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actinium	a radioactive chemical element of the rare-earth group that has eighty-nine protons in each nucleus. (symbol: Ac)
Afro-Eurasia	the land mass of Earth that includes all of Africa, Europe, and Asia.
agro-	soil; earth; field.
air	the mixture of gases that is in the space around the earth. People need air to live.
alien	from a planet other than Earth; extraterrestrial. [1/5 definitions]
alt.²	abbreviation of "altitude," the height of a given object or location above a specified base, such as sea level or earth, esp. the angular distance of a celestial body, measured from the horizon.
altitude	the height of a thing above earth or above sea level.
Antarctic Circle	an imaginary line drawn around the earth parallel to the equator and north of the South Pole.
anthill	a pile of earth made by ants as they dig their underground nest. Anthills are found near the nest entrance.
antipodes	places directly opposite each other on the surface of the earth, as the North Pole and the South Pole.
apogee	the point in the orbit of the moon or of a man-made satellite that is farthest from the earth. (Cf. perigee.) [1/2 definitions]
Apollyon	according to the Book of Revelation, a destroying angel who is the king of the evil spirits that will torture those who do not bear the seal of God when the earth is destroyed; Abaddon.
Arctic Circle	an imaginary line drawn around the earth parallel to the equator which marks the boundary of the Arctic. North of this line there are periods of continuous night in the winter and continuous day in the summer.
astronomer	a scientist who studies the universe beyond the earth.
astronomical unit	a unit of length based on the average distance of the earth from the sun, equal to about 149.6 million kilometers or ninety-three million miles.
astronomy	the branch of science that studies the universe beyond the earth.
atmosphere	the gases that are in the space around the earth and other similar objects in outer space. [1/2 definitions]
atmospheric	of, relating to, or made of the gases that surround the earth.
bacteria	microscopic organisms that often play a role in the decay of living things, the process of fermentation, and sometimes in causing disease. Bacteria are each made up of only one cell, but different kinds of bacteria can take different shapes. These organisms live in all parts of the earth including oceans, deserts, glaciers, hot springs, and in the bodies of most living things. While some kinds of bacteria are dangerous, most kinds are useful because they help in the digestion of food, in the making of soil, in the creation of medicines, and in many other natural processes.
bacterium	singular form of "bacteria." Bacteria are tiny living things made up of one cell each. Bacteria live in most living things and in all parts of the earth. Most bacteria are useful to people, but some are harmful because they can cause disease.
bank¹	a heap or mass of something, such as earth or clouds. [3/6 definitions]

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