An Etymological Dictionary of Astronomy and Astrophysics
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A type of → lunar eclipse that occurs when both the Sun and the eclipsed Moon can be observed at the same time. This is possible only when lunar eclipse occurs just before sunset or just after sunrise. At that case, both bodies will appear just above the horizon at nearly opposite points in the sky. Also called → selenelion and → selenehelion.

→ horizontal; → eclipse.

The angle under which the radius of the Earth at the place of observation would be seen from a celestial body when it is in the horizon (at the instant of rising or setting). The amount varies with the latitude since the Earth is not exactly spherical, and is greatest at equator.

→ horizontal; → parallax.

The angular distance of an object below the horizon when it appears to lie on the horizon.

→ horizontal; → refraction.

In computer science, a scaling in which the processing power is increased/decreased by adding/removing nodes with similar resources. See also → vertical scaling.

→ horizontal; → scaling.

A very bright crescent of light glowing on the lunar horizon at → sunset or just before → sunrise. It has been suggested that → lunar dust is transported electrically high into sky, allowing sunlight to scatter and create glows. On the day side of the → Moon, solar → ultraviolet radiation is strong enough to kick → electrons from → dust grains in the lunar soil. Removal of electrons, which have a negative electric charge, leaves the dust with a positive electric charge. Since like charges repel, the positively charged dust particles get pushed away from each other, and the only direction not blocked by more dust is up. In the 1960s, Surveyor probes filmed a glowing cloud floating just above the lunar surface during sunrise. Later, Apollo 17 astronaut Gene Cernan, while orbiting the Moon, recorded a similar phenomenon at the sharp line where lunar day meets night, called the → terminator.

→ lunar; → horizon; → glow.

A space mission by → NASA whose main goal is to study the → dwarf planet Pluto and it satellites. New Horizons was launched on January 19, 2006; it swung past → Jupiter for a → gravity assist and scientific studies in February 2007, and conducted a six-month-long reconnaissance → flyby study of → Pluto and its moons in summer 2015, culminating with Pluto closest approach on July 14, 2015. It flew 12,500 km above the surface of Pluto, making it the first spacecraft to explore the dwarf planet. Its science payload includes seven instruments: Ralph (visible and infrared imager/spectrometer), Alice (ultraviolet imaging spectrometer), REX (Radio Science EXperiment), LORRI (Long Range Reconnaissance Imager), SWAP (Solar Wind Around Pluto), PEPSSI: (Pluto Energetic Particle Spectrometer Science Investigation), and SDC: (Student Dust Counter). As part of an extended mission, New Horizons has maneuvered for a flyby of → Kuiper belt object 2014 MU69, expected to take place on January 1, 2019, when it is 43.4 → astronomical units (AU) from the Sun.

→ new; → horizon.

For an observer at a given epoch t₀, the boundary between the observable and the unobservable regions of the → Universe. Therefore, the distance to the particle horizon at t₀ defines the size of the → observable Universe. Same as → cosmic horizon.

→ particle; → horizon.

A star found on the red part of the → horizontal branch. According to theoretical models, these stars result from the evolution of stars with a mass around 0.8 Msun, higher than that giving rise to → BHB stars. Upon helium burning in their cores, the remnant envelope of the red giant collapses.

→ red; → horizontal; → branch; → star.

The → apparent horizon formed by the sea.

→ sea; → horizon.

The great circle of the celestial sphere whose poles are the nadir and zenith. Same as astronomical horizon.

→ sensible; → horizon.

The maximum distance a → sound wave could have traveled through the ionized plasma from the → Big Bang until the → recombination era. It is 150 → Mpc, or bout 500 million → light-years. Sound horizon is the equivalent of the concept of → cosmic horizon, where one replaces → electromagnetic wave by → sound wave. The sound horizon is a fixed physical scale at the → last scattering surface. Cosmological models relate the value of sound horizon to the angle it subtends on the sky today. Same as acoustic horizon and sonic horizon. See also → CMB angular power spectrum.

→ sound; → horizon.

A member of a rare class of objects found in → globular clusters to lie about one magnitude above and to the blue part of the → horizontal branch. These stars are identified as post → EHB stars on their way from to the → asymptotic giant branch.

→ supra-; → horizontal; → branch; → star.

A large circle of the → celestial sphere whose plane is perpendicular to the radius of the Earth through the point. Same as → astronomical horizon. The → visible horizon usually lies lower than the true horizon. See also → dip of the horizon.

→ true; → horizon.

That line where Earth and sky appear to meet, and the projection of this line upon the celestial sphere. Also known as the apparent horizon.

→ visible; → horizon.

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

→ zero; → age; → horizontal; → branch; → star.