Big Bang
The Big Bang is the very hot, dense phase of the early universe, everywhere at once, after which the distances between everything began to grow.
What the Big Bang really wasEvery term in one plain sentence. How it works is in the 2-minute article behind it.
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Drake equation
The Drake equation estimates how many civilizations the galaxy may hold by multiplying factors such as the rate of star formation and the share of stars with planets.
How Many Civilizations Could Be in the GalaxyE
Event horizon
The event horizon is the boundary of a black hole beyond which every path, including that of light, leads only inward.
What Happens at a Black Hole's EdgeExpansion of the universe
The expansion of the universe is the growth of distances between galaxies as space itself stretches, not matter flying away from one point.
Why the Universe Is ExpandingF
Fermi paradox
The Fermi paradox is the contradiction between ample stars and time for other civilizations to appear and the complete absence of any trace of them.
Where Are the Aliens If There Are Billions of StarsG
Gas giant
A gas giant is a planet with no solid surface: its gas grows denser with depth, turns into a liquid and, deeper still, behaves like a metal.
Do Gas Giants Have a Solid Surface?Great Red Spot
The Great Red Spot is a giant storm on Jupiter large enough to fit Earth inside, observed continuously since about the 1830s.
What Is That Spot on JupiterH
Hawking radiation
Hawking radiation is the predicted faint glow of black holes that makes them slowly lose mass.
What Is Hawking RadiationM
Martian canals
Martian canals are fine lines on Mars that astronomers saw at the limit of telescope resolution and for half a century took to be artificial.
Who Came Up with the Canals on MarsMaunder minimum
The Maunder minimum is a stretch of nearly 70 years from the mid-17th century when sunspots almost completely disappeared.
Why the Sun Went Quiet for 70 YearsMoon illusion
The Moon illusion is a trick of perception that makes the Moon look huge near the horizon, although its size in photos is the same as overhead.
What Is the Moon Illusion?O
Olbers' paradox
Olbers' paradox asks why the night sky is dark if, in an infinite and eternal universe, every line of sight would end at a star.
Why the Night Sky Is Dark Instead of WhiteOverview effect
The overview effect is a shift in perception described by people returning from orbit after seeing the whole planet, a thin strip of atmosphere and no borders.
What Is the Overview Effect?S
Saturn's rings
Saturn's rings are a rotating disk of ice debris that, so close to the planet, cannot gather into a moon.
Why Saturn Has RingsSol
A sol is a Martian day, about forty minutes longer than a day on Earth.
How Martian Days Differ from Earth'sSolar flare
A solar flare is the release of energy stored for weeks in the Sun's twisted magnetic loops, escaping in minutes as light, X-rays and fast particles.
Why the Sun Has Solar FlaresSolar wind
The solar wind is the stream of charged particles that the Sun constantly blows into space.
How the Solar Wind Affects EarthSpace sickness
Space sickness is nausea in weightlessness caused by a conflict: the vestibular system stops signaling which way is down, while the eyes still see a floor and ceiling.
What Is Space SicknessT
Tides
Tides are the rise of sea level caused by the difference in the Moon's pull on the near and far sides of Earth; high tide comes twice a day.
Why Tides Are Linked to the Moon