TimeAndSpace.Science
Gravity, motion, time and space. A day is Earth turning. A year is it going round. Seasons are that turn being tilted. Built for classrooms and for anyone else who is curious.
Earth
Where is the Sun right now?
Half the Earth is always in daylight, and the other half is night.
The sun is overhead at 7.8° N, 10.4° W — this is the subsolar point. This time of year the sun is moving from summer toward fall, and days in the northern half of the world are getting shorter because Earth is tilting away from the sun in Earth’s orbit.
Open the day and night map →What is the Tropic of Cancer? The dashed gold line north of the equator on the map above — the Sun never stands overhead north of it.
What is the terminator? The curve of night on the map — it leans because Earth does.
Can the moon be up in the daytime? The Moon is up about twelve hours of every twenty-four, and those hours drift through the day.
Time
Where the clock comes from
Nobody invented time — people read it off the sky. A day is Earth spinning once. A year is Earth's orbit, announced by the seasons its tilt creates. A month is the Moon's cycle. Every hour and minute since is bookkeeping for those three motions.
Who invented time? →What is a solar day? Sunrise to sunrise — the day you actually live.
Why do we have seasons? Not because Earth gets closer to the Sun. Because it leans, and the lean faces a different way as it goes around.
What is a synodic month? Full moon to full moon is about 29.5 days, a little longer than one trip around Earth.
Earth
Earth’s tilt makes the seasons
Earth’s axis leans 23.4° and keeps pointing the same way in space all year. So as Earth goes round the Sun, the northern half leans into the light for half the year and away from it for the other half — and the southern half does the opposite. That lean, not distance, is what changes the seasons. Below is the Sun and the Earth from the side, drawn for right now. The yellow line lands where the sun is straight overhead; press a season and watch it move between the tropics.
That line lands at 7.8° N — 15.6° short of the Tropic of Cancer, which is as far north as it can ever get. The northern half of the world is leaning into the light, so more of it falls inside the lit half than outside, and its days are longer than its nights. That lean is the tilt of Earth on its orbit.
Three interactive simulators on that page show it from three different places to stand:
From above · the day & night map From the side · the angle of the sun From beyond the orbit · Earth, Sun & Moon through a year
Why do we have seasons? Not because Earth gets closer to the Sun. Because it leans, and the lean faces a different way as it goes around.
What is Earth’s axial tilt? The axis is not upright. That lean is why the tropics and the polar circles sit where they do.
Earth
The Moon around the Earth
The Sun holds still on the left. Earth and the Moon sit to the right so there is more sky between them. The Moon is about a quarter the width of Earth — that size is true. Distances are not: the simulator says by how much.
27.3-day orbit. This picture is the phases. A month passes in about 24 seconds.
See the Sun, Earth & Moon simulator →Why does the Moon change shape? One half is always lit. The picture above is that lit half, seen from a different angle each night.
What is a synodic month? Full moon to full moon is about 29.5 days, a little longer than one trip around Earth.
What is tidal locking? The same face stays toward us because the Moon turns once per orbit.
Space
The solar system, flat-on
The inner planets on their real orbits. Mercury laps everybody — that difference in speed is the whole story of orbits. Distances are compressed so they fit.
A year every 24 seconds. Positions are real. Planet dots are not — they would be smaller than a pixel.
See the solar system simulator →Why don’t planets fall into the Sun? They are falling. Missing is the rest of the sentence — sideways speed is what keeps the miss going.
Earth
When the tide turns
The Moon pulls harder on the water nearer to it than on 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.
What causes tides? Two bulges of water, Earth turning under them. Most coasts therefore see two highs a day.
Space
What if a moon slowed down?
Nothing spirals in. Take speed away and the far side of the orbit drops toward the planet; add speed and it climbs away — and either way the moon comes back through the point where you changed it.
A circle: falling exactly as fast as the curve carries it away.
How does an orbit work? Falling toward, already moving sideways. Change the sideways and the far side of the path is what moves.
The rest of the site
A hint, not a catalogue. Each tab is its own page.
Time
Clocks, and what they are counting — a world clock, a countdown to the day you are waiting for.
Earth
Your own sky: day and night, sunrise, the Moon, and why the tropics sit where they do.
Space
Where everything actually is — planets, orbits, gravity, and the questions that open the door.
Classroom
Projector mode, questions written for ten-year-olds first, and a way to send the lesson you already run.
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How this site works
Guides: using this in a classroom · how these numbers are worked out · how countdowns work · browser limitations · about this site.