The moon starts dark. We call this the new moon. Sunlight slowly hits the edge. We see a thin slice of light. This slice grows bigger every night. Half of the moon lights up. We call this the first quarter. The light keeps growing until the whole circle is bright. This is the full moon. After the full moon, the light shrinks. It goes back to a half circle. Finally, it becomes a thin slice again before going totally dark. This whole process takes about one month. The gravity of the moon pulls on the oceans. This pulling makes the tides go high and low every day.
We only ever see one side of the moon. The moon spins exactly as fast as it goes around the Earth. This means the same face always points at us. The sun hits the back side sometimes, but we never see it from Earth. You can verify the simple math of orbital mechanics through archival documents preserved within the HathiTrust Digital Library.
Mercury is a small rock near the sun. Long ago, tiny pieces of dust spun around the young sun. The dust crashed together and made Mercury. It was a normal planet with a thick rocky shell. Then a giant space rock hit it very hard. The massive crash blew all the light dirt away into space. Only the heavy iron middle stayed behind. This is exactly why Mercury is completely full of heavy metal today.
Humans did not know much about Mercury at first. They only saw a tiny bright dot near the sun. They watched the dot cross the face of the sun. This simple shadow proved Mercury was closer to the sun than Earth. Later, humans sent a metal machine called Mariner. The machine flew past the planet and took pictures. The pictures showed thousands of deep holes. These holes are craters from rocks hitting the planet over billions of years.
Venus started just like Earth. It had water and clean air. The sun grew brighter over millions of years. The rising heat made the oceans boil completely into the sky. Water vapor is very good at trapping heat. The planet got so hot that the rocks literally baked. The carbon inside the rocks floated out into the air. This made the air incredibly thick. It became a giant boiling oven.
Humans used to look at the thick bright clouds and guess what was underneath. They mistakenly thought Venus was a wet jungle. Soviet engineers built heavy metal ships to land there. They wanted to take pictures of the forest. They totally failed to realize the air pressure was like the deep ocean. The thick air crushed the ships completely. This was a historically ironic failure of basic observation. Later, humans used radar machines to look through the clouds. They found huge dead volcanoes instead of trees.
Earth formed from the same spinning dust. A small planet hit it early on. The broken pieces from the crash flew into space and clumped together. This bright clump became the moon. Ice rocks crashed into Earth and melted. This water made the massive oceans. Tiny life started in the deep warm water and slowly grew.
Humans walked around and mapped the dirt. A smart Greek man measured shadows in two cities. He used simple math to prove the world is a round ball. We did not fall off the edge. Now humans build heavy satellites. We put them high in space to watch the clouds and measure the deep oceans. We know our home perfectly.
Mars formed with a hot iron middle. It had a strong magnetic shield to protect it. It had flowing rivers and deep blue lakes. But Mars is very small. The middle cooled down fast. The magnetic shield broke. The fast wind from the sun blew the thin air away into space. The water froze inside the dirt or floated away forever.
Humans looked at Mars through simple glass lenses. They saw dark straight lines. Some people called them channels. They mistakenly thought alien farmers built them to move water. It was completely false. It was just a trick of the light hitting the lenses. Now humans send small robotic cars to drive on Mars. The cars dig in the dry dirt and find old dried riverbeds.
Jupiter grabbed most of the loose gas when the sun was young. It grew incredibly huge. It almost got hot enough to become a burning star, but it stopped growing. It is just a giant heavy ball of swirling gas. The middle is squeezed so hard the gas acts like thick liquid metal.
Humans saw it as a bright white light. A man named Galileo looked through a glass tube. He saw four tiny dots moving around Jupiter. He proved that things orbit other planets. This changed how humans see the whole sky. Recently, humans sent the Juno machine. It carefully measures the heavy gravity to see what is hidden deep inside the gas.
Saturn formed exactly like Jupiter but smaller. It is mostly light gas. An old moon got too close to the planet. The heavy gravity tore the moon into billions of tiny ice pieces. These pieces spread out and became the famous bright rings.
A man named Huygens looked through a telescope and realized the weird shapes were flat circles. Humans recently sent the Cassini machine. It flew right between the icy rings and the planet. It took incredibly close pictures of the broken ice before burning up in the gas.
Uranus is a giant ball of very cold water and gas. Long ago, a massive flying object hit it. The huge crash knocked the entire planet on its side. Now it rolls around the sun like a ball instead of spinning like a top.
Humans did not see it easily at first. A man named Herschel found it using a large telescope. He thought it was a fast comet. Later, humans did the math and realized it was a whole planet. Only one human machine has ever flown past it. The machine saw a very quiet cold blue world.
Neptune is the farthest planet. It formed closer to the sun and slowly pushed outward into the dark cold. It is full of cold gas and incredibly fast winds. It captured a large heavy moon from deep space. The moon now orbits backward.
Humans found Neptune using pure math. They watched Uranus move. Uranus moved strangely. Mathematicians guessed another giant planet was pulling it. They pointed a telescope exactly where the math numbers said. They found Neptune immediately on the first night.
People used the sun to know when to plant seeds. They used water dripping from bowls to count the hours. Later, they built clocks with swinging metal weights. The weights kept good time. But boats on the ocean needed better clocks to know where they were. A man built a perfect clock using small springs. This let sailors find their exact spot on the map.
Now we use atoms. Atoms vibrate. They vibrate at the exact same speed always. We count the fast vibrations. This is completely exact. Satellites high in space hold these special atomic clocks. They beam the exact time down to Earth. Your phone listens to four different satellites. The phone does simple math with the time signals. It figures out exactly where you are standing. This only works because the atoms shake perfectly.
Imagine you hear thunder. You know the storm is far away because sound takes time to travel. Your phone does this with invisible light signals from space. A satellite yells the exact time. Your phone hears it a tiny bit late. It measures how late the signal was. It does this for four yelling satellites. By seeing how late all four signals are, the phone draws four circles. The exact spot where the circles cross is your location.