Image
The optical representation of an object produced by the reflection or refraction of the light rays by a lens or a mirror.
Inclination of an orbit
The angle between the orbital plane of a body and usually either the plane of the celestial equator or the ecliptic.
Inferior conjunction
The planetary configuration of an inferior planet (Mercury and Venus) when it is between the earth and the sun.
Inferior planet
A planet whose mean distance from the sun is less than the earth's. Its orbit lies between the earth and the sun.
Infrared radiation
Electromagnetic radiation whose wavelength is longer than that of visible red light and shorter than that of radio waves.
Insolation
The amount of the sun's radiation that falls on a unit area of the earth's surface in a unit of time.
Interferometer
An optical instrument that utilizes the principle of the interference of light waves to measure small angles, for example, the diameter of stars.
International date line
An arbitrary line, nearly coinciding with the 180o meridian, across which the date changes by one day.
Interstellar lines
Absorption lines (dark spectral lines) on stellar spectra produced by diffuse gas in space.
Interstellar matter
Microscopic dust particles and diffuse gases in space between the stars.
Irregular galaxy
A galaxy whose shape is not symmetrical-it is neither an elliptical nor a spiral galaxy.
Irregular variable
A star whose energy output is not periodic.
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