The Moon: Earth's Constant Companion
How the Moon Was Born: The Giant Impact Hypothesis
Scientists believe the Moon formed about 4.5 billion years ago, relatively soon after the Earth itself. The most widely accepted theory is the Giant Impact Hypothesis. This theory suggests that a Mars-sized planet, often called Theia, collided with the early Earth. The colossal impact vaporized part of Earth's mantle and threw a massive amount of debris into orbit around our planet. Over time, this debris clumped together due to gravity, eventually forming the Moon we see today.
A Tour of the Lunar Landscape
The Moon's surface is not smooth. Through a telescope, you can see dark patches and bright areas covered with craters. The dark, flat areas are called maria (Latin for "seas"). These are vast plains of solidified lava that formed billions of years ago when asteroids punctured the lunar crust, causing magma to flood the basins. The brighter, heavily cratered regions are the lunar highlands, which are older than the maria.
The surface is also dotted with craters, which are bowl-shaped holes created by the impact of asteroids and comets. Because the Moon has no atmosphere, no weather, and no tectonic activity, these craters remain almost unchanged for billions of years, giving us a record of the Solar System's violent past.
The Moon's Orbit and Its Phases
The Moon orbits Earth in a path that is slightly elliptical, not a perfect circle. This orbit takes about 27.3 days to complete, a period known as the sidereal month. However, the cycle of the Moon's phases, from one New Moon to the next, takes about 29.5 days (the synodic month). The difference occurs because as the Moon orbits Earth, Earth is also orbiting the Sun, so the Moon has to travel a little farther to align with the Sun again.
Moon phases are the different ways the Moon looks from Earth over the course of a month. These phases change depending on the relative positions of the Sun, Earth, and Moon. They are not caused by Earth's shadow; that would be an eclipse. Instead, phases are a result of us seeing different portions of the sunlit part of the Moon.
| Phase | Description | Visibility |
|---|---|---|
| New Moon | The Moon is between Earth and the Sun. The side facing us is dark. | Not visible (except during a solar eclipse). |
| Waxing Crescent | A thin sliver of the Moon's right side becomes illuminated. | Visible in the western sky after sunset. |
| First Quarter | Half of the Moon's disk is illuminated (the right half). | Visible from noon until midnight. |
| Waxing Gibbous | More than half of the Moon is illuminated. | Visible in the afternoon and most of the night. |
| Full Moon | Earth is between the Sun and the Moon. The entire face is illuminated. | Visible all night, rising at sunset and setting at sunrise. |
| Waning Gibbous | The illumination decreases after the Full Moon. | Visible most of the night and in the morning. |
| Third Quarter | The left half of the Moon is illuminated. | Visible from midnight to noon. |
| Waning Crescent | A thin sliver of light remains on the left side. | Visible in the eastern sky before sunrise. |
Gravity in Action: Tides and Eclipses
The Moon's gravity is the primary force behind Earth's ocean tides. Gravity pulls the ocean water towards the Moon, creating a bulge. On the opposite side of Earth, another bulge forms because the Earth is being pulled toward the Moon more than the water on the far side. As Earth rotates, these bulges move around the planet, causing most coastlines to experience two high tides and two low tides every day.
When the Sun, Earth, and Moon align perfectly, we experience eclipses. A solar eclipse happens during a New Moon when the Moon passes directly between the Sun and Earth, casting a shadow on Earth. A lunar eclipse happens during a Full Moon when Earth passes directly between the Sun and the Moon, casting its shadow on the Moon.
Comparing Moons in Our Solar System
Our Moon is special to us, but it is just one of many natural satellites. The Solar System contains over 200 known moons. They come in all shapes and sizes. For example, Jupiter's moon Ganymede is larger than the planet Mercury, while Mars has two tiny, irregularly shaped moons, Phobos and Deimos, which are likely captured asteroids.
Some moons are even more fascinating than the planets they orbit. Saturn's moon Enceladus and Jupiter's moon Europa are believed to have vast subsurface oceans of liquid water beneath their icy crusts, making them prime targets in the search for extraterrestrial life.
| Moon Name | Planet | Notable Feature |
|---|---|---|
| Ganymede | Jupiter | Largest moon in the Solar System; has its own magnetic field. |
| Titan | Saturn | Has a thick atmosphere and lakes of liquid methane and ethane. |
| Io | Jupiter | Most volcanically active body in the Solar System. |
| Triton | Neptune | Orbits in the opposite direction of its planet's rotation (retrograde orbit). |
Common Mistakes and Important Questions
A: Yes! The word "month" comes from "moon." Our calendar month is roughly based on the Moon's cycle of phases, which is about 29.5 days. However, our modern calendar has been adjusted to fit neatly into a solar year.
A: This is due to synchronous rotation. The Moon takes the same amount of time to rotate once on its axis as it does to orbit Earth (~27.3 days). This means the same hemisphere constantly faces Earth. We call the side we never see the "far side," not the "dark side," as it receives just as much sunlight as the near side.
A: Earth occasionally captures small temporary "mini-moons." These are typically tiny asteroids, a few meters across, that get caught in Earth's gravity for a short time (a few months or years) before escaping back into solar orbit. So, while we only have one large, permanent moon, we sometimes have very small, temporary ones.
Footnote
1 Solar System: The collection of eight planets and their moons in orbit around the Sun, together with smaller bodies such as asteroids, comets, and dwarf planets.