Earth & space science · 30 pages
Earth & space science
Plain-language, source-grounded explainers for the everyday building blocks of earth and space science — rocks and the rock cycle, the water, carbon and nitrogen cycles, weather and climate, tectonic plates, the Solar System, the Moon’s phases, eclipses, asteroids and comets. Each page leads with a concise, accurate definition and links across to the wider CASRAI standards and dictionary.
Browse the topic
All 30 earth & space science pages
Weather vs climate
The difference is timescale. Weather is the short-term state of the atmosphere at a place — temperature, rain, wind and cloud over minutes to days. Climate is the long-term average and variability of that weather, usually measured over about thirty years. A cold day is weather; a region being reliably cold is its climate. As one saying goes, climate is what you expect, weather is what you get.
ComparisonComet vs asteroid
The difference is composition and behaviour. An asteroid is a rocky or metallic body, most found between Mars and Jupiter, that has no tail. A comet is an icy body of frozen gases, dust and rock from the cold outer Solar System; as it approaches the Sun, its ices vaporise to form a glowing coma and a long tail. In short: comets are icy and grow tails, asteroids are rocky and do not.
ComparisonWeathering vs erosion
The difference is movement. Weathering is the breakdown of rock, soil and minerals in place — by physical, chemical or biological processes — without moving them anywhere. Erosion is the transport of that loosened material to a new location by wind, water, ice or gravity. Weathering must usually happen first to make material available; erosion then moves it. Deposition is the final step, where the material settles.
ComparisonLunar vs solar eclipse
The difference is which body is in shadow. A solar eclipse happens when the Moon moves between the Sun and Earth, blocking the Sun and casting the Moon’s shadow on Earth — seen by day. A lunar eclipse happens when Earth moves between the Sun and the Moon, so Earth’s shadow falls on the full Moon — seen at night, safely, often turning the Moon red.
ComparisonRotation vs revolution
The difference is the axis of motion. Rotation is the spinning of a body around its own internal axis — Earth’s rotation gives us day and night, taking about 24 hours. Revolution is the movement of one body around another in orbit — Earth’s revolution around the Sun gives us the year, taking about 365 days. In short: rotation is spinning in place, revolution is orbiting around something else.
ComparisonMeteoroid vs meteor vs meteorite
It is the same object at three stages. A meteoroid is a small rocky or metallic body travelling through space. When it enters Earth’s atmosphere and burns up, the bright streak of light it makes is a meteor (a “shooting star”). If any part survives the fiery descent and reaches the ground, that surviving piece is a meteorite. Space rock, then light, then landed stone.
DefinitionThe planets in order
The eight planets in order from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four are small, rocky inner planets; the outer four are giants — Jupiter and Saturn are gas giants, Uranus and Neptune are ice giants. Pluto, once counted ninth, was reclassified as a dwarf planet by the IAU in 2006.
DefinitionIgneous rock
Igneous rock is rock formed when molten magma or lava cools and solidifies. It is one of the three main rock types, alongside sedimentary and metamorphic. Rock that cools slowly underground (intrusive, such as granite) grows large crystals, while rock that cools quickly at the surface (extrusive, such as basalt) has tiny or no crystals. The name comes from the Latin for fire.
DefinitionSedimentary rock
Sedimentary rock forms when sediment — fragments of weathered rock, dissolved minerals or organic remains — settles in layers and is compacted and cemented together over time. It is one of the three main rock types and typically shows distinct layers, called strata. Common examples are sandstone, limestone and shale. Because it forms gently at the surface, sedimentary rock is the main rock type that preserves fossils.
DefinitionMetamorphic rock
Metamorphic rock forms when an existing rock — igneous, sedimentary or older metamorphic — is changed by intense heat, pressure or chemically active fluids, without fully melting. The original rock’s minerals recrystallise into new ones, often producing a layered or banded texture. The name means “change of form”. Examples include marble (from limestone) and slate (from shale). It is one of the three main rock types.
DefinitionRock cycle
The rock cycle is the continuous, slow process by which rocks change from one of the three main types — igneous, sedimentary and metamorphic — into another over geological time. Processes such as melting, cooling, weathering, erosion, deposition, compaction, heat and pressure move rock around the cycle. There is no fixed start or end; any rock can become any other type, driven by Earth’s internal heat and surface forces.
DefinitionFossils
Fossils are the preserved remains or traces of ancient organisms, from bones and shells to footprints and burrows. They form mostly when remains are buried in sediment and, over long ages, are mineralised or leave impressions in rock. Found chiefly in sedimentary rock, fossils are the main evidence for the history of life on Earth and for evolution. Body fossils preserve the organism; trace fossils preserve its activity.
DefinitionPrecipitation
Precipitation is water, in liquid or frozen form, that falls from clouds to Earth’s surface. It includes rain, drizzle, snow, sleet and hail. It forms when water vapour in the air condenses into cloud droplets or ice crystals that grow heavy enough to fall under gravity. Precipitation is the stage of the water cycle that returns water from the atmosphere to the land and oceans.
DefinitionCondensation
Condensation is the change of a substance from a gas into a liquid — most familiarly water vapour turning into liquid water as it cools. It is the opposite of evaporation. In nature, condensation forms clouds, mist, fog and dew, and the water droplets on a cold glass. In the water cycle, condensation is the stage where rising water vapour becomes the cloud that can later produce precipitation.
DefinitionEvaporation
Evaporation is the change of a liquid into a gas at its surface, below its boiling point — most familiarly liquid water becoming water vapour. It happens as faster-moving molecules escape into the air, and is driven by heat, usually from the Sun. It is the opposite of condensation. In the water cycle, evaporation lifts water from oceans, lakes and soil into the atmosphere as vapour.
DefinitionWater cycle
The water cycle, or hydrological cycle, is the continuous movement of water around the Earth, between the oceans, atmosphere and land. Its main stages are evaporation, condensation, precipitation and collection (runoff and groundwater). Powered by the Sun and gravity, it recycles a fixed amount of water endlessly, with no real beginning or end, and it sustains all life by distributing fresh water across the planet.
DefinitionCarbon cycle
The carbon cycle is the continuous movement of carbon between the atmosphere, living organisms, the oceans and rocks. Plants take in carbon dioxide through photosynthesis; respiration, decay and combustion return it to the air; and oceans and rocks store huge amounts over long periods. The cycle keeps atmospheric carbon dioxide in balance, but burning fossil fuels is adding carbon faster than natural processes remove it.
DefinitionNitrogen cycle
The nitrogen cycle is the continuous movement of nitrogen between the atmosphere, soil and living things. Although air is mostly nitrogen gas, most organisms cannot use it directly; bacteria “fix” it into usable forms, plants take it up, animals eat the plants, and decay and other bacteria eventually return nitrogen to the air. Its key stages are fixation, nitrification, assimilation, ammonification and denitrification.
DefinitionMeteorite
A meteorite is a fragment of rock or metal from space that survives falling through Earth’s atmosphere and lands on the surface. Most come from asteroids, with a few from the Moon or Mars. While burning through the air it appears as a meteor; only the piece that lands is a meteorite. Scientists prize meteorites as samples of the early Solar System. The three main types are stony, iron, and stony-iron.
DefinitionAsteroid
An asteroid is a small rocky or metallic body that orbits the Sun, far smaller than a planet and with no atmosphere. Most lie in the asteroid belt between Mars and Jupiter, leftovers from the early Solar System that never formed into a planet. Unlike comets, asteroids are rocky rather than icy and do not grow tails. They range from tiny boulders to hundreds of kilometres across.
DefinitionComet
A comet is an icy body of frozen gases, dust and rock that orbits the Sun. When it nears the Sun, the heat vaporises its ices, releasing gas and dust that form a bright cloud (the coma) around its nucleus and one or more tails that always point away from the Sun. Comets come from the cold outer Solar System and are often described as “dirty snowballs”.
DefinitionMilky Way
The Milky Way is the galaxy that contains our Solar System — a vast, gravitationally bound system of hundreds of billions of stars, plus gas, dust and dark matter. It is a barred spiral galaxy, shaped like a flat disc with curving arms and a central bulge. The faint band of light we see crossing the night sky is our own view of the galaxy’s disc from inside it.
DefinitionGalaxy
A galaxy is a vast system of stars, gas, dust and dark matter, all bound together by gravity. Galaxies range from dwarfs with millions of stars to giants with trillions, and most have a supermassive black hole at their centre. They come in three main shapes — spiral, elliptical and irregular. Our own galaxy is the Milky Way, and the universe contains a great many galaxies.
DefinitionAtmosphere
The atmosphere is the layer of gases that surrounds the Earth, held by gravity. It is roughly 78% nitrogen and 21% oxygen, with small amounts of argon, carbon dioxide, water vapour and other gases. It is arranged in layers — the troposphere, stratosphere, mesosphere, thermosphere and exosphere. The atmosphere makes life possible by providing oxygen, trapping warmth, shielding the surface and driving the weather.
DefinitionTectonic plates
Tectonic plates are the large, rigid slabs that make up Earth’s outer shell, the lithosphere. They float on the hotter, softer rock beneath and move very slowly — only centimetres a year. Where plates meet, their interactions build mountains and cause most of the world’s earthquakes and volcanoes. The theory describing their movement is called plate tectonics, and it underlies much of modern geology.
DefinitionEclipse
An eclipse happens when one body in space moves into another’s shadow, or passes between an observer and a light source. The two familiar kinds involve the Sun, Earth and Moon: a solar eclipse, when the Moon blocks the Sun from Earth’s view, and a lunar eclipse, when Earth’s shadow falls on the Moon. Eclipses occur only when the three bodies line up closely enough.
DefinitionGreenhouse effect
The greenhouse effect is the process by which certain gases in the atmosphere — such as carbon dioxide, methane and water vapour — trap heat radiating from the Earth’s surface, keeping the planet warmer than it would otherwise be. It is a natural and essential process that makes Earth habitable. However, human emissions of greenhouse gases are strengthening the effect, which is the main cause of climate change.
DefinitionBiome
A biome is a large region of the Earth defined by its climate — chiefly temperature and rainfall — and the characteristic communities of plants and animals adapted to live there. Major land biomes include tropical rainforest, desert, grassland, temperate forest, taiga and tundra. A biome is larger and broader than an ecosystem; the same biome type can occur in different parts of the world.
DefinitionThe Moon’s phases
The Moon’s phases are the changing shapes of the illuminated Moon as seen from Earth, caused by our changing view of its sunlit half as the Moon orbits Earth. The Sun always lights half the Moon, but how much of that lit side faces us changes. The eight phases run from new Moon through waxing crescent, first quarter, waxing gibbous, full Moon, then waning back to new — a cycle of about a month.
DefinitionSolar System
The Solar System is the Sun and everything that orbits it under its gravity — the eight planets and their moons, dwarf planets such as Pluto, and countless asteroids, comets and smaller bodies. The Sun holds almost all the system’s mass. It formed about four and a half billion years ago from a collapsing cloud of gas and dust, and it lies within the Milky Way galaxy.
