Glossary
Every concept as a short definition. The question is the link — tap through for the drawing and a deeper pass. Filter the list, or jump A–Z.
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Asteroid belt
What is the asteroid belt? A ring of leftover rock between Mars and Jupiter. It is not the wreckage of a planet. Jupiter’s gravity never let the rubble gather into one world. The dark gaps in it are resonances — the same tug, at the same point, over and over.
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Axial tilt
What is Earth’s axial tilt? Earth’s spin axis is not upright. It leans about 23.4° from the vertical to its orbit. That lean is why we have tropics, polar circles, and seasons — and it barely changes from one year to the next.
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Equirectangular projection
Why is this map flat? This map gives every degree of latitude and longitude the same length on the page, so meridians are straight and the terminator is honest. The cost is size: land near the poles is stretched. The picture says so, in numbers from the drawing.
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Jupiter’s moons
Why does Jupiter have so many moons? Gravity’s reach grows with mass, and Jupiter has more of it than anything in the solar system except the Sun. A wide region around it is a place where a small body stays with Jupiter instead of orbiting the Sun on its own.
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Libration
What is libration? Libration is a small wobble in the Moon’s facing. We still have one near side, but over time we see about 59% of the surface, not 50%. The extra is a peek around the edges.
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Moon phases
Why does the Moon change shape? The Moon makes no light of its own. One half is always lit by the Sun. Phases are your changing view of that lit half as the Moon goes around Earth — about 29.5 days from full moon to full moon.
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Moonrise delay
Why does moonrise get later each night? The Moon moves east along its orbit while Earth turns. Earth has to spin extra — about 50 minutes — to bring the Moon back to the same place on your sky. That extra is why moonrise (and moonset) slip later day by day.
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Olbers’ paradox
Why is the night sky dark? If stars went on forever, every line of sight would hit a star and the night would be as bright as a surface of suns. It is not. The dark is a clue that the universe is not an infinite, unchanging hall of lights.
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Orbit
Why don’t planets fall into the Sun? They are falling. They are also moving sideways. By the time a planet has fallen, the Sun is no longer where it was falling toward. Falling and missing, without end, is an orbit. There is no outward force holding them up.
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Orbit change
What happens when you change one part of an orbit? The shape changes, not just the speed. Slow a planet at one point and it drops onto a longer ellipse that swings closer in and climbs back out. Speed it up and the far side lifts away. It does not spiral into the Sun unless almost all of the sideways speed is gone.
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Orbital motion
How does an orbit work? At every instant a planet has a velocity — where it is already going — and gravity adds a small pull straight toward the Sun. Bend a path by the same small amount at every step and it closes. That closed path is the orbit.
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Orbital tilt of the Moon
Why isn’t there an eclipse every month? The Moon’s path around Earth is tipped about 5° to Earth’s path around the Sun. Most months the full Moon rides above or below Earth’s shadow, and the new Moon rides above or below the Sun. An eclipse needs the Moon at a crossing, not just at a phase.
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Polar circles
What are the polar circles? The Arctic Circle and Antarctic Circle sit at 66.6° north and south — 90° minus Earth’s tilt. Inside them the Sun can stay up all day in summer, or stay down all day in winter.
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Seasons
Why do we have seasons? Not because Earth gets closer to the Sun. When it is summer in the United States it is winter in Australia, on the same planet, at the same distance. Seasons happen because the axis is tilted 23.4° and keeps pointing the same way all year.
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Solar system scale
Why are the planets drawn so close together? Because the real solar system will not fit on a screen. Neptune is about 78 times farther from the Sun than Mercury. Drawn honestly at one scale, the inner four sit in the innermost tenth of the picture. This simulator climbs a zoom ladder so something is always legible, and it tells you what is compressed.
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Subsolar point
What is the subsolar point? It is the one place on Earth where the Sun is straight overhead right now — where a flagpole casts no shadow. It is the yellow dot on the day/night map, and it is where the Sun-centre to Earth-centre line hits the surface.
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Synodic month
What is a synodic month? A synodic month is the time from one full moon to the next, about 29.5 days. It is longer than the Moon’s trip around Earth relative to the stars, because Earth has moved and the Moon must catch up to the same angle with the Sun.
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Terminator
What is the terminator? The terminator is the line between day and night — the circle on Earth where the Sun sits on the horizon. Sunrise is where you cross it going into the light; sunset is where you cross it going out.
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Tidal locking
What is tidal locking? The Moon keeps the same face toward Earth because it turns once in the same time it goes around us. That match is tidal locking — not a dark side that never sees the Sun.
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Tides
What causes tides? The Moon’s gravity pulls harder on the water nearer to it than on the Earth’s centre, and harder on the centre than on the far-side water. The ocean stretches into two bulges. Earth then turns under those bulges, so most coasts see two high tides a day.
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Tropic of Cancer
What is the Tropic of Cancer? It is the circle at 23.4° north. The Sun is straight overhead there once a year, on the June solstice, and never north of it. That number is Earth’s tilt, not a climate zone someone drew.
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Tropic of Capricorn
What is the Tropic of Capricorn? It is the circle at 23.4° south — the southern twin of the Tropic of Cancer. The Sun stands straight overhead there once a year, on the December solstice, and never south of it.
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Twilight
What is twilight? Twilight is the time when the Sun is below the horizon but still lighting the sky. This map draws it as a soft band: from the terminator down to where the Sun is 18° below — the edge of astronomical night.