An Etymological Dictionary of Astronomy and Astrophysics

The splitting of spectral lines into closely-spaced components when the radiating substance is placed in a strong → magnetic field. Distinction is made between the normal and
→ anomalous Zeeman effects, and also the longitudinal and transverse Zeeman effects. In the normal → longitudinal Zeeman effect each spectral line is split into two components with frequencies ν ± Δν. In the normal → transverse Zeeman effect un un-displaced line is observed along with a doublet, i.e. three lines in all, with the frequencies ν and ν ±Δν. In the classical theory of the normal Zeeman effect, the motion of an electron in an atom is regarded as the harmonic oscillation of a linear harmonic oscillator. Arbitrary linearly polarized oscillation of the electron can be resolved into two oscillations: one along the magnetic field and the other in a plane perpendicular to this field. The latter can be further resolved into two oscillations, circularly polarized with opposite directions of rotation that occur in the Larmor precession frequency. Classical theory cannot explain the anomalous Zeeman effect. Both effects are accounted for in quantum mechanics as the result of changes in the energy levels of atomic electrons due to the interaction of their → orbital angular momentum and → spin angular momentum with each other and with the external magnetic field. See also → inverse Zeeman effect.

See also: Named after Pieter Zeeman (1865-1943), Dutch physicist who discovered the phenomenon; → effect.

The point on the celestial sphere directly above the observer’s head, opposite to the direction in which gravity acts. Opposite of → nadir.

Etymology (EN): M.E. cenith, from O.Fr. cenith, from M.L. cenit, senit, incorrect transliteration of Ar. samt (سمت) “path, direction,” abbreviation of samt ar-ras (سمت‌الرأس), literally “road above one’s head.”

Etymology (PE): Sarsu, literally “the way over the head,” from sar→ head + su, → direction.

The angular distance of a celestial body from the zenith. The zenith distance is 90° minus the body’s altitude above the horizon (i.e. the complement of the altitude) and hence is also known as coaltitude.

See also: → zenith; → distance

The number of → meteors expected to be seen under ideal conditions if the → radiant of the → meteoroid stream is at the → zenith of the → observer.

See also: → zenith; → hour; → rate.

A → telescope that is mounted on a → vertical axis or moves only a small amount from the vertical. It is primarily used to determine positional measurement of stars moving near the → zenith. The advantage is that
there is no → atmospheric refraction occurring at the zenith. If a star on one night passes through the center of eyepiece, one must observe it six month later, and see if the star has been offset by the center. A shift would mean a measure of parallax.

See also: → zenithal well.

See also: → zenith; → telescope

Of or relating to the → zenith; located at or near the zenith.

See also: → zenith; → -al.

1. A well used in Antiquity from bottom of which the sky could be observed during the day with a better contrast. The aperture of the well reduced the light diffused by the sky.
   

   2. A vertical tunnel in → Paris Observatory (built in 1667), from the top roof to the underground vaults, 55m deep. This well had the purpose of observing the stars near the → zenith and measure their → parallaxes resulting from the Earth motion around Sun. A long plumb line was attached to the top of the well. Astronomers thought they could measure the stellar shifts with respect to the plumb line. The problem was, however, the lack of stability of the images, because the well acted in fact as a chimney generating turbulence. So that the zenithal well was hardly used.

   See also: → zenith telescope.

See also: → zenithal; → well.

A prefix of the → SI units, denoting 10^-21. Symbol z.

See also: From L. septem, → hepta-, since it is equal to 1/1000⁷.

1. The cardinal number of a class that has no members.
   

2. The only integer that is neither negative nor positive.
   

3. The → identity element for addition.

See also: From Fr. zéro, from It. zero, from M.L. zephirum, from Ar. sifr (صفر) “cipher,” translation of Skt. śūnya “empty place, void, naught.” The rules governing the use of zero appeared for the first time in the Indian mathematician Brahmagupta’s book Brahmasputha Siddhanta “The Opening of the Universe,” written in 628.

A star that has arrived on the → horizontal branch after leaving the → red giant branch. It begins → helium burning in its core.

See also: → zero; → age; → horizontal; → branch; → star.

The position on the → Hertzsprung-Russell diagram for newborn stars which have just started → hydrogen burning in their cores. The ZAMS forms the lower luminosity boundary of of the → main sequence strip. See also → terminal age main sequence (TAMS).

See also: → zero; → age; → main sequence.

Same as → null matrix.

See also: → zero; → matrix.

same as → null method.

See also: → zero; → method.

1. General: A starting point for making a measurement.
   

2. A basic parameter on which a → photometric system relies and must be determined each night. The zero points of → magnitudes and color indices (→ color index) depend on atmospheric transparency that varies from night to night.

See also: → zero; → point.

The lowest possible energy that a → quantum mechanical system may possess. It is the energy of the → ground state of the system. The term “zero point” refers to the observed fact that → vacuum fluctuations persist at → absolute zero temperature. Same as → vacuum energy.

See also: → zero; → point; → energy.

A → polynomial whose → coefficients are all zero. See also → non-zero polynomial.

See also: → zero; → polynomial.

The removal of non-significant zeroes from a number. For example, replacing 531.2300 by 531.23.

See also: → zero; → suppression.

In the → restricted three-body problem, a surface which limits the region of space in which a small body can move. In the expression for the → Jacobi integral, the left side value is always positive or nul; hence the particle motion is confined to the region where U ≤ C_J. The surface that limits this region, defined by U = C_J, is called the zero-velocity surface.

See also: → zero; → velocity; → surface.

Two objects that are in → thermal equilibrium with a third object will be in thermal equilibrium with each other.

See also: → zero; → law; → thermodynamics.

A blue star, also called HD 149757 and HR 6175, which is the nearest, and probably the most widely studied, → massive star. It is variable in several wavelength bands and has a mean visual magnitude of V = 2.58, B - V = 0.01. It lies ~ 222 pc away and has
a formal spectral type of O9.5 Ve and a luminosity of 10⁵ Lsun. ζ Ophiuchi is a very rapid rotator with a v sin i ~ 400 km s^-1, i.e.
~ 85% of the → break-up velocity. It is one of the earliest prototypes of the → Be phenomenon. Moreover, it shows episodes of Hα emission variability, a common feature of Be/Oe stars.
It also shows periodic non-radial pulsations and UV → P Cygni profile variability, as is evident in the periodic behavior of → discrete absorption components (DACs). It has a → mass loss rate of 10^-7 Msun yr^-1 and a → terminal velocity of wind
v_∞ = 1550 km s^-1. ζ Ophiuchi is a well-known → runaway star with a velocity of 30 km s^-1. The interstellar → CH molecule and → CN molecule were first detected toward ζ Ophiuchi. It has been recognized for some time that this star lies close to the blue edge of the → beta Cephei instability strip.

See also: Zeta (ζ), according to the → variable star designation system; Ophiuchi, → Ophiuchus.

Same as → Alnitak.

See also: Zeta (ζ), Gk. letter in the → Bayer designation scheme.

A prefix of the → SI units, denoting 10²¹. Symbol: Z.

See also: From L. septem, → hepta-, by replacing the initial s with z to distinguish from → zepto-.
It is equal to 1000⁷.