Astronomy and related astronomical terms

Sunlight reflected off the earth; earthlight: Earthshine lights up the side of the moon that does not receive direct sunlight.

A parameter of a conic section which describes how much it deviates from a perfect circle: The eccentricity of a circle is zero, an ellipse has an eccentricity between zero and one, a parabola has an eccentricity equal to one, whereas hyperbolae have eccentricities exceeding one.

A binary star in which one component, in orbiting around the other, is totally or partially eclipsed as seen from the earth: Jill was fascinated when she finally was able to view an eclilpsing binary where two stars were circling each other.

A total or partial obstruction of light from a celestial body caused by its passing into the shadow of another body; also, the hiding of one celestial body by another one: An eclipse occurs most notably when the moon passes in front of the sun.

In an eclipse of the sun, it is the light of the sun which is totally or partially cut off from the planet Earth by the moon coming between the two bodies.

The sun's apparent circular path through the heavens or over the celestial sphere in the course of a year: The ecliptic is the route, against the background of stars, that the sun appears to follow each year as it is orbited by the earth.

The ecliptic can be thought of as the plane of the earth's orbit projected on to the celestial sphere or imaginary sphere around the earth.

The ecliptic is tilted at about 23.5° with respect to the celestial equator, a result of the tilt of the earth's axis relative to the plane of its orbit around the sun.

The temperature a black body would have if it radiated the same amount of radiation from the same volume as the object being considered: An effective temperature is the temperature of the surface of a star, conveyed as the temperature of a black body having a similar radius as the star and radiating the identical total amount of energy.

Chunks of rock, usually extending from one side of a crater, that were ejected during he crater's formation: The extent of an ejecta blanket is determined by a number of factors including the size and mass of the impacting body (a meteorite, an asteroid, or a comet), the surface gravity, and the atmospheric pressure.

Where there is no atmosphere and the surface gravity is low, the ejecta blanket may extend to several times the radius of the central crater. On the other hand, on a world where both the atmospheric pressure and surface gravity are relatively high, the ejecta blanket will not reach far beyond the crater rim.

Waves of energy which consist of a combination of electric and magnetic fields at right angles to each other.

Such electromagnetic radiation results commonly from the acceleration of an electric charge, and is propagated in a vacuum at the speed of light.

The whole range of radiation which extends from high-energy gamma rays to low-energy radio waves: The electromagnetic spectrum is the range of wavelengths, or frequencies, over which electromagnetic radiation is propagated.

The longest wavelengths, or lowest frequencies, are those of radio waves, and the shortest wavelengths are those of gamma rays.

A negatively charged, low-mass particle which orbits the nucleus of an atom or exists free in space and in stars: An electron os an elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton; electrons surround the positively charged nucleus of an atom and determine the chemical properties of an atom.

The movement of electrons in an electrical conductor constitutes an electric current.

The variation of a regular planar or solid shape from a perfect circle or sphere: Ellipticity can be described as the departure of the earth, which has a smaller polar than equatorial diameter, from a perfect sphere is more usually described as its oblateness.

Elliptical planetary orbits are more usually described in terms of their eccentricity.

Angular distance in celestial longitude from the sun in the sky: The elongation is the variance between the celestial longitude of the sun and that of a planet or the moon.

Extra radiation at certain specific wavelengths in a spectrum, compared with neighboring wavelengths (colors): An emission line is a spectral line occurring from emissions of electromagnetic radiation by atoms, ions, or molecules in the course of changes from agitated states to conditions of lower energy.

1. A nebula which, as a result of ionized gas within it, shines by its own light.
2. A gas cloud that receives energy from a hot star, allowing it to give off radiation in emission lines; such as, those of hydrogen.

The characteristic reddish radiation of many emission nebulae is mostly from the hydrogen-alpha line.

The spectrum formed by the emission of electromagnetic radiation by a source; such as, a star.