Earth

Earth is the third planet from the Sun and the only planet known so far to host life. It's a blue rocky world with about 70% of its surface covered by ocean.

What kind of planet is it?

Earth orbits the Sun at an average distance of about 149.6 million km. This distance is called 1 AU (astronomical unit) and is used as the yardstick for the distances of other planets. At about 12,742 km across, Earth is the largest of the rocky planets: Mercury, Venus, Earth and Mars.

Inside, Earth is layered like an onion. Beneath the thin outer crust lies the hot, very slowly moving mantle, and deeper still are a liquid outer core and a solid inner core made of iron and nickel. The magnetic field generated by the moving liquid outer core acts as a shield against electrically charged particles streaming from the Sun.

Earth's outer shell is broken into several plates that drift very slowly, a process called plate tectonics. Where plates collide or pull apart, mountains rise and earthquakes and volcanoes happen. Earth is the only planet in the Solar System where active plate tectonics has been confirmed. About 71% of its surface is covered by ocean, and having liquid water spread widely and stably across the surface is another trait unique to Earth.

Atmosphere and weather

Earth's atmosphere is about 78% nitrogen and about 21% oxygen, with the rest made up of argon, carbon dioxide, water vapor and more. All that oxygen is the result of plants and microbes carrying out photosynthesis over a very long time. The ozone layer high in the sky filters out much of the ultraviolet light that's harmful to life.

Water vapor and carbon dioxide in the air create a moderate greenhouse effect that keeps Earth warm. As a result, the average temperature is about 15°C (59°F), staying in the range where water neither freezes nor boils. Calculations suggest that without an atmosphere, the average would drop to around -18°C (0°F).

Earth's axis is tilted about 23.44°, which gives us seasons. While the Northern Hemisphere tilts toward the Sun it's summer there, and while it tilts away it's winter. Because different latitudes receive different amounts of sunlight, air and ocean water circulate, creating all kinds of weather: rain, wind, typhoons and more.

Moons and rings

Earth has one moon: the Moon. It's about 3,474 km across, a little over a quarter of Earth's diameter, and orbits at an average distance of about 384,400 km roughly once every 27.3 days. Because its rotation period matches its orbital period, we always see the same side of the Moon from Earth.

The most widely accepted explanation for its origin is the giant-impact hypothesis: about 4.5 billion years ago, a Mars-sized body slammed into the young Earth, and the debris thrown out came together to form the Moon. The Moon's gravity pulls on the oceans to create tides, and it's also thought to help keep Earth's axis from wobbling too much.

The Moon is drifting away from Earth by about 3.8 cm every year. A moon this large compared with its planet is rare in the Solar System. Earth has no rings.

Exploration

Exploring Earth began with looking back at our home from space.

• Sputnik 1 (Soviet Union, 1957): The first artificial satellite, which opened the Space Age.
• Explorer 1 (USA, 1958): Played a key role in discovering the radiation belts around Earth (the Van Allen belts).
• Vostok 1 (Soviet Union, 1961): Yuri Gagarin became the first human to orbit Earth.
• Apollo 8 and 11 (USA, 1968 and 1969): The Apollo 8 crew captured the famous “Earthrise” photo of Earth rising over the Moon's horizon, and Apollo 11 landed the first humans on the Moon.
• Landsat (USA, 1972–present) and other Earth-observing satellites: Have been recording changes in forests, glaciers, cities and oceans for decades.
• Voyager 1 (1990): Looking back from about 6 billion km away, it captured Earth as a single tiny dot. The image is famous as the “Pale Blue Dot.”
• International Space Station (2000–present): People have lived aboard continuously since 2000 as it circles Earth about 400 km up.

South Korea launched the lunar orbiter Danuri in 2022, which has been observing the Moon's surface and the distant Earth from lunar orbit.

Fun facts

• Earth has the highest average density of any planet in the Solar System.
• Earth isn't a perfect sphere: it bulges slightly at the equator. Its equatorial diameter is about 43 km longer than its pole-to-pole diameter.
• Earth is closest to the Sun in early January, in the middle of the Northern Hemisphere's winter. That's proof that seasons come from the axial tilt, not from our distance to the Sun.
• Earth takes about 23 hours 56 minutes to rotate once relative to the stars. Meanwhile it moves a little along its orbit, so it has to turn for about 4 more minutes before the Sun returns to the same spot in the sky. That's why a day is 24 hours.
• The tides raised by the Moon are very gradually slowing Earth's rotation, so in the distant past a day was shorter than it is now.

See it in Planetica

In Planetica's Solar System view, Earth appears as the third planet from the Sun. Like the other planets, it's placed at its real position for today, calculated from J2000 orbital elements, so where you see Earth on screen is where Earth is on its orbit right now. It's fun to work out which direction the other planets lie in as seen from Earth.

Select Earth and fly in close to see the Moon circling with it. The Moon is drawn larger than life so it's easy to see. Earth's size, gravity, day length, axial tilt, average temperature and orbital period serve as reference values for comparing with the other planets.

Earth's surface patterns aren't a real world map; they're created by Planetica's planet generator. You won't find your hometown on it, but you can feel at a glance where our blue planet sits in the Solar System.

Other planets

← VenusMars →

• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus
• Neptune