Earth's Moon is a rocky body about one-quarter Earth's diameter, with no atmosphere, no liquid water, and extreme temperature swings. Its surface has two main types of terrain: bright, heavily cratered highlands and dark, smooth plains called maria (formed by ancient lava flows). The Moon is tidally locked to Earth — the same side always faces us. The Moon's gravity causes ocean tides on Earth and stabilizes Earth's axial tilt. The leading theory for the Moon's origin is the giant impact hypothesis: a Mars-sized body collided with the young Earth, and debris from the collision coalesced to form the Moon.
Observe the Moon through binoculars or a small telescope to see craters and maria. Compare lunar surface photos to a labeled map identifying major features (Sea of Tranquility, Tycho crater, etc.). Model tides using a round container of water and a magnet moved around the edge. Discuss why the Moon always shows the same face (tidal locking) and what the far side looks like (heavily cratered, very few maria). The Apollo missions provide compelling real-world context — we have actual Moon rocks to study.
You have watched the Moon go through its phases — new, crescent, quarter, full, and back again. Now let us look more closely at what the Moon actually is, what its surface tells us, and how it interacts with Earth.
The Moon is a rocky world about 3,474 kilometers in diameter — roughly one-quarter the size of Earth. It has no atmosphere (so there is no weather, no wind, no sound), no liquid water, and temperatures swing from about +127 degrees Celsius in direct sunlight to -173 degrees Celsius in shadow. Footprints left by Apollo astronauts in 1969 are still there today, perfectly preserved because there is nothing to disturb them.
Look at the Moon through binoculars and you will see two types of terrain. The bright, rough areas are the highlands — the Moon's original ancient crust, heavily pockmarked with impact craters from billions of years of meteorite bombardment. The dark, smooth areas are called maria (Latin for "seas," because early astronomers thought they were oceans). Maria are actually giant impact basins that were flooded by volcanic lava from the Moon's interior roughly 3-4 billion years ago. The lava solidified into dark basaltic rock, creating the smooth plains. The far side of the Moon has very few maria — it is almost entirely cratered highlands, probably because the crust is thicker there and lava had a harder time reaching the surface.
The Moon always shows Earth the same face — a phenomenon called tidal locking. The Moon rotates on its axis at exactly the same rate it orbits Earth (both take about 27.3 days), so the same hemisphere perpetually faces us. This is not a coincidence. Over billions of years, Earth's gravitational pull created a slight elongation in the Moon, and friction from this tidal bulge gradually slowed the Moon's rotation until it locked into sync with its orbit.
The Moon's most obvious effect on Earth is tides. The Moon's gravity pulls on the side of Earth nearest to it more strongly than on the far side, creating a bulge of water (high tide) on the near side and — less intuitively — a second bulge on the far side (because the Earth itself is pulled toward the Moon more than the far-side water is). As Earth rotates through these bulges, most coastlines experience two high tides and two low tides per day. The Moon also stabilizes Earth's axial tilt at about 23.5 degrees, which gives us our regular seasons. Without the Moon's stabilizing influence, Earth's tilt could wander chaotically over millions of years, producing extreme climate instability.
How did the Moon form? The leading theory — the giant impact hypothesis — proposes that about 4.5 billion years ago, a Mars-sized body collided with the young Earth at tremendous speed. The impact blasted an enormous amount of debris into orbit around Earth, and this debris gradually coalesced into the Moon. This theory explains why the Moon's composition is similar to Earth's outer layers, why the Moon has a proportionally small iron core (the impactor's iron merged with Earth's core), and why the Moon orbits in nearly the same plane as Earth's equator.