Astronomy and related astronomical terms

The angular distance between a celestial body and the ecliptic, measured along a line at right angles to the ecliptic, which passes through both celestial poles: The symbol of the celestial latitude is the Greek letter β or beta.

Longitude measured (in degrees) along the ecliptic to the east from the vernal equinox: The Symbol for the celestial longitude is the Greek letter λ or lambda.

The angular distance between a celestial body and the first point of Aries, measured along the ecliptic.

The study of the motion of celestial bodies under the influence of gravitation from one or more other bodies: Celestial mechanics is the branch of astronomy that deals with the calculation of the orbits of celestial bodies, their gravitational attractions, such as those that produce the earth's tides.

It also refers to the orbits of artificial satellites and space probes and is based on the laws of motion and gravity laid down by Isaac Newton (English mathematician, 1642-1727, and physicist; remembered for developing the calculus and for his law of gravitation and his three laws of motion).

The points in the sky where the earth's axis, extended into space, intersects with the celestial sphere: The celestial poles are the points at which the earth's rotation axis pierces the celestial sphere.

All of the stars appear to orbit around this point, whose approximate position in the north is marked by Polaris, the "polestar".

There are three main types of celestial radio emissions:

1. Thermal emissions which occur as a result of the acceleration of electrically-charged particles in a hot gas.
2. Synchrotron or non-thermal, emissions are produced by the acceleration of charged particles, but the acceleration is caused by a magnetic field.
3. Radio spectral-line emissions are concentrated in a narrow band around one specific frequency; just as an optical spectral line corresponds to a single frequency in the visible electromagnetic spectrum.

Radio line emissions usually originate in clouds of hydrogen gas, a relatively common constituent of the universe, which is found in our galaxy's spiral arms, among other places.

The imaginary sphere surrounding the earth, with the stars and other astronomical objects attached to it: The celestial sphere exists on the inside of which the celestial bodies seem to lie.

The positions of bodies, such as stars, planets, and galaxies are specified by their coordinates on the celestial sphere.

One of a group of highly luminous yellow or orange super giant stars, whose brightness varies in a regular manner as the result of stellar pulsations: Cepheid variable is a yellow super giant star that fluctuates regularly in brightness every few days or weeks as a result of pulsations.

The time that a Cepheid variable takes to pulsate is directly related to its average brightness; the longer the period, the brighter the star.

"Cepheus" is a constellation of the northern hemisphere near Cassiopeia (a W-shaped constellation in the northern hemisphere near Polaris) and Draco (a large faint constellation of the northern hemisphere).

A limiting mass, below which a star can become a white dwarf and above which gravity is capable of continuing the collapse to, such as a neutron star: The Chandrasekhar limit is the maximum speculative mass of a white dwarf , about 1.4 solar masses.

A means of converting the electromagnetic energy of photons of a particular wavelength into a digital video signal which can be displayed on a television screen: A charge-coupled device is used for forming images electronically, using a layer of silicon that releases electrons when struck by incoming light.

The electrons are stored in pixels and read off into a computer at the end of the exposure. CCD's have now almost entirely replaced photographic film for applications, such as astrophotography where extreme sensitivity to light is so important.

Since the mid-1970s, a new type of detector called the charge-coupled device has gained popularity in astronomy.

The CCD is an electronic detector that replaces photographic plates or film.

It converts light into a electric charge, which is used to form images on a computer screen.

The charge-coupled device was developed as a replacement for television camera tubes and is commonly used in today's camcorders.

Modern astronomical CCDs are much larger, with as many as 16 million separate detectors, or pixels.

Because they are far more sensitive to light, charge-coupled devices produce better pictures than traditional photographic methods and astronomers can see extremely faint galaxies in almost any part of the sky.

The layer in the sun's atmosphere between the photosphere and the corona: The chromosphere consists mostly of hydrogen gas and is about 10,000 kilometers or 6,000 miles in depth above the visible surface of the sun and many other stars just above the photosphere.

During eclipses, the solar chromosphere glows pinkish red from the hydrogen emissions.

A star, asterism, or constellation that is close enough to the celestial pole that, depending on the latitude at which a person is observing, it never appears to set: Circumpolar stars are those that are within a circumpolar circle in which the size is determined by the viewer's latitude.

A day calculated from midnight to midnight: : The civil day applies to two 12-hour periods, A.M. an P.M., but these are never used astronomically so the 24-hour clock is used.

A small body made of ice, dust, and rocks which orbits the sun: A comet can be a collection of gas, dust, and volatile ice that travel around the sun, generally in very eccentric orbits.

The source of such bodies may be the oort cloud (cloud of comets).

A comet, probably resembling a "dirty snowball", can be between 0.1 and 100 kilometers across, which travels through the solar system in an ecliptical orbit of random inclination to the ecliptic.

A comet grows a tail if it goes close enough to the sun.

A large and exceptionally active comet, which in March, 1997, made its closest approach to Earth since 2000 B.C., coming within 190 million kilometers or 118 million miles: Comet Hale-Bopp has a diameter of approximately 40 kilometers or 25 miles and an extensive gas coma. Hale-Bopp has three tails: one consisting of dust particles, one of charged particles, and a third of sodium particles.

Comet Hale-Bopp was discovered in July, 1995, by two amateur U.S. astronomers, Alan Hale and Thomas Bopp.