Lines Drawn Between Old Fires
In the last lesson you watched the whole sky turn around one quiet point in the north, like a great wheel. Tonight we stop watching it turn and start learning its streets. A city is easy to get lost in until you know a few landmarks. Once you know them, every other street makes sense. The sky works the same way, and the landmarks are patterns of stars.
People have been joining up the stars for thousands of years. Shepherds, sailors and priests saw hunters, bears, scorpions and queens, and they told stories about them by firelight. Each culture drew its own pictures, so by the early twentieth century the sky was a muddle. Neighbouring charts disagreed, and nobody could say for sure where one figure ended and the next began.
The Sky Gets Its Borders
The International Astronomical Union is the world body that settles names in astronomy, and it sorted out the muddle. In 1922 it agreed an official list of constellation names. The Belgian astronomer Eugène Delporte then drew precise boundaries, which the Union adopted in 1928 and published in 1930. The result was 88 official constellations. Think of them less as pictures and more as countries on a map. Every star, and every patch of dark between the stars, now belongs to exactly one of them, just as every field on Earth lies inside some country.
This is the first big idea of the lesson. An official constellation is a whole region of sky with fixed borders. Ursa Major, the Great Bear, is one. It is large, and most of its stars are faint and hard to see.
Patterns Within Patterns
Inside the Great Bear lies a shape you can spot at once: seven bright stars making a saucepan with a bent handle. In Britain it is called the Plough, and in North America the Big Dipper. It is not a constellation. It is an asterism, a familiar pattern of stars that has no official status. If a constellation is a country, an asterism is a famous landmark inside it, or one that sits across a border.
The Summer Triangle shows that second case well. Three bright stars, Vega, Deneb and Altair, make a huge triangle that is easy to see on summer evenings and that stays in the western sky well into autumn. But each star belongs to a different constellation. Vega is in Lyra the Lyre, Deneb in Cygnus the Swan and Altair in Aquila the Eagle. The triangle is a landmark that three countries share, and it belongs to none of them.
Southern-hemisphere skywatchers have the opposite case. The Southern Cross really is an official constellation, called Crux, and it is the smallest of all 88. Two brilliant neighbours, Alpha and Beta Centauri, point the way to it and are known as the Pointers.
The Art of the Hop
Once you know a landmark, you can use it to find others. Skywatchers call this star-hopping. You move from a star you know to one you don't, a jump at a time, the way you might give directions from a church to a pub and then to the post office.
The Plough is where it begins. You may already know that its two end stars, Dubhe and Merak, point up towards the Pole Star. Now try the handle. Its three stars bend in a gentle curve. Follow that curve away from the bowl, keep its bend going, and your eye lands on a bright orange-tinged star. That is Arcturus, the brightest star in Boötes the Herdsman. Carry the curve on in the same direction, then straighten it like a spear thrust towards the horizon, and you reach blue-white Spica in Virgo the Maiden. Generations of stargazers have remembered the route with a little rhyme: follow the arc to Arcturus, then drive a spike to Spica. This pair is best on spring evenings.
On winter nights another landmark rules the sky. Orion the Hunter stands tall, and across his middle three bright stars sit in a short, nearly straight row. That row is Orion's Belt, another asterism. Follow the belt down and to the left, towards the southeast, and you reach Sirius, the brightest star of the night sky, blazing in Canis Major, the Great Dog. Now go back to the belt and follow it the other way, up and to the right. It leads to a reddish star, Aldebaran, the glaring eye of Taurus the Bull. Keep going a little further and you meet the Pleiades, a tiny misty cluster of stars. In the southern hemisphere Orion appears upside down, so the directions flip, but the belt still links Sirius and Aldebaran in one line.
Your Pocket Planetarium
You do not have to learn all this from paper charts. Free planetarium software such as Stellarium, which runs on a computer or in a web browser at no cost, draws the sky for your exact place and time. Many phone apps do the same and will show the view in whichever direction you hold the phone. Search for a name such as Boötes or Cygnus, and the app shows you where it is, with lines joining its stars.
The skill comes in what you do next. Notice which bright stars sit near your target and which way the app is facing. Then turn the screen to its red night mode or dim it right down, and give your eyes several minutes to adjust to the dark. Look up and find the same shape among the real stars. The app shows you the map, but your own eyes have to make the trip.
Tonight's Challenge
Go out on the next clear evening and find the Plough. Trace the arc of its handle as far as it will take you, and see whether Arcturus is still above the horizon. Then turn to the west and look for the three bright corners of the Summer Triangle. If you are up late, wait for Orion to climb in the east and follow his belt to Sirius. Each hop you make is one that people have made, star by star, for thousands of years.
Key takeaways
- There are 88 official constellations, regions of sky with borders set by the International Astronomical Union and published in 1930.
- An asterism is a familiar pattern with no official status, such as the Plough inside Ursa Major or the Summer Triangle, whose stars come from three constellations.
- Follow the arc of the Plough's handle to Arcturus, then drive a spike to Spica.
- Orion's Belt points one way to Sirius and the other way to Aldebaran and the Pleiades.
- Find a constellation in a planetarium app, note the bright stars around it, then dim the screen, let your eyes adjust and match the shape to the real sky.