concentrate in the outer layer of a conductor, caused by the → self-induction of the → conductor and resulting in increased → resistance.

→ skin; → effect.

An important astronautical technique whereby a spacecraft takes up a tiny fraction of the gravitational energy of a planet it is flying by, allowing it to change trajectory and speed. Also known as → gravitational slingshot or → gravitational assist.

Slingshot, from sling, from M.E. slyngen, from O.N. slyngva "to sling, fling" + shot, from M.E., from O.E. sc(e)ot, (ge)sceot; cf. Ger. Schoss, Geschoss; → effect.

Oskar, → effect; falâxan "sling;" from Av. fradaxšana- "sling," fradaxšanya- "sling, sling-stone;"

The → splitting of spectral lines of atoms and molecules due to the presence of an external electric field, which slightly changes the → energy levels of the atom. → Zeeman effect.

Named after Johannes Stark (1874-1957), a German physicist, and Physics Nobel Prize laureate (1919); → effect.

The loss of energy by high-energy electrons in a → galaxy cluster, which distorts the → cosmic microwave background (CMB) radiation through → inverse Compton effect. When photons from the CMB radiation travel through a hot plasma (with a temperature of around 10⁸ K), in which bathe a galaxy cluster, they collide with energetic electrons and some of the energy of the electrons is transferred to the low energy CMB photons. If we look at the CMB radiation through such a plasma cloud, we therefore see fewer microwave photons than we would if the cloud were not there.

Named after Rashid Sunyaev (1943-) and Yakov Borisovich Zel'dovich (1914-1987), Russian astrophysicists; → effect.

A phenomenon occurring when temperature differences exist in an electrical circuit, such as the → Peltier effect, the → Seebeck effect, and the → Thomson effect,

→ thermoelectric; → effect.

The absorption or emission of heat when current is passed through a single conductor whose ends are kept at different temperatures. If current is passed from hotter end to colder end of a copper wire, then heat is evolved along the length of the wire. When current is passed from colder end to the hotter end, then heat is absorbed.

→ Thomson; → effect.

The → Zeeman effect when observed at right angles to the orientation of the magnetic field. Un un-displaced line is observed along with a doublet, three lines in all, with the frequencies ν and ν ± Δν. The two displaced components correspond to a plane of → polarization parallel to the external magnetic field and the un-displaced line to a plane of polarization perpendicular to this field. → longitudinal Zeeman effect.

→ transverse; → Zeeman effect.

A phenomenon in quantum mechanics whereby a particle can penetrate and cross a potential barrier whose energy is greater than the particle's energy. The tunnel effect, forbidden in classical mechanics, is a direct consequence of the wave nature of material particles. Also called tunneling

M.E. tonel, from M.Fr. tonele, tonnelle "funnel-shaped net," feminine of tonnel,diminutive of tonne "tun, cask for liquids." Sense of "tube, pipe" developed in Eng. and led to sense of "underground passage."

Oskar, → effect; tunel, from Fr. tunnel, as above.

The observation whereby when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red. The effect is most commonly known as the → Rayleigh scattering.

Names for John Tyndall (1820-1893), who discovered the effect in 1859.

Double refraction occurring when a strong → magnetic field is applied to a vapor through which light is passing perpendicular to the field.

Named after Woldemar Voigt (1850-1919), a German physicist (1908, Magneto- und Elektro-optik, B. G. Teubner, Leipzig); → effect.

A phenomenon in which the shape of → sunspots flattens as they approach the → Sun's limb due to the → solar rotation. More specifically, when a sunspot approaches the → solar limbs the width of the → penumbra, relative to the → umbra, on the side facing the center of the Sun seems to become narrower than on the side facing the limb. This phenomenon arises from a projection effect, and is due to a geometrical depression (the → Wilson depression) in the layers of constant → optical depth in sunspots (see, e.g., Sami K. Solanki, 2003, Sunspots: An overview, The Astron. Astrophys. Rev., 11, 153).

First noticed by Alexander Wilson (1714-1786); → effect.

The strong correlation between the equivalent width of Ca II → H and K lines of a late-type giant or supergiant star with the absolute visual magnitude of the star.

O. C. Wilson & M. K. Vainu Bappu (1957, ApJ 125, 661); → effect.

A phenomenon that causes a slow variation of the orbital elements of asteroids and meteoroids. It takes place because the surface thermal conductivity of these bodies is not negligible and the rotation of the body about its axis shifts the warmest region from midday to the object's afternoon hemisphere. Consequently the temperature distribution is asymmetric with respect to the Sun direction, and the momentum carried off by the photons emitted in the infrared has a net component along the orbital velocity of the asteroid. This causes a decrease or increase of its orbital energy depending on whether the rotation is prograde or retrograde. The bodies therefore spiral either sunward or outward. The secular drift of the semi-major axis of the orbit is estimated to be of the order of 10^-4 A.U. per million years for a → near-Earth object with a diameter of 1 km. The effect is unimportant for bodies larger than a few km because of their very large mass per unit area (10⁶ g cm^-2 or more) and is especially unimportant for comets that spend little time under intense illumination close to the Sun. Compare with the → Poynting-Robertson effect, which is isotropic. See also → YORP effect.

Named after Ivan Osipovich Yarkovsky (1844-1902), a Russian-Polish civil engineer. Yarkovsky knew nothing of photons and based his reasoning on the → ether concept, but his idea survives the translation to modern physics; → effect.

A phenomenon in which the rotation rate of a small asteroid changes under sunlight absorption. Photons from the Sun are absorbed by a small body and reradiated in infrared. In the process, two forces influence the object: one from the impact of the photons, providing a tiny push, and the other as a recoil effect when the object emits the absorbed energy. In the YORP effect the body's shape has a more effective role than albedo in altering the spin rate. For small asteroids (< 10 km), YORP can cause measurable changes in rotation rate. The effect can even speed up the rotation leading to disintegration. → Yarkovsky effect.

Short for Ivan Osipovich Yarkovsky, John A. O'Keefe, V. V. Radzievskii, and Stephen J. Paddockk, who developed the explanation; → effect.