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

One of two equal or approximately equal parts of a divisible whole.

M.E., from O.E. h(e)alf "side, part," from P.Gmc. *khalbas "something divided" (cf. M.Du. half, Ger. halb, Goth. halbs "half").

Nim, nimé "half," from Mid.Pers. nêm, nêmag "half;" Av. naēma- "half;" cf. Skt.. néma- "half."

The moon when, at either quadrature, half its disk is illuminated.

→ half; → moon.

The length of time required for half of a given quantity of → radioactive material to → decay.

→ half; → life.

The angle between extreme points of the main lobe of an antenna pattern where the sensitivity of the antenna is half the value at the center of the lobe. This is the nominal resolving power of the antenna system.

→ half; → power; → beam; → width.

The thickness of material required to reduce the intensity of an → X-ray beam to one half of its initial value. The HVL is an indirect measure of the photon energies of a beam.

→ half; → value; → layer; → attenuation.

A plate of optical material whose thickness is such that the phase difference between the extraordinary and ordinary rays after passing through the place is exactly one-half cycle. It can serve to rotate the plane of polarization of a light beam.

→ half; → wave; → plate.

A retrograde irregular satellite of Neptune discovered in 2002. Also called Neptune IX. Halimede is about 62 kilometres in diameter.

In Gk. mythology, one of the Nereids, the fifty daughters of Nereus and Doris.

The most famous comet orbiting the Sun once about every 75 years. The last time it appeared was in 1986, and it is predicted to return in 2061. Its earliest recorded sighting is traced back to 240 BC in China. In 1705 Edmond Halley used Newton's new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time. He found that the bright comets of 1531, 1607, and 1682 had almost the same orbits. He concluded that these appearances must belong to a single recurring comet, and predicted its return for 1758. Halley's comet is the first known → periodic comet, hence its → designation 1P/Halley.

Named after the English astronomer Edmond Halley (1656-1742), who first computed its orbit and predicted its return in 1758; → comet.

1) Meteo.: Rings or arcs that seem to encircle the sun or moon and are the result of the refraction of light through the ice crystals that make up cirrus clouds.
2) → halo of galaxy.

Halo, from L. (acc.) halo, from Gk. halos "ring of light around the sun or moon."

Hâlé, loanword from Ar.

The → probability distribution of the → number of galaxies that a host → dark matter halo of a given mass contains. HOD is a powerful theoretical frame to populate dark matter halos with luminous galaxies. More specifically, it describes the bias between galaxies and dark matter by specifying (a) the probability P(N|M) that a halo of → virial mass M contains N galaxies of a particular class and (b) the relative spatial and velocity distributions of galaxies and dark matter within halos.

→ halo; → occupation; → distribution.

The diffuse, nearly spherical cloud of stars and → globular cluster s that surrounds a → spiral galaxy.

→ halo; → galaxy.

The → halo of galaxy belonging to our → Milky Way.

→ halo; → galaxy.

Old stars with very low metallicities (→ metallicity) found in the → halo of the Galaxy. Also called → population II star.

→ halo; → population.

A faint, wide ring around → Jupiter that has the shape of a doughnut. It is about 22,800 km wide and about 20,000 km thick. This ring starts at 100,000 km from the center of Jupiter. The outer edge of the Halo merges into the → Main ring.

→ halo; → ring.

A member of a group of five chemical elements having closely related and similar properties. The halogens are: fluorine, chlorine, iodine, bromine, and astatine. They make up Group 17 of the → periodic table and can be found on the left-hand side of the → noble gases.

From Gk. halo- prefix from Gk. hals "salt" + → -gen.

The brightest star in the constellation → Aries. Hamal is a cool → giant of → spectral type K2 with a → luminosity about 55 times that of the Sun and lies about 65 light-years away.

Hamal, from Ar., shortened form of Ra's al-Hamal (رأس‌الحمل), "the head of the sheep."

One of a set of equations that describe the motion of a → dynamical system in terms of the → Hamiltonian function and the → generalized coordinates. For a → holonomic system with n degrees of freedom, Hamilton's equations are expressed by: q^._i = ∂H/∂p_i and p^._i = - ∂H/∂q_i, i = 1, ..., n.

→ Hamiltonian function; → equation.

Of all the possible paths along which a → dynamical system can move from one configuration to another within a specified time interval (consistent with any constraints), the actual path followed is that which minimizes the time integral of the → Lagrangian function. Hamilton's principle is often mathematically expressed as δ∫Ldt = 0, where L is the Lagrangian function, the integral summed from t₁ to t₂, and δ denotes the virtual operator of Lagrangian dynamics and the → calculus of variations.

→ Hamiltonian function; → principle.

The study of → dynamical systems in terms of the → Hamilton's equations.

→ Hamiltonian function; → dynamics.

A reformulation of classical mechanics that predicts the same outcomes as classical mechanics. → Hamiltonian dynamics.

→ Hamiltonian; → mechanics.