Space and gravity：全部卡片

The Earth spinning on its axis

提示One full turn takes twenty-four hours — and the Sun is not the thing that moves.

为什么Our planet rotates once a day, carrying each place into sunlight and out again. Sunrise is really the horizon dropping away beneath you, not the Sun climbing up.

Eight

提示Pluto was demoted in 2006, which took the count down by one.

为什么Moons orbit planets, and the planets orbit the Sun. Pluto was reclassified as a dwarf planet because it has not swept its orbital neighbourhood clear of other lumps of rock and ice.

The northern half of the Earth is tilted towards the Sun

提示The planet leans at the same angle all year — in June that lean favours us.

为什么The tilt of 23.5° stays fixed as we orbit, so each hemisphere leans sunwards for part of the year. That gives a higher Sun and longer days, which is what makes the warmth.

We see different amounts of its sunlit side

提示Half of it is always lit; what alters is how much of that half faces us.

为什么The Moon takes about a month to circle the Earth, and sunlight always falls on one hemisphere of it. As our viewing angle shifts we see anything from a thin crescent to a full disc.

The Moon

提示Our nearest neighbour slides exactly between us and the star we orbit.

为什么A solar eclipse needs the Sun, Moon and Earth in a straight line with the Moon in the middle, casting a narrow shadow that races across the ground. In a lunar eclipse the Earth sits in the middle instead.

Mass

提示The amount of stuff in an object, unchanged wherever you take it.

为什么Weight is the pull of gravity on that stuff and is measured in newtons, so an astronaut weighs far less on the Moon while being made of exactly the same amount of matter.

gravitational field strength

提示On Earth this value is about 10 newtons for every kilogram.

为什么It explains why weight is your mass with a g attached: a 5 kg bag of shopping presses down on your hand with about 50 N here on Earth.

It becomes about six times smaller

提示The lunar pull is much gentler than the one you grew up in.

为什么The Moon's gravitational field strength is roughly 1.6 N/kg against Earth's 10 N/kg, which is why the Apollo astronauts bounced along. Their mass was exactly the same as on the launchpad.

A star

提示The same sort of thing you see as a pinprick at night, only vastly closer.

为什么The Sun is an ordinary middle-aged star that looks special to us only because it is roughly 250 000 times nearer than the next one along.

They are far further away

提示A torch across a field looks dim, though it is just as bright up close.

为什么Brightness drops off sharply with distance. Some night-sky stars are far bigger and hotter than the Sun, yet trillions of kilometres of space reduce them to pinpricks.

The Milky Way

提示On a dark night it shows as a faint pale band across the sky.

为什么It holds a few hundred billion stars, and our Sun sits in one of its spiral arms, well out from the centre. Billions of other galaxies lie beyond it.

Distance

提示It sounds like a stretch of time, but it is not one.

为什么A light year is how far light travels in a year — about 9.5 trillion kilometres. The nearest star beyond the Sun is just over four of them away, so we see it as it was four years ago.