Earth, Sun, Moon and Planets

The Sun is a star: a huge ball of hot gas. It makes its own light and heat. The stars you see at night do this too.

Why the Sun looks so bright

How bright a light looks depends on its light output and distance. Think of two matching flashlights. Keep one nearby and move the other across a field. The farther light looks dimmer, even though both produce the same light.

The Sun is about 150 million km from Earth. That is about 93 million miles. The next-closest star, Proxima Centauri, is about 40 trillion km away. It is roughly 270,000 times farther away than the Sun. You need a telescope to see it.

Some stars give off much more light than our Sun. Sirius gives off about 25 times as much. Yet it looks faint beside the Sun because it is far away. Its light takes about 8.6 years to reach us. A light-year measures distance: how far light travels in one year. It is about 9.5 trillion km.

Inside the Sun

The Sun is mostly hydrogen and helium gases. Deep in its center, the temperature is about 15 million °C. This is where its energy is made. Its glowing surface is about 5,500 °C.

About 109 Earths would fit side by side across the Sun. About 1.3 million Earths could fit inside it. Other stars can be much larger or much smaller.

Light takes time to travel

Sunlight needs about eight minutes to reach Earth. So the sunlight reaching you now left the Sun eight minutes ago. Light from Proxima Centauri takes more than four years.

Sunlight warms the ground and helps drive weather and the water cycle. Plants use it to make food. Solar panels use it to make electricity. The Sun is Earth's main source of light and warmth.

Stars are still above you during the day. Sunlight spreads through Earth's air and makes the sky bright. That bright sky hides the much fainter stars.

Never look straight at the Sun, even with sunglasses. Never look at it through binoculars or a telescope. Its light can permanently hurt your eyes. Use shadows to study sunlight safely.

Earth, the Sun, and the Moon are nearly spheres. A sphere is a round ball shape. Every point on its surface is the same distance from its center. Earth is slightly wider around its middle, but a globe shows its shape well.

What the evidence shows

Spacecraft have photographed Earth from many directions. They show a round world with a curved edge. Travelers can keep heading around Earth and return to their starting point.

A distant ship offers another clue. As it sails beyond the horizon, its bottom disappears before its mast. Earth's curved surface hides the lower parts first. From a tall tower you can see farther over that curve.

The Moon keeps its round outer edge as its bright shape changes. A ball lit from one side can show a crescent. The boundary between light and dark curves across it. At half moon, that boundary looks nearly straight. A flat plate would look narrow when viewed edge-on.

Gravity pulls material toward the center from all sides. This helps large bodies become round. Smaller space rocks can stay lumpy. On Earth, gravity pulls you toward Earth's center too. People on different sides of Earth each have their own direction of “down.”

Build a size model

Earth is about 12,800 km wide. The Moon is about 3,500 km wide. The Sun is about 1.4 million km wide.

Use a bead 1 cm wide for Earth. Then make the Moon about 3 mm wide. The Sun must be about 1.1 meters wide. You would need a much bigger object than another bead!

Distance needs room as well. The real Moon is about 384,000 km from Earth. That is roughly 30 Earth widths. In the bead model, place the Moon about 30 cm from Earth. Many space pictures squeeze the gaps to fit everything on a page.

Why two unequal worlds look alike

The Sun and Moon look about equally wide in our sky. The Sun is actually about 400 times wider than the Moon. It is also nearly 400 times farther away. That extra distance makes its huge width look much smaller.

Try comparing a nearby small ball with a distant larger ball. Moving the large ball away makes it look narrower. Its real width stays the same. For distant balls, doubling the distance makes the apparent width about half as large. A 2-cm ball can look as wide as a 1-cm ball when it is twice as far away. Always ask whether a space picture shows real sizes, real distances, or just the order of objects.

Earth spins around an imaginary line called its axis. The line runs through the North and South Poles. One full spin is a rotation. Earth turns about once every 24 hours.

Follow sunrise to sunset

The Sun lights the half of Earth facing it. Places on that half have daylight. Places facing away have night. The Sun keeps shining as Earth turns.

Shine a flashlight at a globe in a dark room. Put a sticker on the globe for your town. Keep the flashlight still. Look down from above the North Pole and turn the globe counterclockwise.

1. Start with the sticker in darkness.
2. Turn until the sticker enters the light. This represents sunrise.
3. Keep turning through the lit half. This represents daytime.
4. Turn the sticker into darkness again. This represents sunset and then night.

The flashlight never needs to travel around the globe. Earth's spin carries each place through the light.

Which way does Earth turn?

Earth turns from west to east. Your place turns toward the eastern sky at sunrise. The Sun seems to cross the sky toward the west because you are moving the other way.

A full turn is 360°. Spread that turn over about 24 hours. Earth turns about 15° each hour. At the equator, the ground moves about 1,670 km each hour. You, the air, and things around you move along with Earth. You do not feel a sudden push from this smooth motion.

Daytime here, nighttime elsewhere

Two stickers on opposite sides of the globe face different ways. When one faces the light, the other faces away. This explains how a friend far around Earth can be asleep while you eat lunch.

Time zones group places that use the same clock time. They help clocks match the daily light pattern in different places. You can explain the need for them by turning the globe.

The word “day” can mean a whole 24-hour day. It can also mean just the daylight part. Daylight and darkness do not each last 12 hours every day. Their lengths depend on your season and location.

Keep Earth's two movements separate. Spinning explains the daily light-and-dark pattern. Traveling around the Sun takes a year.

Watch what happens when you turn beside a window. The view seems to slide past in the opposite direction. Earth's spin changes your view of the sky in a similar way.

The Sun’s daily path

The Sun seems to rise in the eastern sky. It climbs, reaches its highest point around midday, and sets in the west. This is apparent movement: movement that you see from your changing viewpoint.

Earth turns eastward. The Sun therefore seems to move westward. The Moon and stars also seem to cross the sky westward during a day or night.

Exactly where the Sun rises changes with the season. Around the equinoxes, it rises due east and sets due west. From the Northern Hemisphere, it rises north of east in summer and south of east in winter.

Where you live also changes the midday view. North of the tropics, the highest Sun is in the southern sky. South of the tropics, it is in the northern sky. This includes most of Australia and all of New Zealand.

Follow stars through one night

Stars keep their familiar patterns as they seem to turn together. They are not racing separately around Earth each night. You are watching from a spinning planet.

Imagine taking two night-sky photographs from the same spot. Take the second two hours after the first. Earth turns about 15° each hour, so your viewpoint turns about 30°. Star positions change in the photographs. That does not mean each star rises 30° higher. A star's path depends on which star it is and where you stand.

A nearly still star

In the Northern Hemisphere, Polaris, the North Star, hardly changes position. It lies almost along Earth's axis above the North Pole. Other northern stars seem to circle it as Earth turns.

Think of a spinning wheel. Its center stays in place while points farther out go around. Polaris is close to the direction of the wheel's center in this comparison.

People in the Southern Hemisphere cannot see Polaris. Their stars seem to circle a point in the southern sky. No bright star marks that point.

Stars remain in the sky during daytime, hidden by the bright sky. The Moon can often be seen during daylight.

A shadow forms where an object blocks light. Sunlight travels in straight lines. The ground behind a stick receives less light, so it looks dark. The shadow points away from the Sun.

Low light makes a long shadow

Place a toy beside a flashlight on a table. Hold the flashlight low. The toy casts a long shadow. Raise the light, keeping the toy still. The shadow becomes shorter.

Outdoors, low morning and late-afternoon sunlight makes long shadows. When the Sun is highest, a standing stick's shadow is shortest. Morning sunlight comes from the east, so the shadow points west. Late-afternoon sunlight comes from the west, so the shadow points east.

North of the tropics, the midday Sun is south. A midday shadow there points north. South of the tropics, that midday direction reverses.

Measure one stick through a day

Use the same upright stick in the same place. Measure from its base to the shadow's tip. Record both the time and length each time. These model results show a possible pattern for a 1-meter stick:

In this example, the Sun is highest at noon. Read down the length column. The shadow gets shorter, reaches a minimum, and grows again.

Put time along the bottom of a graph. Put shadow length up the side, with meters marked clearly. Plot each pair and join neighboring points for a line graph. The line forms a valley. Both 10 a.m. and 2 p.m. give 1.13 m; equal lengths can occur on either side of midday.

Compare days carefully

The shortest shadow does not always come at 12:00 on a clock. Time zones and daylight saving time affect clock readings. Use the shadow's minimum to find when the Sun is highest.

The pattern also changes with the season. In New York City, a 1-meter stick has a midday shadow about 0.3 m long in late June. In late December it is about 2.1 m. The winter Sun stays lower.

A sundial uses a moving shadow to tell time. Its pointer stays still while Earth's turn changes the light's direction.

An orbit is the curved path one object follows around another. Earth orbits the Sun. One complete trip is a revolution, and it takes one year.

Two movements at once

Spin a globe while carrying it around a lamp. The spin represents Earth's daily rotation. The trip around the lamp represents its yearly revolution. Earth does both at the same time.

Earth's orbit is nearly circular. Its distance from the Sun changes from about 147 million to 152 million km. Many drawings stretch the orbit so you can see its shape. That makes the distance change look much larger than it is.

Earth travels about 940 million km around its orbit each year. It moves about 30 km each second. The Sun's gravity, a pulling force, keeps the path curving around the Sun. Gravity also keeps the other planets in their orbits.

Why February sometimes gets another day

Earth's year lasts about 365¼ days. An ordinary calendar year has 365 days. That leaves about a quarter day uncounted each year.

Think through four ordinary years. After one, the missing time is about a quarter day. After two, it is half a day. After three, it is three quarters. After four, it is about a whole day.

A leap year adds February 29, making 366 days. Most years divisible by four are leap years. For example, 2024 and 2028 are leap years.

The real year is a little shorter than 365¼ days. Century years therefore have an extra rule: they must divide evenly by 400. The year 2000 passed that test. The years 1900 and 2100 do not. This small correction keeps the calendar close to the seasons over many centuries.

How people tested an old picture

Many people once pictured Earth fixed at the center. In 1543, Nicolaus Copernicus described Earth and the planets orbiting the Sun.

Around 1610, Galileo used a telescope to watch the sky. He saw moons orbiting Jupiter. This showed that objects could orbit something other than Earth. He also saw Venus go through a full set of phases. Those views fit Venus traveling around the Sun and challenged the old Earth-centered arrangement.

Earth's axis leans by about 23.4°, often rounded to 23.5°. This lean is its axial tilt. The axis keeps pointing nearly the same way in space throughout the year.

Carry the tilt around the Sun

Put a lamp in the middle of a room. Tilt a globe and point its North Pole end toward one wall. Carry the globe around the lamp. Keep the axis pointing toward that same wall.

On one side of the lamp, the Northern Hemisphere leans toward the light. Half an orbit later, it leans away. You never need to tip the axis back and forth. Earth's changing position makes the same steady lean face toward or away from the Sun.

Around June, the Northern Hemisphere tilts toward the Sun. Around December, it tilts away. The Southern Hemisphere has the opposite lean toward the Sun at each time.

Why summer receives more energy

Two changes work together in summer. The Sun climbs higher, so sunlight reaches the ground more directly. Daylight also lasts longer, giving the ground more hours to warm.

Try the first change with a flashlight and paper. Shine the beam straight down and trace its bright patch. Then slant the beam across the paper. The patch spreads into a larger oval. The same light now covers more paper, so each small area receives less.

Low winter sunlight spreads its energy across more ground in this way. Winter days are also shorter. Less energy reaches each patch of ground over the day.

Check the distance idea

You might expect summer when Earth is closest to the Sun. But Earth is closest in early January, during northern winter. At that same time, the Southern Hemisphere has summer.

A change in Earth's distance affects both hemispheres together. It cannot explain summer in one and winter in the other. The steady tilt explains the opposite daylight lengths and sunlight angles.

Picture an Earth with no axial tilt. Each place would have almost the same midday Sun height and daylight length all year. The strong yearly seasons we know would be missing.

Your place on Earth changes how seasons look and feel. Compare places using daylight hours, the Sun's midday height, and the dates of yearly turning points.

Solstices and equinoxes

A solstice marks the longest or shortest daylight of the year. Outside the tropics, it also marks the highest or lowest midday Sun.

Around June 21, the Northern Hemisphere has its summer solstice. Its winter solstice comes around December 21. The Southern Hemisphere has the reverse: December brings its summer solstice.

The equinoxes fall around March 20 and September 22. Neither hemisphere leans toward the Sun then. Day and night each last about 12 hours almost everywhere.

Compare two cities

Start with New York's June value. Fifteen hours of daylight leaves about nine hours of darkness in a 24-hour day. Sydney's June daylight is much shorter because it has winter then.

A monthly daylight graph for New York rises toward June. It falls toward December and then rises again. A Sydney graph peaks near December. The repeating peaks show a yearly cycle with opposite seasons.

Land and oceans take time to warm up. The hottest weeks usually come after the summer solstice, while days are still long. The longest daylight day and the hottest day need not match.

From the equator to the poles

Near the equator, daylight stays close to 12 hours all year. The Sun stays high, and many lowland places stay warm. High mountains can still be cold. Wet and dry seasons are common in many equatorial places.

Near the poles, daylight changes much more. Parts of the Arctic have 24-hour daylight around June and days without sunrise around December. Anchorage, south of the Arctic Circle, still gets more than 19 daylight hours in late June. It gets only about 5½ in late December.

Inside the tropics, the Sun can be overhead on other dates. The highest midday Sun there need not happen on a solstice. Use your location as well as the month when explaining a daylight pattern.

A constellation is a named star pattern, such as Orion or Ursa Major. Its stars look grouped together from Earth. They may actually lie at very different distances.

A smaller named pattern is an asterism. The Big Dipper is an asterism inside Ursa Major.

Face a new part of space

At night, your part of Earth faces away from the Sun. As Earth travels around the Sun, that nighttime direction changes. Stars hidden in the daytime sky can be visible at night months later.

Make a model with a lamp and four star-pattern drawings on different walls. Stand on one side of the lamp. Face away from it and note the drawing you see. Walk halfway around the lamp and face away again. You now see a different wall.

The drawings stayed in place. Your position around the lamp changed your outward view. Earth's orbit changes our night view in the same way.

Look at the same evening time

From the United States, Orion is easy to find on winter evenings. Around June, it lies in the Sun's direction and is hidden by daylight. Scorpius is a familiar summer evening pattern.

Australia has summer during the northern winter. Orion is therefore a summer evening pattern there. The southern view also makes it look turned over compared with the northern view.

Keep a record at the same evening time each month. Different patterns appear, and familiar ones shift position. After a year, the pattern returns. People have used these regular returns to help time planting, harvests, and festivals.

One night or many months?

Earth's rotation explains stars moving across the sky during one night. Earth's orbit explains the changing stars visible at the same time through the year. A star rises about four minutes earlier each night. That adds up to about two hours in a month.

Some stars near the northern sky's turning point stay above the horizon all night. Which ones do this depends on how far north you live. From sufficiently far north, the Big Dipper stays up all year. Its position around Polaris still changes.

The Moon is Earth's natural satellite. A satellite is an object that orbits a planet. The Moon takes about 27⅓ days to go around Earth compared with distant stars. Earth and the Moon travel around the Sun together.

Where moonlight comes from

The Moon makes no light of its own. Sunlight strikes its surface and bounces toward your eyes. This is reflection. Moonlight is reflected sunlight.

The Moon's surface has gray dust, rocks, and many craters. Craters are hollows made when space rocks hit the surface. Its large dark plains are old, cooled lava. People once mistook them for seas.

The Moon has almost no atmosphere and no breathable air. Its sky is black even in daytime. With little air to spread light or hold heat, temperatures change greatly. Sunlit ground can get hotter than 100 °C. During the long night, ground can become colder than −150 °C.

Why we keep seeing familiar markings

The Moon spins once in the time it takes to orbit Earth. These matching motions keep roughly the same face toward us.

Try walking around a chair while always facing it. At the far side, your body faces the opposite wall from where you began. After a whole lap, you have turned once. Keeping your face toward the chair required a turn.

The Moon's far side faces away from Earth. It still receives sunlight. “Far side” tells you which way it faces, not whether it is dark.

Earth's spin makes the Moon seem to cross our sky each day. The Moon also moves along its own orbit. On average, it rises about 50 minutes later from one day to the next.

Satellites made by people

An artificial satellite is launched by people. Weather satellites take pictures of clouds. Other satellites help send television or GPS signals.

The International Space Station travels about 400 km above Earth. It completes an orbit in about 90 minutes. Our Moon is a natural object much farther away.

Many other planets have moons too. Mercury and Venus have none. Mars has two small moons. Jupiter and Saturn each have more than 100 known moons.

In July 1969, Neil Armstrong and Buzz Aldrin became the first people to walk on the Moon. Altogether, 12 astronauts walked there between 1969 and 1972.

The Moon's phases are the changing bright shapes you see from Earth. The Moon stays a whole sphere. Its rocky body does not grow or shrink.

Start with the light

Sunlight lights half the Moon. As the Moon orbits Earth, you see that sunlit half from different angles. Sometimes the whole lit half faces you. Sometimes it faces away. Between those views, only part of it is visible.

Use a lamp for the Sun and a white ball for the Moon. Your head represents Earth. Hold the ball at arm's length. Keep it a little above your head so your head does not block the light.

1. Face the lamp with the ball in front of you. You see the ball's dark half. This represents new moon.
2. Turn a quarter turn, keeping the ball in front. You see half of the bright half. The ball looks half lit: first quarter.
3. Turn until your back faces the lamp. Light reaches the ball's face toward you. This represents full moon.
4. Keep turning. The visible bright part shrinks until you return to new moon.

The lamp always lights half the ball. What changes is how much of that bright half you can see.

Connect the views to positions

At new moon, the Moon is roughly between Earth and the Sun. Its lit half faces mostly away from Earth. It is near the Sun in our sky and hidden in the glare.

At full moon, the Moon lies on the opposite side of Earth from the Sun. Its lit half faces Earth.

At the quarter phases, the directions from Earth to the Sun and Moon form a right angle. We see half of the Moon's disk lit. “Quarter” refers to progress through the cycle, not a quarter of the visible circle.

Why a crescent is not Earth’s shadow

Earth's shadow points away from the Sun. A crescent Moon appears near the Sun's direction in the sky. Earth's shadow cannot reach over to that part of the sky. The crescent is our side view of the Moon's sunlit half.

Clouds can hide the Moon on a particular day. They do not create the steady phase cycle.

People across Earth see the same phase at the same time. A full moon is full for northern and southern viewers. The bright shape can look turned around because viewers stand on different parts of the globe.

You can describe a Moon phase with two clues. How much of its disk is bright? Is that bright part growing or shrinking over several days?

Read the names

Waxing means the bright part is growing. Waning means it is shrinking. A crescent has less than half the disk lit. A gibbous Moon has more than half lit, but is not full.

The phases follow this order:

1. New moon
2. Waxing crescent
3. First quarter
4. Waxing gibbous
5. Full moon
6. Waning gibbous
7. Last quarter, also called third quarter
8. Waning crescent

Then new moon begins the next cycle. First and last quarter both look half lit.

Count weeks through the cycle

One new moon to the next takes about 29½ days. The four main phases are about a week apart.

Suppose new moon falls on September 2. First quarter comes around September 9 or 10. Full moon comes around September 17. Last quarter follows around September 24. The next new moon comes around October 1 or 2. These are estimates because the gaps are not exactly seven days.

The Moon's 27⅓-day orbit is shorter than its 29½-day phase cycle. While the Moon orbits, Earth moves around the Sun. The Moon must go a little farther to reach the same Sun–Earth arrangement again.

Use the changing shape

Suppose your drawings show a thin crescent on Monday. By Thursday, nearly half the disk is bright. The bright part grew, so it is waxing. First quarter is approaching.

A typical upright view from the Northern Hemisphere shows waxing light on the right. The Southern Hemisphere view reverses that. The Moon can also look tilted in the sky. Comparing several days' drawings is a useful way to track growth without relying on left or right.

Choose a time to look

Full moon rises around sunset and sets around sunrise. First quarter is high around sunset and sets near midnight. Last quarter rises near midnight and remains visible in the morning. New moon is near the Sun and usually hidden in its glare.

Keep a month-long diary. Look in the evening for waxing phases. Look in the morning for later waning phases. Record the date, time, and bright shape. Record days when you cannot find the Moon too, so gaps in your drawings stay clear.

The Solar System contains the Sun and everything that orbits it. This includes eight planets, their moons, dwarf planets, and smaller pieces of rock, ice, and dust. The Sun holds more than 99% of the system's mass.

Learn the outward order

From nearest to farthest from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

The sentence “My Very Educated Mother Just Served Us Nachos” gives their first letters. Say each planet name as you use the sentence. The asteroid belt lies between Mars and Jupiter.

The inner four planets are small, rocky worlds. The outer four are giants. All eight orbit the Sun in the same direction. Their paths lie nearly in one flat plane. They also spin on their own axes.

A row of planets in a diagram helps you learn the order. Real planets are spread around their separate orbits. They do not stay lined up beside the Sun.

Tell different space objects apart

A planet is a large, round body orbiting the Sun. It has cleared its orbital neighborhood of other bodies of similar size. Its gravity can gather or move nearby objects. Clearing a neighborhood does not mean the orbit is completely empty.

A dwarf planet is round and orbits the Sun, but shares its region with many other bodies. Pluto was called the ninth planet from 1930 until 2006. It is now classed as a dwarf planet. Ceres, in the asteroid belt, is another dwarf planet.

Asteroids are smaller rocky or metal bodies orbiting the Sun. Many are lumpy. Most lie in the asteroid belt. Comets contain ice and dust. Near the Sun, some ice becomes gas, forming a bright head and tails. The tails point away from the Sun.

A meteoroid is a small piece of rock or dust in space. A meteor is the streak of light made when one rushes into Earth's air. A “shooting star” is this streak, not a falling star.

Planets shine with borrowed light

Stars make their own light. Planets reflect sunlight. A planet can look like a star in our sky, but its position slowly changes against the star patterns. Mercury, Venus, Mars, Jupiter, and Saturn can be seen without a telescope.

Our Sun is one of more than 100 billion stars in the Milky Way galaxy. Many other stars have planets too. A planet outside our Solar System is an exoplanet.

Mercury, Venus, Earth, and Mars have solid, rocky surfaces. They have no rings and few or no moons. Their atmospheres help explain why their temperatures differ so much.

Mercury and Venus

Mercury is the smallest planet and nearest the Sun. It is about 4,900 km wide. Its year lasts only 88 Earth days. The gray, crater-covered ground resembles our Moon.

Mercury has almost no atmosphere to hold heat. Daytime ground can reach about 430 °C. At night it can fall to about −180 °C. Being near the Sun does not keep its nights warm.

Venus is nearly Earth's size, about 12,100 km wide. Its thick carbon dioxide atmosphere traps heat. Its surface averages about 464 °C, or about 870 °F. Venus is the hottest planet, even though Mercury is nearer the Sun.

Clouds of sulfuric acid surround Venus. The bright clouds reflect sunlight, making Venus shine strongly in our sky. People call it the “morning star” or “evening star,” but it is a planet.

Venus spins backward compared with most planets. One spin takes about 243 Earth days. Its trip around the Sun takes about 225 Earth days. A spin therefore takes longer than a year on Venus. Here “spin” means one turn against the stars, not the interval between sunrises. Mercury and Venus have no moons.

Earth and Mars

Earth is the largest rocky planet, about 12,800 km wide. It has oceans of liquid water, air with oxygen, and one large moon. It is the only planet known to have life. Its distance from the Sun and its atmosphere help keep conditions suitable for liquid oceans.

Mars is about 6,800 km wide, roughly half Earth's width. Iron oxide, the same kind of material as rust, colors its dust red. It has thin air, mostly carbon dioxide, and icy polar caps.

Mars averages about −65 °C. One day lasts about 24⅔ hours, close to Earth's day. Its year lasts 687 Earth days, nearly twice as long as ours. Its two small moons are Phobos and Deimos.

Mars has Olympus Mons, the largest known volcano in the Solar System. Robot rovers, including Perseverance, have explored its rocky surface.

Jupiter, Saturn, Uranus, and Neptune are the outer giant planets. They have thick atmospheres, rings, and many moons. None has solid ground like Earth's surface where a lander could stand.

Jupiter and Saturn: gas giants

Jupiter and Saturn are mostly hydrogen and helium. They are called gas giants. Their outer gases grow thicker and hotter deeper inside.

Jupiter is the biggest planet, about 11 Earth widths across. More than 1,300 Earths could fit inside it. Cloud bands circle the planet. The Great Red Spot is a huge storm wider than Earth.

Jupiter also spins fastest: one turn takes about 10 hours. It has more than 100 known moons. Galileo found its four largest in 1610: Io, Europa, Ganymede, and Callisto. Ganymede is the largest moon in the Solar System, even wider than Mercury.

Saturn is second largest, about 9½ Earth widths across. Its bright rings contain countless separate pieces of ice and rock. A ring is not one solid hoop.

Saturn has more than 250 known moons, more than any other planet. Its largest moon, Titan, has thick air and lakes of liquid methane. Saturn has low density: it contains less mass for its size than water does.

Uranus and Neptune: ice giants

Uranus and Neptune are called ice giants. They contain much more water, ammonia, and methane inside than the gas giants. Deep pressure and heat mean these materials are not simply ordinary ice cubes.

Uranus is about four Earth widths across. Methane helps give its atmosphere a pale blue-green color. Its axis leans about 98°, making the planet seem to roll on its side. A year there lasts about 84 Earth years. William Herschel discovered it with a telescope in 1781.

Neptune is a little smaller than Uranus. It is the farthest planet, about 30 times Earth's distance from the Sun. Its year lasts about 164 Earth years. It has the fastest winds measured on a planet.

Neptune is pale greenish blue, slightly bluer than Uranus. Familiar deep-blue pictures had their colors strengthened. Astronomers found Neptune in 1846 after predicting its position from its pull on Uranus.

Compare their rings and motion

All four giants have rings. Saturn's are brightest and easiest to see. The others have thin, dark rings. The giants spin quickly, taking about 10 to 17 hours per turn. Their distant orbits take many Earth years.

Numbers help you compare planets without relying on pictures. Check what each column measures and which units it uses before looking for a pattern.

Read the planet data

Farther planets have longer years. They travel longer paths and move more slowly along them. Size has a different pattern. Jupiter is largest, but Neptune is farthest. Uranus is slightly wider than Neptune.

Temperature generally drops farther from the Sun. Atmospheres matter too: Venus is hotter than Mercury. Spin time also has its own pattern. Jupiter turns in about 10 hours, Earth in about 24 hours, and Venus in about 243 Earth days.

Choose a scale rule

An astronomical unit, or AU, is Earth's average Sun distance: about 150 million km. Mars is about 1.5 AU away, Jupiter about 5 AU, and Neptune about 30 AU.

Let 1 AU equal 10 meters in a model. Earth then goes 10 m from the model Sun. Mars goes about 15 m away. Neptune goes about 300 m away.

Here is the Mars calculation in words. Each AU uses 10 meters. One and a half groups of 10 meters make 15 meters. Measure that distance from the Sun, not from Earth.

Using more precise rounded distances gives Mercury 3.9 m, Venus 7.2 m, Jupiter 52 m, Saturn 96 m, Uranus 192 m, and Neptune 302 m.

Make sizes match distances

At this same scale, the Sun is only about 9 cm wide. Earth is a speck less than 1 mm wide. Jupiter is about 1 cm wide. You need a large outdoor space even with such tiny planets.

If a diagram makes Earth 1 cm wide, Neptune must be about 3.5 km from its Sun at the same scale. Most page-sized diagrams therefore use different scales for width and distance.

Light also reveals the gaps. Sunlight takes about eight minutes to reach Earth, about 43 minutes to reach Jupiter, and about four hours to reach Neptune.