Earth

Shape and Size

Earth is an oblate spheroid, meaning it is slightly flattened at the poles and bulging at the equator due to its rotation. It has a mean radius of approximately 6,371 km (3,959 mi) and a circumference of about 40,075 km (24,901 mi).

Orbit and Rotation

Earth orbits the Sun at an average distance of 149.6 million kilometers (93 million miles), taking approximately 365.25 days to complete one revolution. This slight discrepancy leads to the inclusion of a leap day every four years. Earth’s rotation period is about 24 hours, creating day and night cycles.

Atmosphere

Earth’s atmosphere consists of 78% nitrogen, 21% oxygen, and trace amounts of other gases, including carbon dioxide and argon. It protects life by blocking harmful solar radiation, moderating temperatures, and maintaining a breathable environment.

Biosphere

Earth hosts an estimated 8.7 million species, although the exact number is unknown. The planet’s biosphere includes diverse ecosystems ranging from tropical rainforests to polar ice caps.

History and Evolution

Earth formed approximately 4.54 billion years ago from the accretion of dust and gas. Life emerged within its first billion years, with multicellular organisms appearing around 600 million years ago. Human civilization developed within the last 10,000 years, profoundly impacting Earth’s ecosystems and climate.