Chapter 9 key terms:
1. geocentric model: model of the Universe in which Earth is at the centre
2. heliocentric model: model of the Universe in which the Sun is at the centre
3. astronomical unit (AU): the average distance between Earth and the Sun; about
150 million km
4. meteoroid: a small celestial object made of rock or metal that is smaller than an
5. meteor: a meteoroid that burns up as it travels through Earth's atmosphere; it forms
a bright streak of light in the night sky
6. comet: a large chunk of ice, rock, and dust that orbits the Sun
7. revolve: to orbit a central point or object
8. rotates: moves, or turns, around a centre point
9. axis: an imaginary line around which an object rotates
10. time zone: a vertical section of Earth's surface that has a uniform time
11. solstice: a day when Earth's axis is most inclined toward, or away from, the Sun
12. equinoxes: days when the hours of daylight equal the hours of darkness
13. constellation: a region of stars in the night sky
14. asterism: a pattern of stars in Earth's night sky
15. circumpolar: a star, or constellation, that never sets below the horizon
16. azimuth: the horizontal angular distance from north measured (in °) eastward along
the horizon to a point directly below a celestial object
17. altitude: the angular height of a celestial object above the horizon; measured
vertically (in °) from the horizon
18. gravitational force: the force of attraction between two massive objects; gravity
19. lunar cycle: the sequence of phases made by the Moon during one orbit around
20. phases of the Moon: the changing amount of illuminated Moon surface visible from
21. waxing: increasing in size; a period during which the visible surface of the Moon
22. waning: decreasing in size; a period during which the visible surface of the Moon
23. solar eclipse: when the Moon passes between the Sun and Earth and blocks some
or all of the Sun's light
24. lunar eclipse: when the Moon passes through Earth's shadow and does not reflect
as much sunlight as usual
25. red shift: light from a galaxy shifting toward the red end of the visible spectrum
26. cosmologists: scientists who study the makeup and origins of the Universe
27. Big Bang theory: the theory that the Universe began in an incredibly hot, dense
expansion about 14 billion years ago

Chapter 9 key terms