1. Mercury, Venus, Mars, Jupiter, Saturn
2. Earth centered
3. he thought the planets moved on little circles that moved on bigger circles
4. heliocentric
5. Copernicus
6. four moons revolving around Jupiter and phases of Venus
7. a
8. elongated circle (oval)
9. Brache—late 1500’s—Danish astronomer; observed positions of planets for about 20 years

Kepler---1600—German mathematician; discovered elliptical orbits of planets

10. gravity; inertia
11. tendency of a moving object to continue in a straight line or a stationary object to remain in place
12. a, c, d
13. would travel in straight line away from the sun
14. sun’s gravity pulls on them while inertia keeps them moving ahead
15. space probes

20-2

1. nuclear fusion
2. Hydrogen atoms join to form helium
3. the core
4. helium, light, heat
5. photosphere; inner layer; when you look at image/photograph of the sun

chromosphere; middle layer; at beginning or end of total eclipse

corona; outer layer; during eclipse or with special telescopes

6. solar wind
7. sunspots, prominences, solar flares
8. sunspots—areas of gas on the sun that are cooler than the gases around them

prominences—reddish loops of gas that connect different parts of sunspot regions

solar flares---explosions of hydrogen gas out into space

9. sunspot cycle
10. magnetic storms

20-3

1. the 4 inner planets
2. small with rocky surface
3. 4, 2, 1, 3
4. Mars
5. Mercury, Venus, Earth, Mars
6. a, d
7. crust, mantle, core
8. iron and nickel
9. make inferences about other planets
10. true
11. a, b, d
12. close to sun (hot by day), but almost no atmosphere, so heat escapes at night (very cold)
13. evening star
14. about same size and mass
15. d
16. rotates east to west (opposite of other planets)
17. true
18. greenhouse effect
19. has a slightly reddish tinge due to iron oxide in soil
20. carbon dioxide
21. true
22. wind storms arise and blow the dust around the surface of Mars; darker regions are where dust has been blowing away
23. a, b, d
24. Phobos, Deimos

20-4

1. surfaces
2. Jupiter, Saturn, Uranus, and Neptune
3. 75% hydrogen, 24% helium, and 1% other elements (on average)
4. true
5. Jupiter, Saturn, Uranus, Neptune, Pluto
6. buried so deep inside the gas giants that it has been hard to find out much about them
7. true
8. giant area in Jupiter’s atmosphere with swirling clouds many times bigger than Earth; it appears to be on an ongoing storm similar to a hurricane on Earth
9. a, c, d
10. Ganymede, Callisto, Io, Europa
11. volcanoes
12. chunks of ice and rock, each traveling in its own orbit around Saturn
13. false
14. Titan
15. there are traces of methane in its atmosphere
16. discovered Uranus (first planet discovered since ancient times)
17. about 4 times the diameter of Earth
18. rotates from top to bottom instead of from side to side, the way most of the other planets do
19. 17
20. icy, cratered surfaces; also have lava flows on their surfaces
21. Uranus was not quite following the orbit astronomers thought it should; they hypothesized that the gravity of another planet was affecting Uranus’s orbit
22. c
23. false
24. Triton
25. true
26. so far away
27. a, c,d
28. so small that it may just be the largest of thousands of objects revolving around the sun

20-5

1. chunks of ice and dust whose orbits are usually very long, narrow ellipses
2. nucleus, coma, tail
3. solar winds pushes the gas from a comet away from the sun; gas and dust form the comet’s tail
4. true
5. Halley
6. asteroids
7. between the orbits of Mars and Jupiter
8. species extinction, including dinosaurs
9. b
10. c
11. a
12. from comets or asteroids
13. meteoroids

20-6

1. extraterrestrial life
2. liquid water, suitable temperature range, suitable atmosphere
3. life has been found deep in the ocean, in caves, inside solid rocks, and in hot springs
4. planet most similar to Earth
5. evidence of flowing water in past
6. fossils
7. false
8. biology lab of a Viking lander spacecraft
9. false
10. close-up views from Galileo craft show that Europa’s ice has broken up and re-formed; similar patterns occur in the ice crust over Earth’s Arctic Ocean
11. true

Wordwise

1. photosphere
2. ellipse
3. asteroids
4. retrograde rotation
5. geocentric
6. meteoroid
7. prominences
8. sunspots
9. solar wind
10. corona
11. heliocentric

a description of the solar system in which all of the planets revolve around the sun