An Etymological Dictionary of Astronomy and Astrophysics
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Also known as Gemma, the brightest star in Corona Borealis (visual magnitude 2.23). Alphekka is an A type dwarf lying at about 7 → light-years. Actually it has a faint Sun-like (G5 V) companion, that produces an eclipse of the primary every 17.4 days.

Alphekka, from Ar. Nayyir al-Fakkah "the bright of the broken" (ring of star), from Nayyir "bright" + fakkah "broken," from fakk "to disjoin, unloose".

The time interval between two successive → upper transits of the → true equinox of date.

→ apparent; → sidereal; → day.

The → mean sidereal time corrected for the → nutation and shift in the obliquity of the ecliptic that occurs as a result of the Moon's gravitational effect. Apparent sidereal time differs from mean sidereal time in that the → true vernal equinox point is used.

→ apparent; → sidereal; → time.

The aurora in the Northern hemisphere, also called as Northern Lights.

→ aurora; → north.

The Northern Crown. A small but prominent northern → constellation that lies east of → Arcturus, between → Boötes and → Hercules, and comprises a distinctive arc formed by the stars seven stars. Abbreviation: CrB; genitive: Coronae Borealis.

→ corona; L. borealis "northern."

The → Greenwich Mean Sidereal Time corrected for → nutation. Therefore, it is measured with respect to the → true vernal equinox. GAST and GMST differ by the → equation of the equinoxes.

→ Greenwich Meridian; → apparent; → sidereal; → time.

The → sidereal time related to the angle between the → prime meridian and the → mean vernal equinox, measured in the plane of the equator.

→ mean; → Greenwich Meridian; → sidereal; → time.

The number and fraction of → mean sidereal days elapsed on the → Greenwich meridian since 12h January 1, 4773 BC (mean sidereal).

→ Greenwich meridian; → sidereal; → date.

The integral part of the → Greenwich sidereal date.

→ Greenwich; → sidereal; → day; → number.

Local time measured by the apparent motion of the stars. It is the most useful form of sidereal time since it gives the right ascension of a transiting celestial object at a given location.

→ local; → sidereal; → time.

Same as → sidereal month.

→ lunar; → sidereal; → orbital; → period.

The average time interval between two successive → upper transits of the → mean equinox.

→ mean; → sidereal; → day.

The hour angle of the mean equinox for a given observer.

→ mean; → sidereal; → time.

1) General: Having actual physical existence; genuine, not artificial.
2) Math.: Involving, relating to, or having elements of the set of rational or irrational numbers only.
3) Philo.: Existent or pertaining to the existent as opposed to the nonexistent; actual as opposed to possible or potential.
See also:
→ real dynamical variable, → real equator, → real equinox, → real fluid, → real gas, → real image, → real number, → real object, → real-time operation, → real-time processing, → realism, → realist, → realistic, → reality, → true.

M.E., from O.Fr. reel, from L.L. realis "relating to things; actual," from L. res "matter, thing," of unknown origin.

Hasyâ, from O.Pers. hašya- "true, real;" Av. haiθa- "true, real;" cf. Khotanese hiththo "true, real;" Ossetic aecaeg "true, real, genuine;" Skt. satyá- "true, real, genuine;" PIE base *es- "to be" (Mod.Pers. hastan, astan "to be;" O.Pers./Av. ah-, Skt. as-).

A → dynamical variable which does not have an → imaginary number part.

→ real; → dynamical; → variable.

Same as → true celestial equator.

→ real; → equator.

Same as → true equinox.

→ real; → equinox.

The opposite of an ideal fluid; a fluid which possesses viscosity and therefore exhibits certain frictional phenomena. Viscosity arises due to cohesive forces between molecules and molecular momentum exchange between fluid layers. These effects appear as tangential or shear stresses between moving fluid layers.

→ real; → fluid.

Actual gas whose molecules do not necessarily possess the properties assigned to those of an → ideal gas.

→ real; → gas.

In an optical system, the image of an object produced by the convergence of the light rays that make up the image. → virtual image; → real object.

→ real; → image.