Theme

Why the sky and space intrigue us

Why is the sky blue by day and dark at night? Why does the setting sun turn red, why do stars seem to tremble, why are planets round and how do clouds stay up? Look up: everything overhead obeys precise laws you can grasp without equations.

This page brings together the whys of the sky and space, phenomena everyone has seen a thousand times without always explaining them. Light scattering, the atmosphere filtering, gravity rounding things off: behind the poetry of a sunset lies an elegant mechanics.

Each answer is short, checked and linked to its sources. From the horizon to the cosmos, here is a way to turn the simple act of looking up into a series of small revelations.

A large amber sun dominates a cream sky; below, hills in warm flat tones where the deepest heat pools right at the bottom, offset from the Sun.
Evelie

Why isn't the longest day the hottest?

The longest day falls on 21 June, but the hottest comes in late July. The same lag that makes four in the afternoon hotter than noon, stretched across a whole summer.
Past a certain size, gravity crushes every bump back toward the centre. A sphere is what a massive body always ends up becoming.
Cosmira

Why are planets round?
Droplets so small they fall almost infinitely slowly: a faint upward breath is enough to hold them.
Plumaire

Why do clouds stay up?

Because they are made of droplets so small they fall almost infinitely slowly: a faint upward breath of air is enough to hold them up, despite the hundreds of tonnes of water inside.
Their light crosses kilometres of restless air before reaching the eye. Planets, being closer, don't twinkle.
Cosmira

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.
When the sun sinks, its light has to push through far more atmosphere to reach us. The blue scatters away on the way; the red survives. The sky doesn't change: the path lengthens.
Cosmira

Why are sunsets red?
Olbers' paradox: the universe has an age, light from the most distant stars hasn't had time to reach us, and the light that does is stretched into the invisible.
Cosmira

Why is the sky dark at night?
Blue light zigzags between air molecules and reaches us from everywhere at once. A sky that breathes: not a Rayleigh diagram, not a rural landscape.
Plumaire

Why is the sky blue?