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

A unit of angular size equal to 1/3.600 of a degree.

→ arc; → second.

An angular measurement equal to 1/3600 of a degree or 1/60 of an arcminute. It is denoted by the symbol ''. There are 206,264.5'' in a radian, so that 1'' = 4.848 × 10^-6 radians.

→ arc; → second.

One of the postulates used in the → Bohr model, whereby when an atom is in the steady state an electron travelling in a circular orbit should have → quantized values of the → angular momentum which comply with the condition p = n(h/2π), where p is the angular momentum of the electron, h is → Planck's constant, and n is a positive integer called → quantum number.

→ Bohr; → second; → postulate.

The length of a tropical second (1/31,556,925.97474 of the tropical year) on 1900 January 0.5 → ephemeris time.

→ ephemeris; → second.

A line joining a planet to the Sun sweeps out equal areas in equal intervals of time (year 1609).

→ Kepler; → second; → law.

A one-second added between 60s and 0s at announced times to keep the Coordinated Universal Time (UTC), counted by atomic clocks, within 0s.90 of mean solar time (UT1). Generally, leap seconds are added at the end of June or December.

→ leap; → second.

The distance travelled by light in free space in one second. It is equivalent to 2.997924580 × 10⁸ m or 2.998 × 10⁵ km. This unit of length is mainly used in astronomy, telecommunications, and relativistic physics. Some quantities expressed in this unit are as follows. The mean diameter of the Earth: about 0.0425 light-seconds. The average distance from the Earth to the Moon: about 1.282 light-seconds. The diameter of the Sun: about 4.643 light-seconds. The average distance from the Earth to the Sun: 499.0 light-seconds.

→ light; → second.

A unit of angle equal to one thousandth of an → arcsecond, or 1/3 600 000 degree.

→ milli-; → arcsecond.

A type of pulsar that spins around its axis every few thousands of a second. The prototype 1937+21, discovered in 1982, has a period of 1.56 milliseconds; i.e. it spins more than 600 times per second, the fastest so far found (Backer et al. 1982, Nature 300, 615). These pulsars are distinguished from typical pulsars by the extreme stability of their rotation period. It is thought that they have been rejuvenated by a "spin-up process" involving the accumulation of matter from a companion star. → recycled pulsar.

→ milli-; → second; → pulsar.

For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

→ Newton; → second; → law; → motion.

1) Next after the first in place, time, or value.
2) The unit of time in the → International System of Units; symbol s. It is defined by taking the fixed numerical value of the cesium frequency Δν_Cs, the unperturbed → ground state  → hyperfine transition  → frequency of the → cesium-133 atom, to be 9 192 631 770 when expressed in the unit → hertz (Hz), which is equal to s^-1.

M.E., from O.Fr. second, from L. secundus "following, next in order," from root of sequi "to follow;" PIE base *sek^w- "to follow;" cf. Pers. az from; Mid.Pers. hac "from;" Av. hac-, hax- "to follow," hacaiti "follows" (O.Pers. hacā "from;" Av. hacā "from, out of;" Skt. sácā "with"); Skt. sácate "accompanies, follows;" Gk. hepesthai "to follow;" Lith. seku "to follow."

1) Dovom, dovomin "ordinal number of do, two" (Mid.Pers. do; Av. dva-; cf. Skt. dvi-; Gk. duo; L. duo; (Fr. deux; E. two; Ger. zwei).
2) Sâniyé, from Ar. sâniyat (feminine) "second."

Math: In calculus, limiting an equation to its → second derivative, for example: e^x≅ 1 + x + x²/2. Also called linear approximation. → first approximation.

→ second; → approximation.

An early evolutionary period in the process of star formation which succeeds the → first collapse. When the mass of the → first core has increased by about a factor 2 and the radius has decreased by a similar factor, the central temperature of the core reaches about 2000 K. At this point the → molecular hydrogen begins to dissociate into atoms. This reduces the → adiabatic index (γ) below the critical value 4/3, with the result that the material at the center of the core becomes unstable and begins to collapse. Most of the gravitational energy generated by this collapse goes into the → dissociation of H₂ molecules, so that the temperature rises only slowly with increasing density. In this second collapse phase, as in the first, the density distribution in the collapsing region becomes more and more sharply peaked at center, and the time scale becomes shorter and shorter with increasing central density. The central collapse of the core continues until the hydrogen molecules are nearly all dissociated and γ again rises above 4/3. The central pressure then rises rapidly and once again becomes sufficient to decelerate and stop the collapse at the center. A small core in the → hydrostatic equilibrium then arises, bounded by a shock front in which the surrounding infalling material is suddenly stopped. The initial mass and radius of the second core are about 3 x 10³⁰ g (1.5 x 10^-3M_sun) and 9 x 10¹⁰ cm (1.3 R_sun) respectively, and the central density and temperature are about 2 x 10^-2 g cm^-3 and 2 x 10⁴ K, respectively. The second core will evolve into a → young stellar object (R. B. Larson, 1969, MNRAS 145, 271).

→ second; → collapse.

The beginning of the total phase of a solar eclipse when the leading edge of the Moon touches the eastern edge of the Sun completely obscuring the Sun.

→ second; → contact.

A hydrostatic object predicted to result from the → second collapse of a → molecular cloud in an early stage of star formation.

→ second; → core.

In → calculus, the → derivative of a → first derivative. It is usually written as f''(x), d²y/d²x, or y''.

→ second; → derivative.

A method, used in → calculus, for determining whether a given → stationary point of a → function is a → local minimum or → local maximum.

→ second; → derivative; → test.

A → dredge-up process that occurs after core helium burning, in which the convective envelope penetrates much more deeply, pushing hydrogen burning shell into close proximity with the helium burning shell (→ first dredge-up). This arrangement is unstable and leads to burning pulses. The reason is that the hydrogen shell burns out until there is enough helium for the helium combustion to occur and all the helium is rapidly burnt. Afterward the hydrogen shell again burns outward and the process repeats.

→ second; → dredge-up.

A star whose formation is induced by an older star itself formed previously in the same region. See also → stimulated star formation, → sequential star formation, → triggered star formation.

→ second; → generation; → star.

The surface area of a black hole's horizon can never decrease.

→ second; → law; → black hole; → mechanics.