The moon is completely dark to our eyes. The sun shines entirely on the far back side. The moon sits exactly between the Earth and the sun during this time.
A thin slice of bright light appears on the right edge. The moon moves slightly away from the sun. We finally begin to see a tiny piece of the illuminated half.
Exactly half of the right side shines brightly. The moon has traveled exactly one quarter of the way around the Earth. The line splitting the light and dark is perfectly straight.
Most of the moon shines brightly in the sky. Only a small dark curve remains on the left edge. The light continues to grow every single night.
The entire circle shines with bright white light. The Earth sits exactly between the sun and the moon. The sun completely illuminates the entire side facing us.
The light begins to shrink away. A dark shadow slowly creeps onto the right side of the circle. The moon has finished passing the Earth and moves back toward the sun.
Exactly half of the left side shines brightly. The right side is completely dark. The moon has traveled three quarters of the way around its circular path.
Only a very thin sliver of light remains on the left edge. The moon is almost directly in front of the sun again. Soon it will vanish completely into darkness.
The dark part of a crescent moon glows very softly. Sunlight bounces off the wide Earth oceans and hits the dark lunar dirt. The light bounces back to our eyes like a gentle mirror. You can verify the mathematics of this orbital illumination through the HathiTrust Digital Library.
The moon turns a deep blood red color. The Earth blocks all direct sunlight from hitting the moon entirely. Only the sunset light bleeding through the edges of the Earth atmosphere reaches the lunar surface.
Mercury is a dense iron rock placed incredibly close to the sun. Tiny pieces of hot dust spun around the young solar center long ago. The dust crashed together and slowly built a normal rocky planet. A giant stray space rock hit the young planet very hard soon after. This massive crash blew all the light outer dirt away completely into the void. Only the heavy iron middle stayed behind. This violently stripped core is exactly why Mercury is so heavy today.
Humans did not know much about Mercury at first. They only saw a tiny bright dot appearing briefly near the sun during sunset. They watched the small black dot cross the bright face of the sun. This simple shadow proved Mercury was closer to the sun than Earth. Later humans sent a metal machine called Mariner to fly past the rock. The machine took pictures of thousands of deep holes. These holes are craters from dead rocks hitting the planet over billions of silent years.
Mercury has absolutely no moons. The planet sits far too close to the massive sun to hold anything in orbit. If a moon were to form near Mercury, the immense solar gravity would immediately pull the moon away. The small rock travels alone completely forever.
Venus started exactly like Earth. It had vast liquid water and clean breathable air. The sun grew significantly brighter and hotter over millions of years. The rising heat made the huge oceans boil completely into the thick sky. Water vapor acts very well at trapping heat. The planet got so hot that the rocks literally baked and cracked. The heavy carbon inside the rocks floated out into the air. This made the air incredibly thick and turned the world into a giant boiling oven.
Soviet engineers built heavy metal ships to photograph lush alien forests; they completely failed to realize the thick air would instantly crush solid titanium like a discarded beverage container. Humans used to look at the bright clouds and guess what was underneath. They wrongly thought Venus was a very wet jungle. Later humans used invisible radar waves to look completely through the thick acid clouds. They found huge dead volcanoes instead of strange trees.
Venus also possesses zero moons. The planet rotates incredibly slowly and completely backward compared to the rest of the solar family. Scientists believe a massive early crash completely halted the planetary spin. Any ancient moons likely crashed back into the thick planet millions of years ago.
Earth formed from the same spinning bright dust. The heavy iron sank to the middle while the light rock floated to the top. Ice rocks crashed into the dry Earth and melted entirely. This liquid water made the massive deep oceans. Tiny life started in the deep warm water and slowly grew over immense spans of time. Plants learned to eat light and breathe out oxygen.
Humans walked around and carefully mapped the dirt. A smart Greek man measured simple shadows in two different cities on the exact same day. He used simple math to prove the world is a round ball. We did not fall off the edge. Now humans build heavy satellites and put them high in space. We watch the changing clouds and measure the deep oceans constantly. We know our home perfectly.
Earth has exactly one large moon. A rock the size of Mars hit the young Earth billions of years ago. The broken hot pieces from the massive crash flew high into space and clumped together. This bright clump slowly became the moon. The gravity of the moon pulls on our oceans and makes the daily tides.
Mars formed with a hot spinning iron middle. It had a strong magnetic shield to protect the air. It had flowing blue rivers and very deep lakes. Mars is very small so the middle cooled down incredibly fast. The spinning metal stopped and the magnetic shield broke completely. The fast wind from the sun blew the thin air away into deep space. The water froze solid inside the dirt or floated away entirely.
Some early astronomers squinted through bad glass and decided alien farmers built large water canals; they completely ignored the simpler truth of dry dirt. Humans saw dark straight lines on the red rock and made foolish guesses. Now humans send small robotic cars with wheels to drive on the rusty surface. The cars dig carefully in the dry dirt and find perfectly preserved old riverbeds.
Mars has two very tiny moons named Phobos and Deimos. They are extremely small and look exactly like bumpy potatoes. They are not round balls. Scientists know they are just stray asteroid rocks from deep space. The gravity of Mars caught them when they flew too close. Phobos orbits so fast it rises twice every single day.
Jupiter violently grabbed most of the loose gas when the sun was young. It grew incredibly huge very fast. It almost got hot enough to become a burning star but it eventually stopped growing. It is just a giant heavy ball of swirling gas. The middle is squeezed so incredibly hard the gas acts exactly like thick liquid metal.
Humans saw it as a bright white light in the night sky. A man named Galileo looked through a simple glass tube. He saw four tiny dots moving completely around Jupiter. He definitively proved that things orbit other planets. This changed how all humans see the sky forever. Recently humans sent the Juno machine to measure the heavy gravity and see what hides deep inside the gas.
Jupiter holds dozens of moons but four are incredibly large. Io has hundreds of erupting hot volcanoes. The gravity of Jupiter violently stretches and heats the rock inside Io. Europa is covered completely in thick smooth ice. A deep liquid ocean hides safely beneath the frozen crust. Scientists believe tiny life might swim in that dark ocean.
Saturn formed exactly like Jupiter but slightly smaller. It is mostly very light gas. It would float if placed in a large enough pool of water. An old ice moon got far too close to the gas giant long ago. The heavy gravity violently tore the moon into billions of tiny ice pieces. These pieces spread out flat and became the famous bright rings.
A man named Huygens looked through a telescope and realized the weird shapes were actually flat disconnected circles. Humans recently sent the heavy Cassini machine to Saturn. It flew right between the icy rings and the planet. It took incredibly close pictures of the broken ice before purposely burning up in the thick gas.
Saturn has one giant moon named Titan. Titan has very thick orange air and complete cloud cover. It is the only moon with real rain and large flowing rivers. The rain is not water but entirely freezing liquid gas. Enceladus is another moon. It shoots huge geysers of salt water straight out into empty space.
Uranus is a giant ball of very cold frozen water and gas. A massive flying object hit the planet very hard long ago. The huge crash knocked the entire planet completely on its side. Now it rolls around the sun like a ball instead of spinning upright like a top.
Humans did not see it easily at first. A man named Herschel found it using a large custom telescope. He initially thought it was a fast comet. Later humans did the complex math and realized it was a whole new planet. Only one single human machine has ever flown past it. The machine saw a very quiet and completely featureless blue world.
Uranus has many medium ice moons. They are all named after characters from Shakespeare plays. Miranda is the strangest moon. It looks like it was smashed to pieces and glued back together completely wrong. It has huge deep canyons and giant ice cliffs.
Neptune is the farthest large planet. It formed much closer to the sun and slowly pushed outward into the dark cold edge of the system. It is full of cold heavy gas and incredibly fast loud winds. The winds blow faster than sound travels on Earth.
Humans found Neptune using pure math completely on paper. They watched Uranus move incorrectly in the sky. Uranus moved strangely because something heavy pulled it. Mathematicians guessed another giant planet was pulling the ice giant. They pointed a large telescope exactly where the math numbers said to look. They found Neptune immediately on the first night.
Neptune has one very large moon named Triton. Triton orbits the planet completely backward compared to everything else. This proves it did not form there. The massive gravity of Neptune captured Triton from deep space. Triton is incredibly cold but still shoots dark geysers of frozen nitrogen gas into space.
People used the moving sun to know exactly when to plant seeds. They used water dripping slowly from clay bowls to count the silent hours. Later they built large wooden clocks with swinging heavy metal weights. The weights kept very good time on dry land. Boats on the rough ocean desperately needed much better clocks to know exactly where they sailed. A smart man built a perfect brass clock using small tight springs. This let sailors find their exact spot on the blank map.
Now we use tiny atoms. Atoms vibrate constantly. They vibrate at the exact same speed completely forever. We rigorously count the fast invisible vibrations. This is completely exact and never changes. Satellites high in space hold these special atomic clocks. They beam the exact numerical time down to Earth constantly. Your phone quietly listens to four different satellites. The phone does simple invisible math with the time signals. It figures out exactly where you are standing. This only works because the small atoms shake perfectly.
Imagine you hear loud thunder. You know the dark storm is far away because heavy sound takes time to travel. Your phone does this exact trick with invisible light signals from space. A satellite yells the exact time. Your phone hears it a tiny bit late. It accurately measures how late the signal was. It does this exact measurement for four yelling satellites. By seeing how late all four signals are, the phone draws four large invisible circles. The exact tiny spot where the circles cross is your exact location.