Junior Astro toolkit

The Earth spins on its axis, from west to east

HintThe Sun is not the one doing the moving. Think about what our own planet does once every 24 hours.

The whyOne full turn takes 24 hours, and because we turn eastwards the Sun seems to sweep the other way, east to west. The same spin wheels the stars across the night sky and sets the time of day around the world.

The tilt of the Earth's axis (about 23.5°)

HintNot distance — in June, is the North Pole tilted towards the Sun or away from it? And in December?

The whyIn the UK's summer the northern half of the Earth leans towards the Sun: sunlight lands more directly, so it is more concentrated, and the days are longer. Six months later the same lean points us away. The distance idea fails a simple test — the Earth is actually nearest the Sun in early January, in the middle of our winter, and Australia has summer at exactly that time.

Mars

HintCount outwards past our own planet by one.

The whyMercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune — a favourite mnemonic is "My Very Easy Method Just Speeds Up Naming". The asteroid belt sits between the fourth and fifth planets. Pluto was reclassified as a dwarf planet in 2006, so there are eight planets, not nine — a distinction a quiz question can turn on.

rocky; gas; ice

HintPicture what the small worlds near the Sun are made of, then what the two biggest planets are made of, then what the two coldest, furthest ones are made of.

The whyNear the hot young Sun only rock and metal could stay solid, so the inner worlds are small and dense. Far out, where it was cold, Jupiter and Saturn gathered huge envelopes of hydrogen and helium, while Uranus and Neptune are built more from water, ammonia and methane — what astronomers call “ices” — hence the two different labels — hence the two different labels. The asteroid belt of leftover rubble lies between Mars and Jupiter. Scale matters too: Jupiter alone outweighs all the other planets put together.

We see different amounts of the Moon's sunlit half as it orbits the Earth

HintIt is not our planet's shadow. Picture a ball lit from one side and think about which part you can see as it goes round you.

The whyHalf the Moon is always lit by the Sun; what changes is how much of that lit half faces us. From new Moon (lit side turned away) it grows through crescent, first quarter and gibbous to full (lit side facing us), then shrinks back — a full cycle of about 29.5 days. Quiz tip: a waxing Moon in the UK is lit on the right, a waning Moon on the left.

once

HintThe Moon does spin — count how many turns per trip round us.

The whyBecause the spin and the orbit take the same time (about 27 days — the extra two days of the 29.5-day phase cycle come from the Earth moving round the Sun meanwhile), the same side always points at us. The far side is not a "dark side": it gets just as much sunlight as the near side, only we never see it from here — the first photographs came from a Soviet probe, Luna 3, in 1959. Try it: walk round a chair while always facing it, and count how many times you turn.

A solar eclipse

HintThe Sun's light is blocked for anyone standing in the Moon's shadow — so which body is being hidden?

The whyIt can only happen at new Moon, and a lunar eclipse (the Earth between the Sun and the Moon, the Earth's shadow on the Moon) can only happen at full Moon. They do not happen every month because the Moon's orbit is tilted about 5° to ours, so the three bodies rarely line up exactly. A total solar eclipse is possible only by a lucky coincidence: the Sun is about 400 times wider than the Moon and about 400 times further away, so the two look the same size.

60

HintWeight and the amount of matter in you are two different things, and only one of them depends on where you stand.

The whyMass is the amount of matter in an object, measured in kilograms, and it is the same everywhere. Weight is the pull of gravity on that mass, measured in newtons: weight = mass × g. On Earth g is about 10 N/kg (9.8 to be precise), so she weighs about 600 N; on the Moon g is about 1.6 N/kg, so she weighs about 96 N — roughly a sixth — while her mass is still 60 kg. Mars sits in between at about 3.7 N/kg.

It is by far the closest star to us

HintIt is an ordinary, middle-sized yellow one — the only special thing about it is where it sits compared with the rest.

The whyThe Sun is a fairly average star, one of hundreds of billions in the Milky Way. Its light reaches us in about 8 minutes; light from the next nearest star, Proxima Centauri, takes over four years. Every star you see at night is a distant sun, and many are far bigger and brighter than ours — they only look faint because they are so far away.

reflects

HintThink of a mirror, or of the Moon: neither one makes any glow of its own.

The whyStars are luminous — they make light by nuclear fusion in their cores. Planets and moons are non-luminous: we see them only by the sunlight bouncing off them, which is why Venus is the brightest object in the night sky after the Moon (it is close and covered in reflective cloud). A handy sky-watching clue: planets tend not to twinkle the way stars do, because they show as tiny discs rather than points.

The Earth's gravity keeps pulling it towards the Earth

HintA moving object goes straight unless a force bends its path. Which force acts on the Moon?

The whyPredict first: with no force, Newton's first law says the Moon would travel in a straight line at about 1 km/s. Gravity keeps tugging it sideways towards the Earth, and the straight path bends into a near-circle — an orbit is a fall that keeps missing the ground. The same picture holds for satellites: the International Space Station, about 400 km up, feels roughly 90% of the gravity you do, and circles the Earth every 90 minutes or so. Astronauts float not because gravity is absent but because they and their station are falling together.

A light-year

HintA DISTANCE, not a time — astronomers use it because kilometres get silly.

The whyLight covers about 300 000 km every second, so in a year it goes roughly 9.5 million million kilometres — far too big a number to write out, which is why astronomers use the light-year instead. Proxima Centauri is about 4.2 light-years away, the Milky Way is about 100 000 light-years across, and the Andromeda galaxy is about 2.5 million light-years off, so we see it as it was 2.5 million years ago.