Earth's surface is organized by an invisible grid of latitude and longitude. Latitude measures how far north or south a point is from the Equator, ranging from 0° at the Equator to 90° at the poles, and lines of latitude are called parallels. Longitude measures how far east or west a point is from the Prime Meridian, running from 0° at Greenwich, London, to 180° east or west, and these lines are called meridians. The Equator itself is an imaginary line about 40,075 kilometers long that divides the Earth into the Northern and Southern Hemispheres, while the Prime Meridian divides it into the Eastern and Western Hemispheres. Other key reference lines include the Tropic of Cancer at about 23.5°N, the northernmost latitude where the Sun can appear directly overhead during the June solstice, and the Tropic of Capricorn at about 23.5°S, the southernmost such latitude during the December solstice. Beyond the Arctic Circle at 66.5°N and the Antarctic Circle at 66.5°S, there is at least one day of continuous daylight and one of continuous darkness each year. The International Date Line roughly follows the 180° meridian in the Pacific: crossing westward advances the date by one day, crossing eastward sets it back. The world is divided into 24 standard time zones, each about 15° of longitude wide, although many countries use offset zones that result in more than 24 in practice.
Beneath the surface, the Earth's lithosphere is broken into massive tectonic plates that float on the semi-fluid asthenosphere. There are seven major plates—the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American—plus several minor ones. Their interactions shape the planet: earthquakes, volcanic activity, and mountain formation all result from plate movement. The Ring of Fire is a horseshoe-shaped zone around the Pacific where about 75 percent of the world's volcanoes and 90 percent of its earthquakes occur, stretching some 40,000 kilometers. At convergent boundaries, where plates collide, mountain ranges like the Himalayas can rise, or deep ocean trenches like the Mariana Trench can form through subduction. The San Andreas Fault in California marks a transform boundary where the Pacific and North American plates slide past each other over a length of about 1,200 kilometers, and the Great Rift Valley in Africa shows what happens at divergent boundaries as the continent slowly splits apart.
The Earth has five major climate zones—tropical, dry, temperate, continental, and polar—based on the Köppen classification system. Tropical climates have average monthly temperatures above 18°C year-round and include rainforests, monsoons, and savannas. Biomes shaped by these climates include the tundra, a treeless biome with permafrost, mosses, and lichens in polar regions and high mountains; the savanna, with grasses and scattered trees and distinct wet and dry seasons; the taiga or boreal forest, the world's largest terrestrial biome, dominated by coniferous trees across northern North America, Europe, and Asia; and the temperate deciduous forest, with seasonal leaf loss and four distinct seasons. Permafrost, ground that stays frozen for at least two consecutive years, covers about 25 percent of the Northern Hemisphere's land surface and underlies much of the tundra and taiga.