ASTR 103 - Astronomy

Glossary - I

igneous

Adjectival description of molten lava or basalt rock that has solidified.

image

 

inclination

angle between the orbital planes of two bodies; or the angle of the equator of a body with respect to its orbital plane.

inertia

reluctance of a body (proportional to its mass) to change its state of motion or rest when a force acts on it.

inertial force

force needed to overcome the inertia of a body in order to change its motion.

inertial frame of reference

coordinate system of an observer moving uniformly in space to which all his measurements are referred.

inferior conjunction

position of an inferior planet, as viewed from Earth, when it is between the Sun and Earth.

inferior planet

a planet whose orbit lies inside Earth's orbit (Mercury or Venus).

inflationary epoch

 

infrared

Portion of the electromagnetic spectrum extending from the visible red end toward the longer wavelengths up to about one millimeter.

intensity

The rate at which radiant energy propagates along the ray of electromagnetic radiation.

interference

phenomenon produced when waves of the same wavelength from different parts of a radiating source reinforce or interfere with each other to form a fringe pattern.

interferometer

apparatus employed to detect and measure interference from two or more coherent wave trains from the same source and, in astronomy, to measure the angular width of minute celestial sources and to determine their position on the sky with great accuracy.

interplanetary medium

the matter composed of gas and some dust lying between the planets.

interstellar cloud

the clumping of interstellar gas and dust into immense clouds having higher densities of matter than the space between clouds (see also dark and diffuse clouds).

interstellar dust

 

interstellar gas

 

interstellar medium

the region of gas and dust particles in the Milky Way system between the stars.

interstellar molecules

 

interstellar reddening

selective scattering of light from a distant object by dust particles in the interstellar medium. It is much larger in the blue region of the visible spectrum than in the red region.

inverse-square law of light

relationship between the apparent brightness of a light source and the distance from the source; the apparent brightness varies inversely as the square of the distance.

ion

atom that has lost or gained one or more electrons than its normal complement.

ionization

stripping an atom of one or more of its electrons.

ionization potential

 

ionosphere

Upper atmospheric region from about 50 to 400 kilometers where the air is ionized into discrete layers (D,E,F1, and F2 layers) by the Sun's ultraviolet and X-ray radiation.

ion tail (of a comet)

Filamentary tail of a comet (separate from the dust tail), resulting from the interaction of the solar wind with ions in the comet's head.

irregular galaxy

galaxy without symmetrical form.

isotope

Atom similar to another atom with the same number of protons but with a different number of neutrons in the nucleus.