The Moon is Earth's only natural satellite, a rocky world that shapes our planet and has been visited by humans.

Key idea: The Moon orbits Earth at a specific distance and speed, and Earth's gravity keeps one side of the Moon permanently facing us.

The Moon travels around Earth at an average distance of about 384,399 kilometers. To give you an idea, that's roughly 30 times the width of Earth. It takes about 27.3 days for the Moon to complete one full orbit around Earth compared to the stars. However, to return to the same phase we see from Earth (like a full moon to a full moon), it takes a bit longer, about 29.5 days. This longer period is called a lunar month.

The strong pull of gravity between the Earth and the Moon keeps them bound together. This gravitational pull is stronger on the sides of Earth and the Moon that face each other. These forces are what cause Earth's ocean tides. They also caused the Moon to become 'tidally locked,' meaning it always shows the same side to Earth.

Key idea: The Moon is a relatively large and massive satellite with low surface gravity and a barren, dusty surface.

The Moon is quite large for a satellite. Its diameter is about 3,474 kilometers, which is roughly one quarter the size of Earth. Imagine the width of the entire contiguous United States; that's about how wide the Moon is! Its mass is about 1.2% of Earth's mass. Among all the moons in our Solar System, it's the fifth largest and fifth most massive. What makes it truly stand out is that it's the largest and most massive moon compared to its parent planet (Earth).

If you were to stand on the Moon, you'd feel much lighter. The Moon's surface gravity is about one sixth of Earth's gravity. This means if you weigh 150 pounds on Earth, you'd only weigh about 25 pounds on the Moon! Its surface is rocky, with a very thin layer of dust called regolith. It barely has an atmosphere, water, or a magnetic field.

“If you were to stand on the Moon, you'd feel much lighter.”

Key idea: The Moon most likely formed from debris ejected after a Mars sized object collided with early Earth.

Scientists believe the Moon formed about 4.51 billion years ago, not long after Earth itself. The most accepted idea is called the giant impact hypothesis. This theory suggests that a body about the size of Mars, sometimes called Theia, crashed into the early Earth. This massive collision blasted a huge amount of material from Earth into space.

This ejected material then came together and formed the Moon. This idea helps explain why the Moon's composition is so similar to Earth's, yet also has some differences. For example, the Moon has fewer heavy metals than Earth. This theory also explains the high amount of spin (angular momentum) in the Earth Moon system.

“This massive collision blasted a huge amount of material from Earth into space.”

Key idea: The Moon's surface features include numerous impact craters and dark plains of ancient lava called maria.

The Moon's surface is covered with impact craters, which are bowl shaped holes left by asteroids and comets crashing into it. Because the Moon has almost no atmosphere, there's no wind or rain to wear these craters away, so they stay preserved for billions of years.

You can also see large, dark, flat areas on the Moon called maria (pronounced 'mah ree ah'). These are ancient plains of solidified basaltic lava. Billions of years ago, the Moon was volcanically active. Lava flowed into large impact basins, cooled, and hardened, creating these dark 'seas' that we can see from Earth. Most of these maria are on the side of the Moon that faces us.

Key idea: Water ice exists in permanently shadowed craters at the Moon's poles, which is crucial for future lunar exploration.

Even though liquid water cannot stay on the Moon's surface in direct sunlight, scientists have found evidence of water ice. This ice is mostly hidden in very cold, permanently shadowed craters at the Moon's poles. These areas never see sunlight, so the ice can remain frozen for billions of years.

Finding water on the Moon is very important for future space missions. If astronauts can use this lunar water for drinking, breathing, or even making rocket fuel, it would make building a Moon base much more practical and less expensive than bringing all the water from Earth.

Key idea: The Moon's gravity causes Earth's ocean tides and is slowly increasing the length of our days while pushing the Moon further away.

The Moon's gravity has a powerful effect on Earth. The most noticeable effect is the ocean tides. The Moon's pull creates bulges of water on Earth: one on the side facing the Moon and another on the opposite side. As Earth rotates, different parts of the world pass through these bulges, causing high and low tides.

Over billions of years, this constant tugging has also slowed down Earth's rotation. This means our days are slowly getting longer. In return, the Moon is slowly moving farther away from Earth, by about 38 millimeters (about 1.5 inches) each year. This is about the same speed your fingernails grow!