You may have noticed something interesting when staring at the night sky and admiring the stars. Careful observation reveals that some stars are always in the night sky. These stars are called circumpolar stars. What is a circumpolar star? Read on to find out.
These wonderful celestial objects move in a circular path around both the North and South Poles. They never disappear below the horizon. The photo above shows the north celestial pole around which circumpolar stars move.
This differs from the stars in the sky that rise and set in the sky each night. The motion of stars has been a cause of fascination for astronomers for centuries.
You’re in the right place if you’re curious about circumpolar stars. This article will explore all there is to know about these unique stars. You will learn what makes a star circumpolar and how to spot one in the night sky. We will also look at their importance throughout history.
Learning more about stars is a great way to deepen your appreciation of the night sky. Not only do you learn about the stars but you also learn about Earth’s rotation.
Circumpolar Stars: What Are They?
Circumpolar stars are a peculiar phenomenon, and chances are you’ve probably observed one when looking at the night sky. However, due to their slow rotation, it takes careful observation to recognise them for what they are.
These stars that never set are located near each of the Earth’s poles of rotation. Most stars move across the sky as the Earth rotates on its axis. They rise in the east and set in the west.
However, the closer the star is to one of Earth’s celestial poles the less it will move in the sky. Stars found exactly at the celestial pole will remain stationary in the sky.
Circumpolar Stars: Where are they Found?
In the northern hemisphere, the circumpolar stars in the sky are located close to the north celestial pole. Polaris is the most famous of these circumpolar stars. The star is located very close to the north celestial pole (you may be able to identify it in the image higher on the page or in the image below). Explorers and navigators have used Polaris for centuries to find true north. For more information on Polaris, you can see What Color Is Polaris?
In the southern hemisphere, the circumpolar stars in the sky are located close to the south celestial pole. Many famous circumpolar stars are in the south, including Alpha and Beta Centauri, The Southern Cross, and Achernar. Like in the northern hemisphere, they have played an important role for navigators and are part of many indigenous stories. Unlike in the northern hemisphere, no prominent star exists at the south celestial pole.
The height of a star above the horizon depends on two factors; your latitude and your location on Earth. If the observer is in the northern hemisphere, the closer they are to the North Pole, the higher in the sky the circumpolar star will appear. For the southern hemisphere the closer to the South Pole the higher they will be.
None of the stars seen from the equator are circumpolar. This is because the celestial equator (The projection of the equator onto the sky) intersects with the horizon at a 90-degree angle. As a result, all of the stars rise and fall each day.
Finding A Circumpolar Star
So, chances are you’re probably wondering how to spot some circumpolar stars for yourself. Despite what you might initially think, it’s a somewhat easy process. You just need to know how to do it!
Finding a circumpolar star first involves knowing basic information about the Earth’s rotation. You will also need to know the positions of some stars.
Polaris is the easiest circumpolar star to locate for those in the northern hemisphere. Polaris is situated incredibly close to the north celestial pole. Also, thanks to its brightness, it is quite easy to see. It is why it’s often used as a reference point for finding other stars.
The height of the celestial pole can be calculated if you know the latitude of your location. The angle between the horizon and the pole will be the same as your latitude. For example, if you are at 42 degrees latitude the angle between the horizon directly north of you (or south in the southern hemisphere) and the pole will be 42 degrees.
Locating the North Celestial Pole
To locate Polaris, you’ll need to find the Big Dipper. The Big Dopper is a well-known asterism (a group of stars that form a recognizable shape) in the constellation Ursa Major.
Once you’ve found it, look for the last two stars at the end of the Big Dipper’s bowl. These two stars point toward Polaris.
Locating the South Celestial Pole
Finding a circumpolar star in the southern hemisphere is a little more difficult. This is because the south celestial pole is harder to locate. Unlike in the northern hemisphere, there are no bright stars near it (Why are some stars brighter than others?).
The easiest way to locate the south celestial pole is to locate the Southern Cross. As the name suggests, the Southern Cross is a group of stars that form a distinctive cross shape. The longer axis of the cross points toward the south celestial pole. The pole is found around four times the length of the long axis away from the bottom of the Southern Cross. You can also imagine a line perpendicular to the line between the two ‘pointer’ stars (Alpha and Beta Centauri). Where this line intersects the line extended from the long axis of the longer Southern Cross axis is approximately the south celestial pole. This is shown in the image below.
How to find the South Celestial Pole
Off topic, but if you are in the southern hemisphere locate the Jewel Box in the Southern Cross and point your telescope at it. It is a wonderful sight.
If you want to make your life slightly easier, you can download a smartphone app. A good app allows you to locate stars easily. They will also allow you to located the celestial poles very easily. So, with this guide, you’ll spot circumpolar stars quickly!
Circumpolar Star Move in the Sky
It would appear that stars in the sky do not move from year to year. However, the celestial poles do move in the sky.
The axis that the Earth rotates on ‘wobbles’ cyclically over a period of hundreds of thousands of years. The movement is caused by Milankovitch Cycles.
The effect of this movement is that in the future Polaris will not be located near the north celestial post. Polaris will be the closest to the north celestial pole in 2100. After then it will drift further and further away. In about 20,000 years the star Thuban will be the north star.
Importance Of Circumpolar Stars In Culture And History
Throughout history, circumpolar stars have been a prominent part of various cultures. For example, Thuban was used in ancient Egypt to align the Great Pyramid of Giza with the true north.
Meanwhile, in many Polynesian cultures, circumpolar stars were used as a way of marking the seasons, as well as navigation.
The Inuit people of the Arctic region believed that the North Star was once a great hunter. They believe the great hunter could climb on a rope and become a star in the sky.
In the past, an astrolabe was used to locate objects in the sky.
Final Thoughts
Circumpolar stars are a unique and fascinating phenomenon that has captivated professional and amateur astronomers for years. They have had a part to play in many different cultures and civilizations around the world.
Hopefully, you now have an understanding of what makes a star circumpolar. With a little practice, you will easily identify circumpolar stars. This should increase your appreciation for the universe and night sky!
Hi, my name is Jason Anderson, and I am a Physics Professor. Ever since I was a kid, I’ve been fascinated with space, the universe, the moon, you name it. I spent hours and hours at the planetarium close to my hometown, wondering what else could be out in the universe.
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.
