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aphelion

the point along a planet's orbit at which it is farthest from the Sun

circle

a closed curved line on which all points are of equal distance from its center

drawing compass

a mechanical device used to draw circles

ellipse

A shape like a stretched-out circle; an oval

focus (foci)

the point at the center of a circle

gravity

the attractive force between objects

orbit

the elliptical path that a satellite travels as it moves around a star or planet

perihelion

the point along a planet's orbit at which it is nearest to the Sun

radius (radii)

the distance on a circle from the focus to the circumference

ratio

a relationship between two measurements

relative distance

the distance between objects when compared with the distance between surrounding objects

satellite

an object in orbit around a larger object

system

a group of items that form a unified whole

diameter

the distance from one side of a circle, through the focus, to the other side

planet

a large, spherical satellite of a star