Astro Bob: Draco is waiting for you, let’s go hunting for dragons!

If you can see the Little Dipper even faintly, you are ready to meet the dragon. While not particularly bright, Draco’s form is so distinctive that it’s easier to find what one would expect. It also helps that he’s tucked between the two Dippers, which frame the constellation nicely. To locate the dragon use the pointers again, but this time stop at the very first star they point to – it’s the tip of Draco’s tail.

Use the two stars at the bottom of the Big Dipper bucket, called the pointers, to find the North Star, a good starting point for locating the Little Dipper. The star in Draco’s tail shines just to the right of the Dipper bowl. The dragon wraps around the Little Dipper, ending at the dragon’s head, located near Vega. (Stellarium)

Once you find it, move the tail up between the two bowls of the Dipper, then over and to the side of the Little Dipper. From there, shoot straight at a small group of diamond-shaped stars that outline the dragon’s head called the diamond. The rhombus – a narrow rhombus – is the brightest and easiest part of the constellation. Star patterns that are easy to recognize within a constellation or shared by two or more constellations are called asterisms. The highest stars of this asterism, named Rastaban and Eltanin, form the eyes of the dragon.

Coiled dragon Ladon looks spooky in this mythological depiction of a Urania Mirror star map published in 1824. Polaris is circled in red.  (Alexander Jamieson and Sidney Hall)

Coiled dragon Ladon looks spooky in this mythological depiction of a Uranian Mirror star map published in 1824. Polaris is circled in red. (Alexander Jamieson and Sidney Hall)

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Once you find Draco’s curvy outline, I think you’ll agree that with just a little imagination it really looks like a real dragon. How this beast came to inhabit the sky is an interesting story. . . or tales, in fact.

In Greek mythology, there is often more than one story behind the origin of a constellation. As part of one of his twelve labors, Hercules the strong man was to pick golden apples from a special tree given to Zeus’ wife, Hera. Ladon the dragon was guarding the tree. As Hercules distracted and then killed the dragon with an arrow, his fellow conspirator, Atlas, plucked out the apples.

When Hera learned of Ladon’s death, she placed him in the sky between the Dippers as a tribute. In another story, the Roman goddess Minerva killed the dragon in a great battle and threw it into the sky where it froze in place, still twisting, at the celestial north pole. Make your choice.

The only twist Draco is doing now is the slow spinning of the sky as the Earth spins on its axis. There are a lot of double stars and galaxies in the dragon, but we’ll focus on two stars of particular interest, Thuban (THOO-ban) and Kuma. The first is a discrete star of magnitude 4 located directly in front of the bowl of the Little Dipper.

Thuban, the third star in Draco's tail, was the pole star nearly 5,000 years ago when the pyramids were built.  Which star is at the celestial north pole changes due to precession (explanation below).  (Stellarium)

Thuban, the third star in Draco’s tail, was the pole star nearly 5,000 years ago when the pyramids were built. Which star is at the celestial north pole changes due to precession (explanation below). (Stellarium)

Five thousand years ago, when the Egyptian pyramids and Stonehenge were built, Thuban was the pole star. When your ancestors looked at the sky back then, it was their Polaris. And just like our pole star, Thuban stayed at one place in the sky as the northern constellations circled around him overnight.

We are very fortunate to have a bright and easy to spot North Star. But it hasn’t always been so, and it won’t necessarily be the case in the future thanks to something called precession. We all know the rotation and revolution of the Earth around the Sun. What is less known is that the planet is shaking like a top. The oscillation causes the axis of the planet to draw a circle in the sky over a period of 26,000 years. Our planet is slightly larger around the equator than around the poles. The combined gravitational forces of the sun and the moon on this equatorial bulge are responsible for its cyclical tilt.

The oscillation of the Earth, called precession, causes its axis to draw a circle in the sky over a period of 26,000 years.  The stars that are on this circle temporarily become pole stars.  Currently, Polaris is the pole star, but Errai in Cepheus will earn this title around 4000 AD.  In the distant future, 12,000 years from now, Vega will become the pole star.  (Left: NASA. Right: Tau'olunga / CC BY-SA 2.5)

The oscillation of the Earth, called precession, causes its axis to draw a circle in the sky over a period of 26,000 years. The stars that are on this circle temporarily become pole stars. Currently, Polaris is the pole star, but Errai in Cepheus will earn this title around 4000 AD. In the distant future, 12,000 years from now, Vega will become the pole star. (Left: NASA. Right: Tau’olunga / CC BY-SA 2.5)

You can think of the axis as a finger pointing towards the pole star. The oscillation points the finger in different directions around a circle. At present it points to Polaris, but from around 3900 BC to 1900 BC it was pointing to Thuban. Around 500 BC, he had approached Kochab, the shining star in the bucket of the Little Dipper, and was heading from there to Polaris.

Currently, the pole star is less than one degree from the celestial north pole, the direction in the sky that the Earth’s axis points to. The star will pass closest to this point in 2102, just 1/2 ° true north. Then the finger will move away from Polaris and point to the star Errai in Cepheus the King. Hang on. The axis will return to Thuban in 20,350 AD

After thinking about Thuban, take your binoculars and focus them on Kuma, also known as Nu Draconis, the weakest star in the Rhombus at magnitude 4.9. If you can’t see it because of light pollution or moonlight, just point the binoculars at the diamond, and it will be the star in the lower left.

Each star of the Rhombus asterism has a name.  Enjoy them all, but be sure to watch the double star Kuma through binoculars.  (King Bob)

Each star of the Rhombus asterism has a name. Enjoy them all, but be sure to watch the double star Kuma through binoculars. (King Bob)

Kuma looks like a single star to the naked eye, but splits neatly into two identical white suns separated by 62 arc seconds (1 / 30th of the diameter of the full moon) in binoculars. The distance between them is 1,900 times the distance from Earth to the sun, and it takes them about 44,000 years to orbit their center of gravity. This means that for each orbit the Earth performs (very) about two precession cycles.

So many things go round and round in this universe. Rather than turtles, it’s more like wheels all the way down.

“Astro” Bob King is a freelance writer for the Duluth News Tribune. Learn more about his work at duluthnewstribune.com/astrobob.

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