Space and gravity

15 degrees

提示Divide a full circle by the number of hours in a day.

为什么A full rotation is 360°, and 360 ÷ 24 = 15. That is why time zones are roughly 15° of longitude wide: the Sun reaches its highest point an hour later for every 15° you travel west, which is also how sailors once used a clock to work out their longitude.

It would fly off in a straight line

提示With no sideways pull, nothing is left to bend its motion.

为什么The Sun's gravity constantly bends each planet away from the path it would otherwise take, and that is what curves an orbit into a near-circle. Remove the pull and the planet would simply carry on at a steady speed in whatever direction it was already moving. The same reasoning explains why the Moon keeps circling the Earth rather than drifting away.

Northern, towards, longer — a higher Sun for our half of the world

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.

Its orbit is tilted, so the shadow usually misses the Earth

提示The three bodies must line up in height as well as left to right.

为什么The Moon's path is tilted by about 5° to the plane of the Earth's orbit, so at most new moons its shadow passes above or below us and nothing is seen. Only when the Moon crosses that plane at new moon does the shadow land on the surface. The shadow is a few hundred kilometres wide at most, which is why any one town waits centuries between total eclipses.

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.

About 8 minutes

提示Light covers 300 000 km every second, and the Sun is 150 million km away.

为什么The Sun is about 150 million km away and light travels at roughly 300 000 km per second, so 150 000 000 ÷ 300 000 gives about 500 seconds — a little over 8 minutes. You always see the Sun as it was 8 minutes ago. Light from the nearest star beyond the Sun, Proxima Centauri, takes just over four years to arrive; it is a small, dim red dwarf rather than a Sun-like star.

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.

About 25

提示Divide the separation by the width, keeping both in light years.

为什么2 500 000 ÷ 100 000 = 25, so our two galaxies sit about twenty-five galaxy widths apart. Galaxies are packed far more closely, in their own terms, than stars are: neighbouring stars stand millions of star widths apart. That is why galaxies collide and merge, as ours and Andromeda will, while the stars inside them almost never hit one another.

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.