Earth

Lore & Background

Earth formed about 4.5 billion years ago from gas and dust in the early Solar System. The formation of the ocean and the subsequent development of life occurred during the first billion years of Earth's history. Life spread globally and has been altering Earth's atmosphere and surface, leading to the Great Oxidation Event two billion years ago. Humans emerged 300,000 years ago in Africa and have spread across every continent on Earth. Humans depend on Earth's biosphere and natural resources for their survival, but have increasingly impacted the planet's environment. Humanity's current impact on Earth's climate and biosphere is unsustainable, threatening the livelihood of humans and many other forms of life, and causing widespread extinctions.

In Their Own Story

The rain had stopped, but the smell of sulfur still clung to the flagstones. Adam stood in the courtyard of the old monastery, his coat heavy with water and something darker, and watched the last of the gate's light gutter out like a candle in a draft. Behind him, the nave was a ruin of splintered pews and black scorch-marks on the vaulted ceiling. A single bell still hung, cracked down one side, and when the wind shifted it gave one low, tired note. He did not look back at the gate. He never looked back at the gate. He rolled his shoulders, felt the familiar ache in the joints, and walked toward the tree line where the first green of the valley was just beginning to show through the morning mist. Earth was still here. That was enough. That had to be enough.

Reader's Guide

Earth has a dynamic atmosphere composed primarily of nitrogen and oxygen. Water vapor is widely present, forming clouds that cover most of the planet. The water vapor acts as a greenhouse gas and, together with other greenhouse gases such as carbon dioxide, creates conditions for liquid surface water and water vapor to persist. This process maintains the current average surface temperature of 14.76°C. Differences in captured energy between geographic regions drive atmospheric and ocean currents, producing a global climate system with different climate regions and weather phenomena. Earth's crust consists of slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes. Earth has a liquid outer core that generates a magnetosphere capable of deflecting most destructive solar winds and cosmic radiation.

Did You Know?

• Earth is about eight light-minutes (1 AU) away from the Sun and orbits it, taking a year (about 365.25 days) to complete one revolution.
• Earth rotates around its own axis in slightly less than a day (about 23 hours and 56 minutes).
• Earth's axis of rotation is tilted with respect to the perpendicular to its orbital plane around the Sun, producing seasons.
• Earth is orbited by one permanent natural satellite, the Moon, which orbits Earth at 384400 km—1.28 light seconds—and is roughly a quarter as wide as Earth.
• The Moon's gravity helps stabilize Earth's axis, causes tides, and gradually slows Earth's rotation.