About the Moon

What Is the Moon?

The Moon is Earth's only natural satellite, orbiting our planet at an average distance of 238,855 miles (384,400 kilometers). It is the fifth-largest satellite in the solar system and the largest relative to its host planet, with a diameter of 2,159 miles (3,474 km) - about 27% of Earth's diameter.

The Moon has no atmosphere, no liquid water on its surface, and no magnetic field. Its surface gravity is about one-sixth of Earth's, meaning an astronaut who weighs 180 pounds on Earth would weigh only 30 pounds on the Moon.

How the Moon Formed

The leading scientific theory for the Moon's formation is the Giant Impact Hypothesis. Approximately 4.5 billion years ago, a Mars-sized body called Theia collided with the proto-Earth. The colossal impact ejected a vast cloud of debris into orbit around Earth, which gradually coalesced to form the Moon.

Evidence supporting this theory includes:

• The Moon's composition is remarkably similar to Earth's outer layers
• The Moon has a relatively small iron core compared to its size
• Oxygen isotope ratios in lunar rocks closely match Earth's
• Computer simulations reproduce the Moon's orbital characteristics

The Moon's Surface

The lunar surface is divided into two primary terrain types:

The Moon's surface is covered in a layer of fine, powdery debris called regolith, created by billions of years of meteorite impacts. This regolith ranges from 2 meters deep on the Maria to 20 meters deep in the highlands.

The Moon's Orbit

The Moon orbits Earth in an elliptical path, taking approximately 27.3 days to complete one orbit (sidereal period). However, because Earth is also moving around the Sun, the time between identical moon phases (synodic period) is about 29.5 days.

Key orbital facts:

• Perigee (closest approach): ~225,623 miles (363,104 km)
• Apogee (farthest point): ~252,088 miles (405,696 km)
• Orbital inclination: 5.145° relative to the ecliptic
• Orbital speed: ~2,288 mph (3,683 km/h)

The Moon is tidally locked to Earth, meaning it rotates on its axis at exactly the same rate it orbits Earth. This is why we always see the same face of the Moon. However, due to a phenomenon called libration, we can actually see about 59% of the Moon's surface over time.

Why Moonrise and Moonset Times Change

The Moon rises and sets at different times each day because of its orbital motion around Earth. Each day, the Moon moves about 13.2° eastward in its orbit, which means Earth must rotate an additional ~50 minutes to "catch up" to the Moon's new position.

This means:

• Moonrise occurs approximately 50 minutes later each successive day
• Moonset also shifts by roughly 50 minutes daily
• The exact shift varies based on your latitude and the Moon's orbital geometry
• Near the equinoxes, the shift can be more or less than 50 minutes at high latitudes

Use our moon calculator to find the exact moonset time for your location today.

The Moon's Influence on Earth

Exploring the Moon

Humanity's exploration of the Moon represents one of our greatest achievements:

• 1959: Luna 2 (USSR) becomes the first spacecraft to reach the Moon
• 1966: Luna 9 achieves the first soft landing on the Moon
• 1969: Apollo 11 lands the first humans on the Moon (Neil Armstrong and Buzz Aldrin)
• 1972: Apollo 17 marks the last crewed lunar mission (to date)
• 2009: LCROSS confirms water ice in permanently shadowed craters
• 2019: Chang'e 4 (China) lands on the far side of the Moon
• 2024: Multiple nations and private companies begin new lunar landing programs

In total, 12 humans have walked on the Moon, all during NASA's Apollo program (1969-1972). They brought back 842 pounds (382 kg) of lunar samples that continue to provide scientific insights today.