Earth's Internal Structure
Earth is not a solid ball β it has distinct layers, each with different composition and properties. From outside to inside:
- Crust: The thin, solid outer layer. Oceanic crust is about 5β10 km thick; continental crust is 25β70 km thick. Made mainly of silicate rocks.
- Mantle: The thickest layer (~2 900 km). Made of semi-solid (plastic) rock that flows very slowly. Convection currents in the mantle drive plate tectonics.
- Outer Core: A liquid layer of iron and nickel (~2 200 km thick). Its movement generates Earth's magnetic field.
- Inner Core: A solid ball of iron and nickel (~1 200 km radius). Despite being the hottest part (~5 100Β°C), immense pressure keeps it solid.
Earth's magnetic field is produced by electric currents flowing in the liquid outer core. It acts like a shield, deflecting harmful solar wind particles away from Earth's surface β crucial for protecting life.
The Moon
The Moon is Earth's only natural satellite. It orbits Earth approximately every 27.3 days (sidereal period). Key properties:
- Diameter: about 3 474 km (roughly ΒΌ of Earth's diameter)
- Distance from Earth: approximately 384 400 km
- No atmosphere β so no weather, no sound, and extreme temperature swings (-173Β°C to +127Β°C)
- Surface covered in craters formed by meteorite impacts (no atmosphere to burn them up)
- Surface gravity: about 1/6 of Earth's gravity (β1.62 m/sΒ²)
- The Moon is tidally locked β it rotates once per orbit, so we always see the same side
A 60 kg person on Earth would weigh only ~10 kg on the Moon due to its weaker gravity.
Moon Phases
As the Moon orbits Earth, the angle between the Sun, Earth, and Moon changes. This changes how much of the sunlit side of the Moon we can see from Earth β these are the phases of the Moon. The full cycle takes about 29.5 days (synodic period).
| Phase | What we see | SunβMoon angle |
|---|
| New Moon | Moon not visible (Sun and Moon on same side) | 0Β° |
| Waxing Crescent | Thin sliver visible on right | ~45Β° |
| First Quarter | Right half lit | 90Β° |
| Waxing Gibbous | More than half lit on right | ~135Β° |
| Full Moon | Entire face lit | 180Β° |
| Waning Gibbous | More than half lit on left | ~225Β° |
| Last Quarter | Left half lit | 270Β° |
| Waning Crescent | Thin sliver on left | ~315Β° |
Waxing means the lit portion is growing; waning means it is shrinking. In the Southern Hemisphere, the lit side appears on the left during waxing phases (opposite to the Northern Hemisphere).
Tides
Tides are the periodic rise and fall of sea levels caused primarily by the Moon's gravitational pull on Earth's oceans. The Sun also contributes, but its tidal effect is less than half that of the Moon (despite being much larger, it is much farther away).
The Moon's gravity pulls the ocean water on the side closest to it outward, creating a high-tide bulge. At the same time, inertia creates another bulge on the opposite side. As Earth rotates, most coastlines experience two high tides and two low tides every ~24 hours.
Spring Tides vs Neap Tides
- Spring tides: Occur at new moon and full moon, when the Sun, Earth, and Moon are aligned. The gravitational forces combine β tides are higher than usual (largest range). The name comes from the Old English word meaning "to rise up," not the season.
- Neap tides: Occur at first and last quarter, when the Sun and Moon are at right angles. Their gravitational forces partially cancel β tides are weaker with a smaller range.
Eclipses
- Solar eclipse: The Moon moves between Earth and the Sun, casting a shadow on Earth. Only visible in a narrow path. Total solar eclipse = Moon completely blocks the Sun.
- Lunar eclipse: Earth moves between the Sun and Moon, casting its shadow on the Moon. The Moon may turn red ("blood moon") as Earth's atmosphere refracts red light onto the Moon. Visible from the entire night-side of Earth.
Why don't we get eclipses every month? The Moon's orbit is tilted about 5Β° relative to Earth's orbit around the Sun, so alignment only happens occasionally.