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A black hole is matter packed so tightly that not even light escapes. We know they exist from their effects on nearby stars and gas, from X-rays, gravitational waves and one direct image.
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The Moon's gravity pulls unevenly across Earth, stretching the oceans into two bulges. Earth rotates through them, giving most coasts two high tides a day.
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Earth's orbit takes about 365.24 days, not a whole number. An extra day roughly every four years keeps the calendar in step with the seasons.
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Dark matter is unseen mass inferred from its gravity, making up roughly 27% of the universe. Its existence rests on many observations, but what it is remains unknown.
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Seasons are caused by the tilt of Earth's rotational axis, about 23.5 degrees, not by changes in Earth's distance from the Sun.
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Astronomers combine the expansion rate, the cosmic microwave background and the ages of old stars. They agree on roughly 13.8 billion years, though the methods differ slightly in detail.
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The universe has a finite age, so light from very distant stars has not reached us, and expansion stretches the light that does arrive beyond what eyes can see.
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Venus is hotter because its dense carbon dioxide atmosphere traps heat in a runaway greenhouse effect, while airless Mercury loses heat to space.
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Yes, by current estimates. About 3 trillion trees outnumber the Milky Way's roughly 100 to 400 billion stars, though both figures are estimates.
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Venus spins once in about 243 Earth days but orbits the Sun in about 225, so its rotation outlasts its year. Why it spins so slowly and backward is not fully settled.
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In 2006 the IAU defined 'planet' to require clearing the orbital neighbourhood. Pluto meets the other criteria but not that one, so it became a dwarf planet.
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The Fermi paradox is the clash between the apparent likelihood of alien civilizations and the absence of any evidence for them. Proposed answers say life is rare, civilizations are short-lived or quiet, or we have not looked enough.
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Light moves at full speed, but dense solar plasma scatters it constantly in random directions. The resulting zig-zag makes the trip very long, with estimates ranging from tens of thousands to millions of years.
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Today the Moon's distance is measured by timing laser pulses bounced off reflectors left on its surface, accurate to about a millimeter. Ancient astronomers got a rough figure from eclipses.
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Hawking's calculation implies that black holes destroy information as they evaporate, which conflicts with quantum mechanics. Most physicists now think information survives, but how is debated.
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The Carrington Event was the most intense geomagnetic storm in recorded history, peaking on 1–2 September 1859. It disrupted telegraphs and produced auroras seen far from the poles.
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Astronomers split starlight into a spectrum and read the dark or bright lines, which act as a unique fingerprint for each chemical element.
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The sky looks blue because gas molecules in the atmosphere scatter short-wavelength blue light far more strongly than red light. Sunsets look red and orange because blue light is scattered out of the direct beam over the longer path.
Not factually verified
