Astronomy and related astronomical terms

A nebula that shines as the result of the scattering of the light of a star or group of stars nearby.

Such scattering is usually caused by dust within the nebula.

A telescope that uses lenses to gather light from faint objects.

The front, objective, lens is usually made of two or more components, with the eyepiece at the other end being the point at which the observer sees the image of the object.

The topmost layer of the moon and earth-like planets, which has been broken up by meteoric bombardment.

The density of a substance relative to that of water.

Thus, a material with a relative density of 5.5 has a density 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 he relative motion and position of an observer on his own observations.

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 (depends 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.

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 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 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.

The national astronomical observatory of the U.K., 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 stars are commonly found in globular clusters.

A bright zodiac constellation in the southern hemisphere, 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.

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.