The other day I got an awesome question on Twitter from one of my lovely blog readers. He wanted to know if the North Star is always directly about our north pole.
Honestly, that’s a great question! If you also have a space question, feel free to send it on Twitter (tag: @Astronomicca). Then I can write about your question next time.
Great space question, @JonasDamith!Tweet
Okay. Let’s jump right in and see if we can figure this one out.
The North Star Has a name: Polaris
Let’s start by investigating what the North Star actually is.
The North Star is not the name of one particular star. Instead, it’s the title we use for whichever star is directly above the Earth’s north pole. Right now, it’s the star Polaris who has the title as the North Star. Polaris is short for Stella Polaris and it means “polar star” in Latin.
The reason for this name is of course it’s location: The star above the (north) pole. If we draw a straight line that starts at the northernmost part of Earth and just continues throughout space, we will eventually bump into Polaris (ouch!).
Due to the location of the North Star, all stars move across the night sky, except Polaris. The daily rotation of Earth will make it seem as if all stars move in a circle around Polaris. Those of you who have done night sky photography will know that when the exposure time goes above some 15-ish seconds, then stars will start to leave trails and hence appear as lines instead of dots. This is because Earth rotates around its own axis. However, if you photograph Polaris it will just stay put and not move at all.
You can see Polaris with the naked eye. If you locate the Ursa Minor or Little Bear constellation (Danish: Lille Bjørn, den ligner en mini-udgave af Karlsvognen) and extend the arm you will find Polaris at the end.
The North Star Changes – But Perhaps Not Like You Think
So, back to Jonas’ question. Is Polaris always the North Star? Does Polaris always have this glorified title?
The short answer is…
Polaris is not always directly above the Earth’s north pole.
It’s tempting to think that it’s a one-time-thing. That, maybe, because of how Earth moves through space Polaris is our North Star right now, but it never was in the past and it never will be again in the future. Well, think again. Polaris is the Earth’s North Star every 26,000 years and to understand why this is we need to take a close look at the Earth’s wobble.
Earth Has A Wobble – But What Is A Wobble?
You read that correctly. Earth has a wobble. Earth is wobbling right now as you are reading this, however at a low frequency so no one notices.
This wobble is essentially caused by the changing direction of Earth’s rotation axis. Okay, brain-explosion. Let’s look at a drawing.
Earth’s rotation axis moves in a circle and hence creates this sort of “spinning top”-like motion. It takes Earth 26,000 years to complete one circular motion, and therefor Polaris has the North Star-title once every 26,000 years.
It happens to be so, that in this circular orbit directly across from Polaris we find the bright star Vega. Because Vega is also on the path of Earth’s axis’ circular motion, it has the magnificent consequence that in 13,000 years Vega will be the new North Star.
So, dear Jonas, to answer your question: the North Star is not always the North Star. Sometimes it’s Vega. But next time that happens, in 13,000 years, we will not be around to experience it.