Finding Jupiter or the Southern Cross is easy if you know where to look — and that's easy too once you know how the movement of the Earth affects our view of the sky.
If you live in a city you might struggle to identify anything in the night sky besides the Moon — all those stars can look pretty random.
So how do you make sense of what's going on up there? It helps to know how living on a big spinning planet affects what part of the sky we see at any one time.
Once you know what the Earth is up to, the movement of the stars and planets over one night or an entire year starts to make sense.
You'll know where to look for planets and constellations, when to look for your star sign, and why cities on the same latitude see the same stars at night.
::::::::Which way do the stars
move across the sky?:::::
You can gaze at the night sky for five or 10 minutes and nothing much will happen. But if you could speed the whole night up and watch the sky in fast motion, you'd see the stars moving across the sky as one.
Stars rise in the east and set in the west, just like the Sun and Moon do. It's because the Earth spins from west to east, so everything in the sky comes into view as we spin towards it and leaves our view as we spin away from it.
But depending on where you are, not all stars rise and set. If you're in the southern half of Australia, looking south at the sky in fast motion, the stars there (including the Southern Cross) go around the sky in a circle.
For example in April, the Southern Cross is on its side in the early evening, but it becomes more upright towards midnight, like in the animation below.
The stars aren't really circling a point in the sky — we are. The point is the South Celestial Pole, the spot in the sky that lies directly above the South Pole.
The Earth spins on an axis that runs from the North Pole to the South Pole — it's like we're turning on a giant imaginary skewer.
If you look up at the sky from the North or South Pole, you'll always see the stars going in circles overhead, never rising or setting at the horizon. At the North Pole the stars circle the North Star, which lies exactly where the "skewer" is pointing.
If you went to the equator you'd see something quite different. The equator lies at a right angle to the Earth's axis of spin (the "skewer"), so every star rises in the east, goes straight across the sky and sets in the west with mathematical precision.
The downside is you can't see the celestial poles or the stars circling around them because the rest of the planet blocks your view.
In between the poles and the equator — at latitudes like southern Australia — you get a bit of both. You're far enough south to see the stars circling the South Celestial Pole, but because you're not at a right angle to the axis of spin, the stars don't go straight overhead as they travel from east to west.
Instead they move further north as they rise in the east, and further south as they set in the west. In the Northern Hemisphere, the stars curve towards south as they rise.
::::::::Where should you look
to see the planets?:::::::::
The stars aren't the only things in the night sky — you can usually see a planet or two with the naked eye as well.
The planets (including Earth) all orbit the Sun in roughly the same plane. And that means from Earth it looks like the Sun and all the planets follow an imaginary line across the sky called the ecliptic.
That's great for sky watchers because if you know where the Sun travelled during the day, you know where to start looking for the five bright planets — Jupiter, Saturn, Mars, Mercury or Venus — at night.
But unlike the stars, planets don't follow a simple cycle. When they appear is dictated by their orbits — the bright planets take between 88 days (Mercury) and 29 years (Saturn) — and their position in relation to the Sun and the Earth at any given time.
So that means that even though Jupiter might appear in the evening sky in April this year, it won't appear in same place at the same time next month, or April next year.
Why do we see different constellations at different times of the year?
As well as the daily spin of the planet, the Earth's annual orbit around the Sun means we see slightly different sections of the universe from one night to the next. It also means we'll be back where we started in a year's time, looking at the same stretch of sky.
If you look at the sky from the same place after just six months — halfway through the Earth's solar orbit — some of the constellations will still be there, some new ones will have appeared and others disappeared.
The best way to get a handle on the regular rhythm of the stars appearing and disappearing is to look at the zodiac constellations.
:::::::::When can you see your
'. star sign' in the sky?:::::
There are 12 zodiac constellations — the ones we're all familiar with from astrology.
Like the Sun and the planets, the zodiac constellations all lie in the ecliptic, so they're visible from every place on Earth where you can see the sun rise and set. This explains why they feature so prominently in ancient cultures.
The zodiac constellations span the entire sky, so we never face all 12 of them at once.
If you look up on a dark night, you should be able to see at least four zodiac constellations at any one time, lying across the ecliptic. During the night, as the Earth spins, each of these constellations will sink in the western sky, while others will rise in the east. All up, if you were to watch the sky for an entire night, you'd see up to 10 of the 12 zodiac constellations. The few that you can't see are in the patch of the sky blocked out by the Sun.
So learning to spot the zodiac constellations, knowing what order they appear in (the same order given in horoscopes) and knowing where the ecliptic is, means you should be able to pick out an impressive four or more constellations that everyone's heard of.
If you've read a horoscope you might expect to see Aries prominent in the March/April sky, and Leo in the July/August sky and so on, but this isn't the case.
For example, you can't see Sagittarius in the night sky during mid-December because at this time of year the sun is positioned between Earth and the constellation, blocking our view. The same goes for the rest of the zodiac constellations; ancient astrologers assigned each zodiac sign to the time of year when the sun blocks that constellation from our view.
So your "star sign" is actually your "sun sign".
::::::Does everyone see the
same sky at night? :::::::
As well as the Earth's position in space, the area of sky we can see at night is determined by our latitude — how far north or south of the equator we are.
Places at the same latitude see the same view of the night sky. So while Adelaide and the Chilean capital Santiago are separated by the Pacific Ocean, they see the same constellations at night because the Earth is spinning them past the same stretch of southern sky.
But people living on the same longitude can see quite different skies.
Residents of Adelaide and Tokyo are on the same longitude, and see some of the same parts of the sky — the area around the ecliptic, like zodiac constellations and planets.
But Tokyo is too far north to see objects near the South Celestial Pole, like the Southern Cross. And Adelaidians can't see things in the far northern sky, like Polaris, the North Star.
It's because the earth is spherical, and the bulge around its middle blocks the north and south extremes from view.
:::::What things are only visible in the southern hemisphere?::::::
People in the Southern Hemisphere get an exclusive view of the Large and Small Magellanic Clouds — two of our neighbouring galaxies that can be identified with the naked eye. We also see some of the brightest globular clusters (spherical clumps of stars that orbit galaxies) such as 47 Tucanae in the constellation Tucana and Omega Centauri in the constellation Centaurus.
Dark sky features such as dark nebulae (clouds of interstellar dust that block light from the stars behind them) are also more prominent in the Southern Hemisphere. The darkest of the dark nebulae is a feature called the Coalsack. Tucked in near the Southern Cross, the Coalsack forms the head of our best known Indigenous constellation, the Emu in the Sky.
Get out there!::::::::
So now that you have the know-how, all that's left to do is to wait for nightfall and set up yourself up in a comfortable position under the heavens — and make sure to keep your eyes open! We can't promise that the sky won't continue to be a mystery, but hey, that's half the fun!
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