rare earth metal

Any of a large class of chemical elements including scandium (atomic number 21), yttrium (39), and the 15 elements from 57 (lanthanum) to 71 (see lanthanides). The rare earths themselves are pure or mixed oxides of these metals, originally thought to be quite scarce; however, cerium, the most plentiful, is three times as abundant as lead in the Earth's crust. The metals never occur free, and the pure oxides never occur in minerals. These metals are similar chemically because their atomic structures are generally similar; all form compounds in which they have valence 3, including stable oxides, carbides, and borides.

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or labyrinth of the ear

Part of the ear containing organs of hearing and equilibrium. The bony labyrinth has three sections (semicircular canals, vestibule, and cochlea); within each structure is a corresponding part of the membranous labyrinth (semicircular ducts, two saclike structures in the vestibule, and cochlear duct). Sound vibrations are transmitted from the middle ear through the membrane-covered oval window to fluid in the snail-shell-shaped cochlea, whose motion stimulates hair cells in the cochlea. The hair cells trigger nerve impulses that travel to the brain, which interprets them as sound. The vestibule and semicircular canals also have organs with hair cells. Those in the vestibule indicate the head's position with respect to the rest of the body (see proprioception). The three semicircular canals, at right angles to each other, signal motion of the head in three-dimensional space. Continued stimulation after motion stops causes a mismatch with visual input, experienced as dizziness or motion sickness.

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Inner-grazer is a class of solar orbits. This diagram shows the six possible types of orbits one planet can have in relation to another. The Sun is shown in the middle (the orange dot) and the outer planet's orbital band is in yellow. The orbit of the other planet is a ring whose inner radius is the planet's perihelion and its outer radius the aphelion.

• Middle column, top: The minor planet's orbit enters the other planet's orbit from the inside without traversing it — it is an inner-grazer.