About this universe
Cosmira is the great dark pricked with ancient lights. Here you ask why stars twinkle, why the sky is dark at night, and why planets are round. The same light that travels all the way to us is the one that, lower down, paints the sky blue.
The whys of this universe
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Why does the Moon look so big when it rises?
It doesn't grow: your brain sees it as bigger near the horizon, an optical illusion that no one has fully explained yet.
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Why does the Moon always show the same face?
Because the Moon spins on itself exactly once per trip around the Earth. This perfect match is no accident: Earth's gravity slowed the Moon's spin over billions of years until it locked, through the very same force that lifts our tides.
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Why are planets round?
Past a certain size, a body's own gravity becomes strong enough to flatten every peak and fill every hollow toward its centre: the shape it settles into is always a sphere. None of them is perfectly round, though: spin pulls the equator outward, some 21 kilometres farther from Earth's centre than the poles.
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Why do stars twinkle?
Because their light crosses kilometres of restless air before reaching your eye, and the atmosphere makes that tiny pinpoint dance. Planets, being closer, don't twinkle.
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Why are sunsets red?
Because the light from a sun near the horizon has to pass through a far thicker layer of atmosphere than at noon. The blue is scattered out of the line of sight by the molecules of the air; the red passes through almost untouched. Add dust or volcanic ash and the red saturates further, the way Krakatoa reddened skies worldwide after 1883.
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Why is the sky dark at night?
Because the universe has an age: light from the farthest stars hasn't had time to reach us yet. And the light that does arrive has been stretched so far by cosmic expansion that it has slipped out of the visible range.