When you’re new to stargazing, at some point, you will have looked up at the clear night sky. And probably felt some heady mix of awe, wonder, and confusion. A planisphere can help.
Even if you’re not feeling dwarfed by our infinite and expanding universe, there are so many stars, and planets, and far too many satellites, that you might wonder how you’re ever supposed to find all the amazing things you want to look at.
Luckily, along with our easy to follow posts on What’s Up in the Night Sky over on Astrotourism WA, you can learn how to find your way around the night sky, with a little patience and some help from a dedicated map: a planisphere.
What is a planisphere and what does it do?
An astronomy planisphere is a flat, circular star chart, usually made of flexible plastic or card, with a rotating oval window cut into it. And it’s your map of the stars.
The planisphere’s star chart bit shows the sky visible from your latitude, and the window reveals what bit of the sky is visible at the time you choose.
Then, all you have to do to use it is line up the current date (or the date you want to go stargazing) on the outer ring with the current time on the inner ring. The planisphere’s window shows you what’s up.
How to set up a planisphere
Turn the wheel until the date and time line up with either your current time/date, or when you plan to go out observing.
Note: most planispheres use the 24-hour clock, so 8pm is 20:00.
Once you’ve got your date and time aligned, hold the planisphere overhead, with the direction you’re facing at the bottom. If you’re looking north, the word NORTH should be at the bottom edge. If you’re looking south, SOUTH is at the bottom.
This might feel backwards, but we promise it makes sense when you try it. If you hold a map above your head and turn to face north, east is on your left, matching its real-life position.
Now the stars you see through that little oval window should be the stars in front of you right now. If you’ve set it to a future date, you’re going to need to have to wait until it matches.
Why a planisphere's latitude is important
Before you rush out and buy a planisphere, you should know that planispheres are made for the different hemispheres and some even get as specific as latitudes.
The starry sky seen in our southern hemisphere has constellations in different positions from the northern hemisphere, and some can’t be seen at all from the other side of the planet.
Not only is a planisphere made for London wrong for Perth, but a planisphere made for Perth is less useful if you’re all the way down in Invercargill, New Zealand. That’s because while Perth sits at a latitude of around 32°S, Invercargill is more like 46.4°S.
Most planispheres are accurate from +5° to −5° of their design latitude: this means a planisphere designed for 30°S is still pretty accurate for Albany at 35°S and the 26th parallel on the northern border of South Australia and the Northern Territory.
Even in Broome, just south of the 18th parallel, you’ll get a slightly different stargazing experience than Perth. All of this to say, latitude is why some astronomical sights that are probably too low on the horizon for you to see from Perth are better seen at more northerly latitudes, and phenomena like the Aurora Australis are more clearly visible in southerly places like Hobart.
The moral is: use the right planisphere for your hemisphere, and the most accurate one you can get for your latitude.
How to look up from a planisphere to the sky
A hard part of using a planisphere is matching what’s on the page to what you see. If you start with one of the brighter objects or what you know, even if that’s only one star, or constellation, you can find almost everything else relative to that.
Another tip for bridging that planisphere-to-sky gap is a truth universally acknowledged in stargazing. Let your eyes adapt to the dark to reach their peak sensitivity, and use a red (or orange) torch so you don’t lose your night vision.
Also, save using the edges of the planisphere until last. Because of the planisphere’s design, the oval window cuts off near the horizon, and stars that are close to the horizon are harder to see if you don’t have dark skies and an unobstructed view.
Our advice: start near the centre of the window, where the stars are higher in the sky and easier to spot.
Once you know your way around the night sky, you might want to check outone of Quasar Astronomy’s downloadable resources (like its yearbook), or find a good star atlas (one hat goes into more detail than a Planisphere).
We also recommend some good mobile apps for stargazing, (making sure you use the app or your phone’s night mode so you don’t ruin your night vision), and you can use Stellarium on your laptop before going outside, or as a mobile app for real-time viewing.
How to read the sky by yourself
Our southern hemisphere sky has many advantages for learning how to read it.
Milky Way season, when the Galactic Centre is high in the sky, is between mid-autumn and mid-spring. The time from April to August is generally a sweet spot for dark skies, earlier evening viewing, and lower humidity. It’s hard to miss or mistake the Milky Way, so you’re already on your way.
The Large and Small Magellanic Clouds (often abbreviated LMC and SMC) are satellite galaxies near ours. You can see both the small and large galaxies with the naked eye in the southern hemisphere, and they’re not visible at all in the northern hemisphere.
Most famous of all the southern sky’s sights is the Southern Cross asterism, in the Crux constellation. If that sounds confusing, remember an asterism is just a group of stars forming a pattern or shape that isn’t a constellation.
The Southern Cross is a key landmark that most of us have learnt, even if we don’t know many others, because if you can find it, you can find south. In more southerly parts of Australia, Crux is circumpolar (meaning it’s visible all night, every day of the year).
The two bright stars beside Crux are the Pointers, Alpha (α) Centauri and Beta (β) Centauri. They’re appropriately named because of how they point toward the Southern Cross, helping you know you’re looking at Crux, rather than another vaguely cross-shaped asterism.
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