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The planet closest to the Sun is Mercury, then Venus, and next is Earth.
We’re number three! That should be an Earthling’s loudest cheer. Earth is just the right distance from the Sun for water to remain liquid, rather than freeze or evaporate, which is very important for supporting life.
We’re not the smallest! That’s our next loudest cheer. Small planets have weak gravity, and weak gravity can’t hold an atmosphere, so gases just fly out into space. Earth’s gravity is strong enough to hold on to a thin layer of gases, or atmosphere, which is the air we breathe. It protects us from deadly solar rays and keeps the planet from freezing solid.
We’re just right! If Earth were a tiny bit “wrong,” life could not exist.
Cross Section of Earth
Crust: Rock
Mantle: Semisolid rock
Inner core: Solid—nickel and iron
Outer core: Liquid—nickel and iron
◀ When you listen to a weather report, what temperature do you hope to hear? Something like 72°F would sound like a nice day, but how about 105°F, or 0°F? Those temperatures are hard for life to handle, but they’re mild compared with other planets. Life survives in a very narrow range of temperatures.
▲ You’re standing on Earth’s hard outer crust, and just 30 miles below your feet is hot, partly molten (or melted) rock. Picture Earth as the size of a basketball. The crust would be as thin as a hair! Mighty Mount Everest would be too tiny to see.
Check It Out!
If you had to show that the Earth is round, how could you do it, and what evidence would you use to prove it? A photograph doesn’t count!
The masts of a ship on the horizon can be seen before the ship, so the ship must be below the horizon. That would not be possible on a flat Earth. Also, during a lunar eclipse, Earth’s shadow is curved.
◀ What would Earth be like if things were a little bit different? Just look up at the Moon, an airless desert with gravity too weak to hold an atmosphere. Meteors explode with full force, instead of burning up on the way to the surface. The temperature is either freezing or boiling. That’s why there’s no life on the Moon.
Twelve humans have set foot on the Moon, all of them Apollo astronauts. They all walked on the Moon between 1969 and 1972. No one has landed there since. Apollo astronauts are the only humans who have seen the entire face of the Earth at once. Astronauts on the space shuttles orbited too close, so they could see only part of the Earth. ▶
Solar System Mystery
▲ Where did the Moon come from? Was it a chip off the old Earth, or did Earth’s gravity grab a passing asteroid? Most likely not. Maybe the Moon formed out of space rocks around Earth, or maybe a planet smashed into Earth, splitting the crust and sending molten rock shooting out into space. The rock hardened and then its gravity pulled in stray rocks from the explosion. Do you have another idea? Here’s a clue: Moon rocks are like Earth rocks, but they also contain elements that are rare on Earth and common in space.
▲ The Sun’s most direct and hottest rays hit the equator, but the heat doesn’t stay at the equator. That’s because wind, like this hurricane, spreads the heat around. Clouds help trap the heat.
◀ Is the Sun at the center of our solar system? Of course it is, but this question was hotly debated for thousands of years. Ptolemy (left) was a second-century Egyptian who claimed the Sun, the Moon, and the planets orbited Earth. Astronomers didn’t prove him wrong until the sixteenth century, and Copernicus was the first one to do it.
