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

The aberration of light for an object moving with → relativistic speed. In contrast to the classical case, the → Lorentz transformation between the → rest frame of the observer and that of the object must be used. Relativistic aberration is expressed by the equation: cos φ' = (cos φ - )/(1 - cos φ.v/c), where φ is the classical → aberration angle, v the speed of the Earth, and c the → speed of light. It can also be expressed as: tan φ' = -(c/v)(1 - v²/c)^1/2.

→ relativistic; → aberration.

The Doppler effect when the relative motion of the source and the observer is comparable to the speed of light. In that case the classical Doppler formula should be corrected for effects of the special theory of relativity (Lorentz transformation).

→ relativistic; → Doppler effect.

A physical phenomenon described only by the theories of → special relativity or → general relativity.

→ relativistic; → effect.

In → special relativity, the mass that is assigned to a body in motion. Einstein demonstrated that the velocity of the observer has an effect on the way inertial mass is measured. The relativistic mass is expressed by m_t = m₀ / [1 - (v/c)²]^1/2, where m₀ is the → rest mass, v is the velocity, and c the → velocity of light. The experimental evidence for this phenomenon is observed in the mass of high-speed electrons accelerated by magnetic fields. This formula shows that it is impossible for a mass to be accelerated to the velocity of light in a vacuum.

→ relativistic; → mass.

A particle which has a speed comparable to the velocity of light.

→ relativistic; → particle.

A → general relativistic phenomenon in which the line joining the → apsides of an orbit gradually rotates in a → prograde direction. Also called the → orbital precession and → perihelion precession. Although the general relativistic precession of Mercury's orbit is extremely small, the similar precession observed in the orbit of the → binary pulsar PSR B1913+16 amounts to 4.23 degrees per year, i.e. 36,000 times greater than the → advance of perihelion of Mercury.

→ relativistic; → precession.

A quantum theory that incorporates special relativity, for example, → quantum electrodynamics.

→ relativistic; → quantum mechanics.

A redshift caused by the → relativistic Doppler effect. → gravitational redshift.

→ relativistic; → redshift.

Same as → relativistic velocity.

→ relativistic; → speed.

The change in the direction of the → rotation axis of a → pulsar in a → binary pulsar. In such a system, → geodetic precession leads to a relativistic → spin-orbit coupling, analogous of → spin-orbit coupling in atomic physics. In consequence, the pulsar spin precesses about the total → angular momentum, changing the relative → orientation of the pulsar toward Earth (Damour & Ruffini, 1974). As a result, the angle between the pulsar → rotation axis and our → line of sight changes with time, so that different portions of the emission beam can be observed leading to changes in the measured pulse profile. In extreme cases, the precession may even move the beam out of our line of sight and the pulsar may disappear as predicted for PSR 1913+16 for the year 2025.

→ relativistic; → spin; → precession.

The velocity of a body when it is a significant fraction of the → speed of light.

→ relativistic; → velocity.

To withstand the action or effect of; to strive against, or oppose.

M.E. resisten, O.Fr. resister, from L. resistere "to resist, to stand back, withstand," from → re- "against" + sistere "take a stand, stand firm," cognate with Pers. istâdan, as below.

Baristâdan "to withstand, to stand in opposition," from bar- opposition prefix, → object, + istâdan "to stand," → solstice.
Istâdegi kardan, composite verb, from istâdegi "resistance," state noun from istâdan, + kardan "to do, to make," → -or.

1) General: The act or power of resisting, opposing, or withstanding.
2) Electricity: A property of a conductor by virtue of which the passage of current is opposed, causing electric energy to be transformed into heat. It is the quantity that determines the current that results due to a voltage in a simple circuit. For a fixed voltage, if the resistance increases, the current decreases. It is given by the equation R = ΔV/I, where ΔV is the voltage difference and I the current. Resistance has the SI units volts per ampere, called → ohm.

From → resist + -ance a suffix used to form nouns either from adjectives in -ant or from verbs.

Baristgari, from baristgar, → resistant.
Istâdegi "resistance," state noun from istâdé "stood," p.p. of istâdan "to stand," → resist.

1) Characterized by or showing resistance.
2) A person or thing that resists.

Present participle of → resist, with -ant a suffix forming adjectives and nouns from verbs.

Baristgar, from barist present stem of baristâdan, → resist, + -gar, agent suffix, → -or.
Istâdegigar, from istâdegi, → resistance, + -gar.

Capable of or inclined to resistance; resisting.

From resist, → resistance, + → -ive.

Baristmand, from barist present stem of baristâdan, → resist + -mand possession suffix, → -ist.

A property of a → conductor which is defined as the ratio of the → electric intensity (E) to the → current density (J): ρ = E/J. The greater the resistivity, the greater the intensity needed to establish a given current density, or the smaller the current density for a given intensity. A "perfect" conductor would have zero resistivity, and a "perfect" → insulator an infinite resistivity.

From → resistive + → -ity.

An electrical component used to introduce a known value of resistance into a circuit.

→ resist; → -or.

An expert in science, especially one of the physical or natural sciences. → scholar.

From → science + -ist an agent noun suffix.

Dânešmand, from dâneš, → science, + -mand suffix of possession.

A female person having the same parents as another person.

M.E. suster, sister, from O.E. sweostor, swuster or Old Norse systir, in either case ultimately from PIE *swesor; cf. Skt. svásar-, Av. x^vaηnhar-, Pers. xâhar, xwâhar, as below, Gk. eor, L. soror (Fr. sœur), O.C.S., Rus. sestra, O.Ir. siur, Sw. syster, Dan. soster, O.Sax. swestar, M.Du. suster, Du. zuster, O.H.G. swester, Ger. Schwester.

Xâhar, xwâhar, from Mid.Pers. xwah(ar) "sister;" Av. x^vaηnhar- "sister;" cf. Khotanese hvar- "sister;" cognate with E. sister, as above.

Of, relating to, or subject to the theory of → special relativity.

→ special; → relativistic.