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Astronomy & Telescopes Glossary - I
Inclination - The angle between the plane of orbit and the plane of the ecliptic.
Index Catalog (IC) - A star classification system that supplements to the New General Catalog, which lists nebulae and star clusters with an IC number.
Inferior planets - The planets Mercury and Venus are called inferior planets because they are closer to the Sun than Earth.
Inner planets - The small, rocky planets that orbit close to the sun: Mercury, Venus, Earth, and Mars.
Iridium flare - Bright flashes of light in the night sky that are due to sunlight reflecting off of the mirrors of orbiting Iridium communications satellites.
ISS - Abbreviation for the International Space Station.