Astronomy and related astronomical terms

The densness of a substance relative to that of water: Thus, a material with a relative density of 5.5 has a compactness of about 343 pounds per cubic foot or 5,500 kilograms per cubic meter.

Theories of physics proposed by Albert Einstein in 1905 to explain the influence of the relative motion and position of an observer on his own observations: Relativity refers to the nature of physical occurrences of light, mass, time, and gravity on the relative motion between a viewer and the object being viewed.

They say, among other things, that space and time can not be considered separate ideas. The perception of space-time is different for a person standing still on Earth than it is for someone moving very fast away from or toward it.

What we see is "relative" to, or dependent on, our acceleration as we move.

The General Theory of Relativity, published in 1916, defined gravitation as a function of four-dimensional space-time.

The ability of a telescope to separate two closely spaced sources of radiation, such as stars: The resolving power of an optic instrument is its capability to produce single or separate images of closely situated objects.

A body is said to be in resonance when it is affected by a force applied with a certain frequency, the resonant frequency, at which the body is seriously perturbed from equilibrium: An example of resonance in astronomy is provided by the Kirkwood gaps.

1. The apparent backward movement, or reversal of direction, by outer planets in the solar system: Retrograde motion is simply an optical illusion, created by the fact that the Earth is orbiting the Sun much faster than the outer planets are.

2. The clockwise, or east to west, motion of a body, and hence the reverse of direct motion: As the majority of bodies in the solar-system orbit around their governing bodies, that is the Sun or a planet in direct motion, the occurrence of retrograde motion usually indicates some peculiarity.

The angular distance measured eastward along the celestial equator, between a celestial object and the first point of Aries: The symbol of the right ascension is α (alpha), and this coordinate is usually expressed in units of time.

The minimum distance between the center of one body and another orbiting around it at which the second can withstand tidal forces generated by the first: A Roche limit can be exemplified by the center of a planet in which the tidal force is more powerful than than the common gravitational pull or magnetism between two neighboring revolving objects.

The national astronomical observatory of the U.K.: The Royal Greenwich Observatory was founded in 1657 at Greenwich, S.E. London, England, to provide navigational information for sailors.

After World War II, it was moved to Herstmonceux Castle, Sussex. In 1990, it was transferred to Cambridge.

It also operates telescopes on La Palma in the Canary Islands, including the William Herschel Telescope, commissioned in 1987.

One of a family of pulsating giant stars, with periods of less than one day: The period of particular examples does show changes, both abrupt and slow. Such RR Lyrae stars are commonly found in globular clusters.

A bright zodiac constellation in the southern hemisphere: Sagittarius is represented as a centaur aiming a bow and arrow at neighboring Scorpius.

The Sun passes through Sagittarius from mid-December to mid-January, including the winter solstice, when it is farthest south of the equator. The constellation contains many nebulae and globular clusters, and open star clusters.

Kaus Australis and Nunki are its brightest stars. The center of our galaxy, the Milky Way, is marked by the radio source Sagittarius A.

A moon or a man-made body in orbit around a planet: For clarity, natural satellites are commonly described as "moons" to distinguish them from man-made or artificial satellites.

Concept for providing large amounts of electricity for use on the Earth from one or more satellites in geosynchronous Earth orbit: The satellite power system sends the Sun's energy to a satellite in an orbiting geostatiopn where it is then converted to electric energy and beamed to Earth

A very large array of solar cells on each satellite would provide electricity, which would be converted to microwave energy and beamed to a receiving antenna on the ground. There, it would be reconverted into electricity and distributed the same as any other centrally generated power, through a grid.

This system is also termed Space Solar Power (SSP).

1. A camera mounted in a Schmidt telescope: A Schmidt camera is a telescope-camera able to take pictures covering a wide area of the sky without optical problems by virtue of having a specially shaped corrector plate near its upper-end.
2. A wide-angle photographic telescope used in astronomy which has a special internal mirror to correct optical aberrations: A Schmidt telescope is a type of reflecting telescope; more accurately, a large camera, in which the coma produced by a spherical concave mirror is compensated for by a thin correcting lens placed at the opening of the telescope tube and has a usable field of 0°.6.

The Schmidt telescope has a corrector lens that prevents distortions of the image which is produced by its large spherical mirror.

Something called "spherical aberration" occurs when the uncorrected mirror does not focus all of the light rays at the same point.

A mass limit for the isothermal, helium core of a main-sequence star above which the star must rapidly increase in radius and evolve away from the main sequence: The Schönberg-Chandrasekhar limit is a limit on the mass of a main sequence star's core above which the star will leave the Main Sequence to become a red giant.

This takes place when the helium core makes up 10 to 15 per cent of the star's mass.

The maximum mass of a star's helium-filled core can support the overlying layers against gravitational collapse. Once the core hydrogen is exhausted, it is believed to be 10 to 15% of the total stellar mass.

If this limit is exceeded, as can only happen in massive stars, the core collapses, releasing energy that causes the outer layers of the star to expand to become a red giant.

It is named after Indian-born (Lahore, India, now Pakistan) American astrophysicist, Subrahmanyan Chandrasekhar (1910-1995) and the Brazilian astrophysicist, Mario Schönberg (1914-1990), who were the first to point out this limit and derive it.

The twinkling of a point source of light as a result of the turbulence of the atmosphere through which the source's light passes: Scintillation is the sparkling of the stars caused when changes in the density of the Earth's atmosphere produce uneven refractions of starlight.

Scintillation is a rapid variation in the light , or blinking, of a celestial body caused by turbulence in the Earth's atmosphere.