Earth

A color image of Earth from Apollo 17, showing Africa, Antarctica, and the Indian Ocean.

Earth is the third planet from the Sun in the Solar System and the only known celestial body to support life. It has a dense atmosphere composed primarily of nitrogen and oxygen. About 71% of Earth's surface is covered by oceans, giving it a predominant blue appearance. Scientists estimate that Earth formed approximately 4,600 million years ago.

Description

Earth orbits the Sun between Venus and Mars at an average distance of 149.6 million kilometers (1 astronomical unit). Its single natural satellite, the Moon, significantly influences Earth's tides and stabilizes its axial tilt. Earth has an equatorial diameter of 12,756 kilometers.

Structure

The interior of Earth is divided into five distinct layers:

• Inner core: A solid metallic sphere primarily composed of iron and nickel.
• Outer core: A liquid layer also composed of iron and nickel.
• Mantle: A viscous layer of silicate materials.
• Asthenosphere: The upper part of the mantle, partially molten and ductile.
• Crust: The outermost layer, consisting of two types:
• Oceanic crust
• Continental crust

Above the crust lies the atmosphere, which becomes less dense with increasing altitude.

Movements

Rotational and Orbital Motions

Earth exhibits two main motions: rotation and revolution.

• Rotation: Earth rotates around its axis once every 24 hours, resulting in the cycle of day and night.
• Revolution: Earth orbits the Sun in an elliptical path, completing one revolution in approximately 365.25 days. The additional 0.25 days lead to a leap year every four years.

Long-Term Motions

Earth also undergoes long-term motions over millennia:

• Precession: A slow conical motion of Earth's axis caused by gravitational forces from the Sun and the Moon.
• Nutation: Small oscillations of Earth's axis with a period of approximately 18 years, primarily due to the Moon's gravitational influence.

Internal Motions

Illustration of convection currents.

Internal motions within Earth, such as convection currents in the mantle, drive the movement of heat and energy. This process involves:

• Cooler, denser material sinking.
• Heating near the core, causing material to rise.
• Continuous cycling of material, which drives plate tectonics.

The Moon

Earth has one natural satellite, the Moon, which influences tides, stabilizes Earth's axial tilt, and contributes to the planet's dynamic evolution.