Mirror, Mirror on the Universe

algebra - a tool used by mathematicians to describe patterns and relationships with symbols

analogy - a similar or like situation often used to explain a difficult concept

center - the middle; the center of a circle is the same distance from all points on the circumference

dimensions - different directions in space such as forward, sideways, and up for our three-dimensional world

distance - how far it is from one point to another as measured by a ruler

epoch - a period of time considered in terms of noteworthy and characteristic events

expansion rate - how rapidly something expands; the amount expanded divided by the time it takes

galaxy - a collection of stars attracted to each other by gravity; often billions of stars

gravity - an attractive force between stuff the physicists call mass

light year - the distance light travels in one year

linear relationship - when one quantity changes in the same proportion as another; for example, if water runs from a faucet at a steady rate, the amount of water collected in a cup will double if the time doubles and the amount of water collected is proportional to or linearly related to the time

measurement unit - a standard quantity agreed upon by many people, such as the second, which is used to measure time

Milky Way - our galaxy

millimeter - a measurement unit of distance about the thickness of your fingernail

model - a simple or complex plan dreamed up to explain observations

patterns - an organized relationship between objects such as the cells of a honeycomb

recession - moving away

relationships - connections between quantities

solar system - the Sun and all the planets, comets, asteroids, and space objects bound to the Sun by the force of gravity

space - the expanse in which the solar system, stars, and galaxies exist

speed - distance moved per unit of time

star - a big ball of burning gas held together by gravity

Sun - the closest star to Earth

time - actually not definable, the connection between past, present and future measured by a clock

universe - all existing things, including the Earth, the heavens, the galaxies, and all therein

velocity - speed in a given direction