The Moon is Earth’s natural satellite and the brightest object in our night sky. This cheat sheet covers the Moon’s surface features, how it formed, and how its motion affects what we see from Earth. Students need these ideas to understand phases, eclipses, tides, and evidence from lunar rocks and craters.
The Moon has highlands, maria, craters, rays, and regolith that record a long history of impacts and volcanism. Its phases come from the changing angle between the Sun, Earth, and Moon, not from Earth’s shadow. The leading formation model is the giant impact hypothesis, which says the Moon formed from debris after a Mars-sized object struck early Earth.
Key Facts
- The Moon’s average distance from Earth is about 384,400 km.
- The Moon’s gravity is about 1/6 of Earth’s gravity, so weight on the Moon = Earth weight divided by 6.
- The Moon rotates once in about 27.3 days and orbits Earth once in about 27.3 days, which is why the same side always faces Earth.
- The cycle from one new moon to the next is about 29.5 days because Earth also moves around the Sun during the Moon’s orbit.
- Moon phases are caused by the changing amount of the Moon’s sunlit half that we can see from Earth.
- A lunar eclipse happens when Earth is between the Sun and Moon, and Earth’s shadow falls on the Moon.
- A solar eclipse happens when the Moon is between the Sun and Earth, and the Moon’s shadow falls on Earth.
- The giant impact hypothesis states that the Moon formed from rock and debris thrown into orbit after a large object hit early Earth.
Vocabulary
- Crater
- A bowl-shaped depression on the Moon made mostly by impacts from meteoroids, asteroids, or comets.
- Maria
- Dark, flat areas on the Moon formed by ancient lava flows that filled large impact basins.
- Highlands
- Bright, rough, heavily cratered regions of the Moon that are older than most maria.
- Regolith
- The loose layer of dust, broken rock, and tiny glass fragments covering the Moon’s surface.
- Synchronous Rotation
- The matching of the Moon’s rotation period and orbit period, causing the same lunar hemisphere to face Earth.
- Giant Impact Hypothesis
- The model that the Moon formed from debris after a Mars-sized body collided with early Earth.
Common Mistakes to Avoid
- Thinking Moon phases are caused by Earth’s shadow is wrong because Earth’s shadow only causes lunar eclipses, not the monthly phase cycle.
- Calling the dark maria oceans is wrong because maria are solid basalt plains formed by ancient lava, not liquid water.
- Assuming the far side of the Moon is always dark is wrong because the far side receives sunlight during half of each lunar month.
- Confusing rotation with revolution is wrong because rotation means spinning on an axis, while revolution means orbiting another body.
- Forgetting that the Moon has weaker gravity is wrong because objects still have weight on the Moon, but their weight is about 1/6 of their Earth weight.
Practice Questions
- 1 An astronaut weighs 600 N on Earth. About how much would the astronaut weigh on the Moon if Moon weight = Earth weight divided by 6?
- 2 The Moon’s average distance from Earth is 384,400 km. If a spacecraft travels 10,000 km per hour, about how many hours would the trip take?
- 3 The Moon takes about 29.5 days to go from one new moon to the next. About how many complete phase cycles occur in 1 year of 365 days?
- 4 Explain why the same side of the Moon always faces Earth, but the far side is not always dark.
Understanding The Moon Features & Formation
The Moon keeps nearly one face pointed toward Earth because its spin became locked to its orbit long ago. Earth’s gravity pulled more strongly on the nearer side of the young Moon. Over time, this force slowed and adjusted the Moon’s rotation until one turn took the same time as one trip around Earth.
The far side is not permanently dark. It receives sunlight during part of every orbit, just as the near side does. Spacecraft have photographed it, and it has many more craters with fewer dark lava plains.
Phases are easiest to learn by building the geometry in your mind. Half of the Moon is always lit by the Sun, except during an eclipse. From Earth, the visible lit part changes as the Moon moves along its path.
A new moon is near the direction of the Sun, so its bright half faces mostly away from us. At full moon, Earth lies roughly between the Sun and Moon, so we see most of the bright half. A useful observation is that a waxing Moon shows more light each night, while a waning Moon shows less.
In the Northern Hemisphere, the bright part usually grows from right to left. This appearance is reversed in the Southern Hemisphere.
Eclipses do not occur every month because the Moon’s orbital path is tilted compared with Earth’s path around the Sun. Usually, the Moon passes a little above or below the straight alignment needed for an eclipse. An eclipse becomes possible when a new moon or full moon occurs near a point where the two orbital paths cross.
During a solar eclipse, only a narrow track on Earth may lie in the Moon’s darkest shadow. During a lunar eclipse, people across much of Earth’s night side can see the event. The Moon can look red in a total lunar eclipse because Earth’s atmosphere bends reddish sunlight into its shadow.
The Moon affects Earth through gravity. Its pull is stronger on the side of Earth closer to the Moon than on the far side. This small difference stretches oceans into two broad tidal bulges.
As Earth turns, many coastlines move through these bulges and usually experience two high tides each day. The Sun contributes to tides too. When the Sun, Earth, and Moon line up near new moon or full moon, their pulls combine and produce larger spring tides.
When the Sun and Moon are at right angles relative to Earth, smaller neap tides occur. Local coast shape, sea floor depth, and weather can greatly change the actual tide height.
Lunar landscapes preserve evidence that Earth has mostly erased. On Earth, wind, water, plate movement, and living things wear down old craters. The Moon has almost no atmosphere or flowing water, so impact marks can remain for billions of years.
Dark maria formed when lava flooded huge impact basins and cooled into dark rock. Brighter highlands are older crust with many overlapping craters. Scientists estimate relative ages by counting craters, since older ground has generally collected more impacts.
Samples brought back from the Moon show similarities between lunar rocks and Earth’s outer layers. Those clues support the idea that both bodies share material from a violent early event.