Senior 4 practice book · Unit 10 of 10

General Structure of the Solar System

50 questions that complete the Senior 4 quiz for this unit: 23 core and 27 challenge. Easy and hard questions are mixed. Try each question, then tap “Show answer”.

Common misconceptions

The Earth has seasons because it is much closer to the Sun in summer.The seasons come from the 23.5° tilt of the Earth's axis; the Earth is actually closest to the Sun in early January, during the northern winter.
Astronauts float in the space station because there is no gravity in space.At the station's height gravity is still about 90% of its value on the ground; astronauts float because they and the station are falling round the Earth together.
The stars of a constellation are close together in space.They only lie in the same direction as seen from Earth; one star of a constellation can be many times farther away than another.
The Sun is a huge fire burning gas.The Sun shines because of nuclear fusion of hydrogen into helium in its core; ordinary burning would have used it up within a few thousand years.

What this unit covers

Topics marked new are not tested much in the quiz, so this book gives them extra questions.

Members of the Solar System: the eight planets in order, dwarf planets and moons
Inner (terrestrial) and outer (Jovian) planets
Origin of the Solar System: the nebular hypothesis
Asteroids, comets, meteors and meteorites
Astronomical distances: astronomical unit, light year, light travel time
Kepler's laws of planetary motion
Gravity and orbits: the Sun's pull as centripetal force, orbital speed
Phases of the Moon
Solar and lunar eclipses
Earth's rotation and revolution: day and night, seasons, the year
Stars and the celestial sphere: constellations, right ascension and declination, motion of the sky
The Sun as a star: nuclear fusion, photosphere, sunspots, solar wind
Tides and the Moon's gravity
Exploring space: telescopes, satellites and space probes

Go to the questions

Questions (1–50)

Easier and harder questions are mixed, just like in a real exam. The 27 harder ones are marked ★ Challenge.

1True or false
It is safe to look at a partial solar eclipse through ordinary sunglasses.
Show answer
Answer: False
Even a thin crescent of Sun can burn the retina; use certified eclipse glasses or a pinhole projector.
Common mistakeMany people think dark glasses are enough; they let through far too much light and infrared.
2True or false · ★ Challenge
The Earth is closest to the Sun in early January, when it is winter in the Northern Hemisphere.
Show answer
Answer: True
Perihelion is about 3–4 January; the northern winter happens because that hemisphere is tilted away from the Sun.
Common mistakeThis surprises learners who think distance causes the seasons.
3True or false
Planets nearer the Sun move faster in their orbits than planets farther out.
Show answer
Answer: True
The Sun's pull is stronger close in, so a faster speed is needed to stay in orbit; Mercury moves at about 48 km/s, Neptune at about 5 km/s.
Common mistakeSome learners think big outer planets must move faster; their orbits are longer but their speeds are lower.
4True or false · ★ Challenge
The constant T²/r³ in Kepler's third law is the same for the Moon orbiting the Earth as for the Earth orbiting the Sun.
Show answer
Answer: False
The constant depends on the mass of the central body; the Sun is far heavier than the Earth, so the values differ.
Common mistakeUsing planet values (in AU and years) for a moon of the Earth is a common error; compare only bodies orbiting the same central body.
5Multiple choice · ★ Challenge
At about what time does a full Moon rise in Kigali?
1. AAt about sunset, near 6 pm
2. BAt about sunrise, near 6 am
3. CAt about midnight
4. DAt about midday, near noon
Show answer
Answer: A. At about sunset, near 6 pm
At full moon the Moon is opposite the Sun in the sky, so it rises as the Sun sets and sets as the Sun rises.
Common mistakeChoosing midnight confuses rising with being highest; the full Moon is highest in the sky around midnight.
6True or false
Ceres, the largest body in the asteroid belt, is classified as a dwarf planet.
Show answer
Answer: True
Ceres is round and orbits the Sun but shares its orbit with the asteroids, so it is a dwarf planet.
Common mistakeSome learners call every asteroid-belt object an asteroid; Ceres is big enough to be round, which makes it a dwarf planet.
7Multiple choice · ★ Challenge
When do spring tides (the largest tidal range) occur?
1. AAt first and last quarter
2. BOnly in the spring season
3. COnly during solar eclipses
4. DAt new moon and full moon
Show answer
Answer: D. At new moon and full moon
At new and full moon the Sun, Earth and Moon are in line, so the Sun's tidal effect adds to the Moon's.
Common mistakeChoosing "spring season" is a word trap: "spring" here means "springing up", and spring tides happen twice every month.
8True or false
Most meteors seen as "shooting stars" burn up completely high in the atmosphere and never reach the ground.
Show answer
Answer: True
Most meteoroids are tiny grains that vaporise from friction with the air; only rare larger ones reach the ground as meteorites.
Common mistakeSome learners think every shooting star lands somewhere; nearly all of them vaporise 70–100 km up.
9Multiple choice · ★ Challenge
What is the main cause of the seasons in countries far from the equator?
1. AThe change in the distance between the Earth and the Sun through the year
2. BThe tilt of the Earth's axis, which changes the Sun's height and day length
3. CThe Moon's position, which blocks more sunlight in some months
4. DChanges in how much energy the Sun produces in each month
Show answer
Answer: B. The tilt of the Earth's axis, which changes the Sun's height and day length
Because the axis is tilted 23.5°, each hemisphere leans towards the Sun for half the year (high Sun, long days) and away for the other half.
Common mistakeChoosing distance is the classic misconception; the Earth is nearest the Sun in January, which is winter in the north.
10True or false
Neap tides, with the smallest range, happen at first and last quarter, when the Sun and Moon pull at right angles.
Show answer
Answer: True
At right angles the Sun's tidal effect partly cancels the Moon's, giving smaller tides.
Common mistakeSome learners link neap tides to full moon; full moon gives spring tides.
11Multiple choice · ★ Challenge
A comet moves on a long, thin elliptical orbit. Where in its orbit is its tail longest?
1. AClose to the Sun, where heating and the solar wind are strongest
2. BFar from the Sun, where it moves most slowly along its long orbit
3. CHalfway along its orbit, between nearest and farthest
4. DThe tail has the same length everywhere along its orbit
Show answer
Answer: A. Close to the Sun, where heating and the solar wind are strongest
Near the Sun the ices evaporate fastest and the solar wind and radiation pressure are strongest, so the tail grows longest.
Common mistakeChoosing "far from the Sun" links the tail to slow motion; far out, the comet is frozen and usually has no tail at all.
12True or false
The solar wind is a stream of charged particles from the Sun, and it causes the auroras seen near the Earth's poles.
Show answer
Answer: True
The Earth's magnetic field guides the particles towards the poles, where they make the upper air glow.
Common mistakeSome learners think the solar wind is moving air; space has no air, it is a flow of charged particles.
13Multiple choice · ★ Challenge
Kigali is about 2° south of the equator. In which months is the Sun almost directly overhead at noon there?
1. AOnly in June
2. BOnly in December
3. CNever, since Kigali is south of the equator
4. DAround March and September
Show answer
Answer: D. Around March and September
The overhead Sun crosses the equator at the equinoxes (about 20 March and 22 September), so places very near the equator have it overhead close to those dates.
Common mistakeChoosing December uses the southern-tropic date; that is when the Sun is overhead at 23.5° S, far south of Rwanda.
14Fill in the blank
A natural body that orbits a planet, such as Ganymede orbiting Jupiter, is called a ______.
Show answer
Answer: moon (natural satellite)
Moons orbit planets; planets orbit the Sun.
Common mistakeWriting "asteroid" is wrong: asteroids orbit the Sun directly, not a planet.
15Multiple choice · ★ Challenge
Sunspots look dark in photographs of the Sun. Why?
1. AThey are holes through which we can see the cold inside of the Sun
2. BThey are shadows of planets passing in front of the Sun
3. CThey are cooler than the gas around them, though still very hot
4. DThey are clouds of dust floating above the Sun's surface
Show answer
Answer: C. They are cooler than the gas around them, though still very hot
Strong magnetic fields slow the flow of heat, so a sunspot is about 1500 K cooler than the surrounding photosphere and looks dark by contrast.
Common mistakeChoosing planet shadows confuses sunspots with transits of Mercury or Venus, which are small round dots that move across quickly.
16Multiple choice · ★ Challenge
Why do the stars appear to move across the night sky from east to west?
1. AThe Earth rotates from west to east
2. BThe stars orbit the Earth every day
3. CThe Sun pulls the stars to the west
4. DThe Earth orbits the Sun in a day
Show answer
Answer: A. The Earth rotates from west to east
As the Earth turns eastward, the whole sky appears to turn the opposite way, so stars rise in the east and set in the west.
Common mistakeChoosing "the stars orbit the Earth" is the old geocentric idea; the motion is only apparent.
17Fill in the blank
The Earth's axis is tilted at about ______° to the perpendicular to the plane of its orbit.
Show answer
Answer: 23.5
The 23.5° tilt causes the seasons and sets the latitudes of the tropics.
Common mistakeWriting 90° describes an axis lying in the plane of the orbit, which is not the case for Earth.
18Multiple choice · ★ Challenge
Why does the Moon usually look dark red during a total lunar eclipse instead of vanishing?
1. AThe Moon gives out its own red light once the sunlight is blocked by the Earth
2. BEarth's atmosphere bends red light into its shadow while blue is scattered
3. CLight from Mars is reflected onto the Moon during the eclipse
4. DThe Earth's shadow is made of red light rather than darkness
Show answer
Answer: B. Earth's atmosphere bends red light into its shadow while blue is scattered
Sunlight passing through Earth's atmosphere is refracted into the shadow; the blue part is scattered out on the way, so mainly red light reaches the Moon.
Common mistakeChoosing "its own light" is wrong: the Moon only reflects light; the red colour is refracted sunlight.
19Multiple choice
What causes day and night?
1. AThe Earth moving around the Sun once every year
2. BThe Sun moving around the Earth once every day
3. CThe Moon blocking the Sun for half of each day
4. DThe Earth spinning on its axis once every 24 hours
Show answer
Answer: D. The Earth spinning on its axis once every 24 hours
As the Earth rotates, each place turns into the sunlight (day) and then away from it (night).
Common mistakeChoosing the yearly orbit mixes rotation with revolution; the orbit gives the year and the seasons, not day and night.
20Multiple choice · ★ Challenge
Why does a geostationary communication satellite stay above the same point on the equator?
1. AIts orbital period is 24 h, in the same direction as the Earth spins
2. BIt hovers in one place because its rocket engines exactly balance its weight
3. CIt is so far away that gravity no longer acts on it at all
4. DIt is tied to the Earth by the Earth's magnetic field lines
Show answer
Answer: A. Its orbital period is 24 h, in the same direction as the Earth spins
It goes round once a day eastwards above the equator, so it turns with the Earth and appears fixed in the sky.
Common mistakeChoosing "no gravity" is wrong: gravity is what keeps it in orbit; without it the satellite would fly off.
21Multiple choice
How does the Sun produce its energy?
1. ABy burning hydrogen gas with oxygen in its atmosphere
2. BBy nuclear fission of uranium in its hot outer layers
3. CBy nuclear fusion of hydrogen into helium in its core
4. DBy slowly cooling down from when it formed long ago
Show answer
Answer: C. By nuclear fusion of hydrogen into helium in its core
In the core, at about 15 million K, hydrogen nuclei fuse into helium and a little mass becomes a great deal of energy.
Common mistakeChoosing fission mixes the Sun with nuclear power stations; the Sun joins light nuclei, it does not split heavy ones.
22Multiple choice · ★ Challenge
Why was Pluto reclassified as a dwarf planet in 2006?
1. AIt does not move in an orbit around the Sun at all
2. BIt is too small to have pulled itself into a round shape
3. CIt has no moons, and every planet must have at least one
4. DIt has not cleared its orbit of other similar bodies
Show answer
Answer: D. It has not cleared its orbit of other similar bodies
Pluto orbits the Sun and is round, but it shares its region (the Kuiper belt) with many similar icy bodies, so it fails the third condition for a planet.
Common mistakeChoosing "no moons" is wrong twice: Pluto has five moons, and Mercury and Venus are planets with none.
23Multiple choice
What is the main cause of ocean tides?
1. AThe Moon's pull, which is stronger on the side of the Earth nearer to it
2. BThe wind blowing the sea water towards the land twice in every day
3. CThe heating of the oceans by the Sun during the daytime
4. DThe spinning Earth throwing the sea water outwards at the equator
Show answer
Answer: A. The Moon's pull, which is stronger on the side of the Earth nearer to it
The Moon pulls the near side of the Earth more than the centre, and the centre more than the far side, raising two bulges of water.
Common mistakeChoosing the wind is wrong: tides keep a regular 12-hour rhythm whatever the weather.
24Multiple choice · ★ Challenge
The average densities of four planets are: A 5.4 g/cm³, B 0.7 g/cm³, C 5.5 g/cm³, D 1.6 g/cm³. Which two are most likely outer (Jovian) planets?
1. AA and C
2. BB and D
3. CA and B
4. DC and D
Show answer
Answer: B. B and D
Outer planets are mostly hydrogen, helium and ices, so their densities are low (about 0.7–1.7 g/cm³); rocky inner planets are about 4–5.5 g/cm³.
Common mistakeChoosing A and C assumes the biggest planets must be the densest; the giants are big but made of light gases.
25Fill in the blank
The visible surface of the Sun, at about 5800 K, is called the ______.
Show answer
Answer: photosphere
The photosphere is the layer from which most visible light escapes.
Common mistakeWriting "core" is wrong: the core is at about 15 million K and cannot be seen.
26Short answer · ★ Challenge
Explain why most coasts have about two high tides every day.
Show answer
Model answer: The Moon's uneven pull raises two bulges of water, one on the side facing the Moon and one on the opposite side. As the Earth rotates once a day, each place passes through both bulges, so it has two high tides (about every 12 h 25 min, since the Moon also moves along its orbit).
Common mistakeSaying there is one bulge facing the Moon leads to one tide a day; the far-side bulge is just as important.
27Short answer · ★ Challenge
Explain why the collapsing solar nebula flattened into a spinning disc.
Show answer
Model answer: The cloud was turning slowly. As gravity pulled it inwards it spun faster (conservation of angular momentum, like a skater pulling in the arms). Material could fall freely along the spin axis but not across it, and collisions between particles cancelled out up-and-down motions, so the cloud flattened into a disc around the spin axis.
Common mistakeSaying "the Sun's heat pushed the gas into a disc" is wrong; the flattening comes from rotation and gravity, before the Sun was shining.
28Multiple choice
Which list gives the planets in order, starting nearest the Sun?
1. AMercury, Venus, Earth, Mars, Saturn, Jupiter, Uranus, Neptune
2. BMercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
3. CVenus, Mercury, Earth, Mars, Jupiter, Saturn, Neptune, Uranus
4. DMercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Uranus
Show answer
Answer: B. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Rocky Mercury, Venus, Earth and Mars come first, then the asteroid belt, then Jupiter, Saturn, Uranus and Neptune.
Common mistakeSwapping Uranus and Neptune is the most common slip; a memory sentence such as "My Very Easy Method Just Speeds Up Naming" helps.
29Short answer · ★ Challenge
Venus is almost twice as far from the Sun as Mercury, yet its surface is hotter. Explain why.
Show answer
Model answer: Venus has a very thick atmosphere of carbon dioxide. Sunlight warms the surface, but the CO₂ traps the infrared radiated by the ground (a runaway greenhouse effect), so the surface reaches about 460 °C. Mercury has almost no atmosphere, so its heat escapes and its night side becomes very cold.
Common mistakeAnswering "Venus is bigger, so it holds more heat" misses the cause; it is the atmosphere, not the size, that traps the heat.
30Fill in the blank
The time from one full moon to the next, about 29.5 days, is called a lunar (synodic) ______.
Show answer
Answer: month
The cycle of phases takes about 29.5 days, which is where the idea of the month comes from.
Common mistakeWriting "year" or "week" ignores the length: 29.5 days is roughly one calendar month.
31Short answer · ★ Challenge
Describe the Earth's motion if the Sun's gravity suddenly disappeared, and explain your answer.
Show answer
Model answer: The Earth would move off in a straight line at constant speed (about 30 km/s), along the tangent to its orbit at that moment. With no force, by Newton's first law its velocity would no longer change direction.
Common mistakeSaying the Earth would fly straight outward, away from the Sun, is the centrifugal idea; it would leave along the tangent.
32Multiple choice
What provides the centripetal force that keeps the planets moving in their orbits?
1. AThe push of the solar wind
2. BMagnetic forces from the Sun
3. CA centrifugal force outward
4. DThe Sun's gravitational pull
Show answer
Answer: D. The Sun's gravitational pull
The Sun's gravity pulls each planet towards the Sun, continually turning its velocity into a curved path.
Common mistakeChoosing the solar wind is wrong: it pushes outward and is far too weak to bend a planet's path.
33Fill in the blank · ★ Challenge
Jupiter's largest moon, Ganymede, orbits 1.07 × 10⁹ m from Jupiter's centre with a period of 7.15 days. Its orbital speed is about ______ km/s (2 s.f.).
Show answer
Answer: 11
T = 7.15 × 86 400 s ≈ 6.18 × 10⁵ s; v = 2πr/T = 2π × 1.07 × 10⁹ ÷ 6.18 × 10⁵ ≈ 1.1 × 10⁴ m/s ≈ 11 km/s.
Common mistakeLeaving the period in days gives a speed in m/day; change days to seconds first.
34Multiple choice
During a total solar eclipse, what do people in the penumbra of the Moon's shadow see?
1. AA total eclipse
2. BA lunar eclipse
3. CA partial eclipse
4. DNo eclipse at all
Show answer
Answer: C. A partial eclipse
In the penumbra only part of the Sun is hidden by the Moon, so the eclipse is partial.
Common mistakeChoosing "no eclipse" forgets that the penumbra is a region of partial shadow, not full sunlight.
35Short answer · ★ Challenge
Explain why a lunar eclipse can be seen from the whole night side of the Earth, while a total solar eclipse is seen only along a narrow track.
Show answer
Model answer: In a lunar eclipse the Moon itself goes dark inside the Earth's large shadow, so anyone who can see the Moon (the whole night side) sees the eclipse. In a solar eclipse the Moon's small umbra only just reaches the Earth, covering a spot about 100–200 km wide that sweeps along a narrow path.
Common mistakeSaying solar eclipses are rare because the Sun is small misses the point: the Moon's shadow on Earth is small.
36Multiple choice
A star is 50 light years away. When we look at it tonight, we see it:
1. Aexactly as it is right now
2. Bas it will be in 50 years
3. Cas it was 50 years ago
4. Das it was 50 days ago
Show answer
Answer: C. as it was 50 years ago
Its light has taken 50 years to reach us, so we see the light that left it 50 years ago.
Common mistakeChoosing "as it is now" forgets that light, though fast, takes time to cross great distances.
37Short answer · ★ Challenge
Rwanda launched its first satellite, RwaSat-1, in 2019, and also uses communication satellites to bring internet to remote schools. Give two other ways in which satellites benefit people in Rwanda.
Show answer
Model answer: Weather satellites help forecast heavy rain and drought for farmers. Earth-observation satellites monitor crops, soil moisture, forests and floods. Navigation (GPS) satellites guide vehicles and help map land. Communication satellites carry television, phone and internet signals. (Any two.)
Common mistakeWriting "satellites make it rain" is a misconception; they observe and forecast weather, they do not control it.
38Multiple choice
Kepler's first law says each planet moves in an ellipse with the Sun at one focus. What does this mean for the Earth's distance from the Sun?
1. AIt is exactly the same all year long
2. BIt doubles from January to July
3. CIt changes a little during the year
4. DIt is greatest during the rainy seasons
Show answer
Answer: C. It changes a little during the year
With the Sun at a focus, not the centre, the distance varies; for Earth it changes by only about 3%.
Common mistakeChoosing "doubles" exaggerates the ellipse; most planetary orbits are very nearly circles.
39Short answer · ★ Challenge
The Earth is closest to the Sun in early January and farthest in early July. Use Kepler's second law to say when the Earth moves fastest in its orbit, and explain why.
Show answer
Model answer: It moves fastest in early January. The line from the Sun to the Earth sweeps out equal areas in equal times; when the Earth is close, this line is short, so the Earth must cover a longer arc in the same time to sweep the same area.
Common mistakeLinking the faster speed with the July "summer" (in the north) mixes up seasons with distance; Earth is fastest when nearest, in January.
40Multiple choice · ★ Challenge
The Earth moves at about 30 km/s in an orbit of radius 1.5 × 10¹¹ m. What is its centripetal acceleration towards the Sun?
1. A6.0 × 10⁻⁹ m/s²
2. B6.0 × 10⁻³ m/s²
3. C2.0 × 10⁻⁷ m/s²
4. D9.8 m/s²
Show answer
Answer: B. 6.0 × 10⁻³ m/s²
a = v²/r = (3.0 × 10⁴)² ÷ 1.5 × 10¹¹ = 9.0 × 10⁸ ÷ 1.5 × 10¹¹ = 6.0 × 10⁻³ m/s².
Common mistakeChoosing 6.0 × 10⁻⁹ m/s² uses v = 30 without changing km/s to m/s.
41Multiple choice
Why are some large telescopes, such as the James Webb Space Telescope, placed in space?
1. AIn space they are much closer to the stars, so the stars look a lot larger
2. BAbove the atmosphere, images are not blurred and infrared is not absorbed
3. CIn space there is no gravity, so the mirrors can be made of paper
4. DIn space the stars shine much brighter, because there is no daytime
Show answer
Answer: B. Above the atmosphere, images are not blurred and infrared is not absorbed
The atmosphere blurs images and absorbs much infrared and ultraviolet light; a space telescope avoids both.
Common mistakeChoosing "closer to the stars" ignores the scale: even the 1.5 million km to the James Webb telescope is nothing compared with the distance to a star.
42Short answer · ★ Challenge
The Moon takes 27.3 days to orbit the Earth, but 29.5 days pass from one new moon to the next. Explain the difference.
Show answer
Model answer: While the Moon goes once round the Earth, the Earth (with the Moon) moves about one-twelfth of the way round the Sun. The Sun is now in a slightly different direction, so the Moon has to travel about two more days to line up again between the Earth and the Sun for the next new moon.
Common mistakeSaying "the Moon slows down" is wrong; the extra time comes from the Earth's motion round the Sun.
43Short answer
State the three conditions a body must meet to be called a planet (International Astronomical Union, 2006).
Show answer
Model answer: It must (1) orbit the Sun, (2) have enough mass for its own gravity to pull it into a nearly round shape, and (3) have cleared the neighbourhood around its orbit of other bodies.
Common mistakeAdding "it must have an atmosphere" or "it must have moons" is a common error; neither is a condition.
44Short answer · ★ Challenge
Explain why Kigali, at about 2° south of the equator, has days of nearly 12 hours all year and no hot summer or cold winter, unlike cities in Europe.
Show answer
Model answer: Near the equator the tilt of the axis hardly changes the Sun's noon height or the length of day during the year, so the energy received is nearly the same in every month. In Europe the tilt makes the Sun high with long days in summer and low with short days in winter. Rwanda's seasons are rainy and dry seasons, set by winds and rain, not by temperature.
Common mistakeSaying "Rwanda is closer to the Sun" is wrong; the difference comes from the angle of the sunlight, not distance.
45Fill in the blank
Jupiter is about 5.2 AU from the Sun (1 AU = 1.5 × 10¹¹ m). This distance is ______ × 10¹¹ m.
Show answer
Answer: 7.8
d = 5.2 × 1.5 × 10¹¹ = 7.8 × 10¹¹ m.
Common mistakeDividing 5.2 by 1.5 (3.5) instead of multiplying is a common slip; 1 AU is 1.5 × 10¹¹ m, so multiply.
46Multiple choice · ★ Challenge
Two moons orbit the same planet. Moon B's orbit radius is 4 times that of moon A. Using Kepler's third law, how does B's orbital period compare with A's?
1. A8 times as long
2. B4 times as long
3. C16 times as long
4. D2 times as long
Show answer
Answer: A. 8 times as long
T² ∝ r³, so T ∝ r^3/2 = 4^3/2 = 8.
Common mistakeChoosing 16 uses T ∝ r²; Kepler's law gives T² ∝ r³, so take the square root of 4³ = 64.
47Short answer
Explain why we see different constellations in the evening sky in June and in December.
Show answer
Model answer: As the Earth orbits the Sun, the night side of the Earth faces a different direction in space. In June the night side faces one part of the sky; six months later the Earth is on the other side of the Sun, so the night side faces the opposite part of the sky, and the constellations in that direction are seen.
Common mistakeSaying "the stars move in their own orbits" is wrong; the change comes from the Earth's position in its orbit.
48Short answer · ★ Challenge
Each second the Sun turns about 4.0 × 10⁹ kg of mass into energy (E = mc², c = 3.0 × 10⁸ m/s). Calculate the power output of the Sun.
Show answer
Model answer: E per second = mc² = 4.0 × 10⁹ × (3.0 × 10⁸)² = 4.0 × 10⁹ × 9.0 × 10¹⁶ = 3.6 × 10²⁶ J, so the power is about 3.6 × 10²⁶ W.
Common mistakeForgetting to square c gives 1.2 × 10¹⁸ W, far too small; in E = mc² the speed of light is squared.
49Multiple choice
Which observation best supports the nebular hypothesis?
1. AAll planets have the same density and are of the same size
2. BThe planets all lie at equal distances apart from the Sun
3. CEvery planet has exactly the same number of moons
4. DAll planets orbit in the same direction, nearly in one plane
Show answer
Answer: D. All planets orbit in the same direction, nearly in one plane
A single spinning, flattened disc of gas and dust would give planets that orbit the same way and lie close to one plane.
Common mistakeChoosing equal densities contradicts the evidence: inner and outer planets differ greatly, which the hypothesis also explains.
50Short answer · ★ Challenge
The space probe Voyager 1 is about 160 AU from Earth (1 AU = 1.5 × 10¹¹ m, c = 3.0 × 10⁸ m/s). How many hours does a radio command take to reach it?
Show answer
Model answer: d = 160 × 1.5 × 10¹¹ = 2.4 × 10¹³ m. t = d/c = 2.4 × 10¹³ ÷ 3.0 × 10⁸ = 8.0 × 10⁴ s. In hours: 8.0 × 10⁴ ÷ 3600 ≈ 22 h.
Common mistakeForgetting to convert AU to metres gives a time of about 5 × 10⁻⁷ s, which is clearly impossible for such a distant probe.