1. The History of the Universe and the Earth

2. The process of assigning specific numerical dates to geological formations, typically through radiometric dating methods. Click here to reveal the definition!

3. A theory that the universe began 10 to 20 billion years ago with an explosion from a single point and continues to expand outward. Click here to reveal the definition!

4. A large deposit, typically of iron or sulfur, that reacted with atmospheric oxygen during the first 2.5 billion years of Earth’s history and removed it from the air. Click here to reveal the definition!

5. An isotope resulting from radioactive decay. Click here to reveal the definition!

6. Establishing the equivalence of rocks of similar age in different areas, based on the fossils they contain. Click here to reveal the definition!

7. A collection of stars and related objects held together by gravitational attraction; includes between one million and one trillion stars, as well as dust and gases. Click here to reveal the definition!

8. A force that acts without physical contact to pull two objects toward each other. Click here to reveal the definition!

9. The time required for one-half of the atoms of a radioactive substance to decay. Simulation of many identical atoms undergoing radioactive decay, starting with either 4 atoms per box (left) or 400 (right). The number at the top is how many half-lives have elapsed. Note the law of large numbers: With more atoms, the overall decay is less random. Click here to reveal the definition!

10. A species of genus of fossils that provides especially precise correlation of rock strata. Click here to reveal the definition!

11. An element that can have different mass numbers; isotopes of the same element have the same number of protons but different numbers of neutrons. Click here to reveal the definition!

12. The law of planetary motion that states that objects revolve around the sun in elliptical orbits, and that the Sun is at one focus of the ellipse. Click here to reveal the definition!

13. The law of planetary motion that states that a line joining the Sun and a planet in orbit around the Sun will “sweep” an equal area of its elliptical orbit during any equal time period. Click here to reveal the definition!

14. The law of planetary motion that states that the amount of time needed for a planet to orbit the Sun depends on the mass of the Sun and the planet’s average distance from the Sun. Click here to reveal the definition!

15. The distance light travels in one year, about 9.5 X 10 12 km. Click here to reveal the definition!

16. A large cloud of gas and dust in outer space, capable of forming a star through the actions of gravity. Click here to reveal the definition!

17. An isotope in the process of radioactive decay that gives rise to a daughter isotope. Click here to reveal the definition!

18. The spontaneous disintegration of an unstable atomic nucleus with accompanying emission of radiation. Click here to reveal the definition!

19. Measurement of the amount of naturally occurring radioactive isotopes in rocks in relation to their daughter isotopes; performed to assign absolute dates to rocks. Click here to reveal the definition!

20. The process of determining the order in which geologic formations were made; does not assign dates to those formations. Click here to reveal the definition!

21. A colored or black line that is part of an object’s visible light spectrum; created through the use of a spectroscope. Click here to reveal the definition!

22. An instrument that separates light given off by a star or other object into a spectrum. Click here to reveal the definition!