An Introduction to Circumpolar Constellations
Did you know that the night sky above our heads has poles, just like we do on Earth?
There are two celestial poles, and, just like on Earth, they are the north celestial pole and the south celestial pole.
Polaris is a star you may have heard of, more commonly called the pole star. Why is Polaris called the pole star?
If you were to stand at the north pole in the Arctic and looked directly up at night, Polaris is the star you'd be looking at. It marks the point in the sky which is directly above the north pole, known as the north celestial pole.
The south celestial pole is the point in space directly above Earth's south pole.
For northern hemisphere observers, Polaris holds a special position in our night sky because it appears not to move at all. Instead, all the other stars appear to rotate around it.
Constellations which rotate around Polaris without falling below the observer's horizon are called circumpolar constellations. They're in the sky every minute of every day of the year, even when the sun is up... we just can't see them then!
Since they never set, circumpolar constellations make a great starting point for learning your way around the night sky.
What you may not realise is that the number of constellations you see as circumpolar depends on your location.
(By the way, you can learn how to take a picture of stars like the one above by clicking the link.)
How to Work Out if a Star is Circumpolar
You've seen that Polaris is directly overhead at Earth’s north pole, but the further south you are, the lower Polaris appears in the sky.
At Earth's equator Polaris skirts the horizon.
Cross the equator into the southern hemisphere and Polaris never rises. Instead you can now see the southern celestial pole above the horizon.
Keep travelling southwards and the southern celestial pole gets higher in the sky until, at the south pole in the Antarctic, it is directly overhead.
Calculation for Circumpolar Constellations
To calculate if a star is circumpolar in your location, you need two pieces of information:
- Your latitude on Earth, i.e. how many degrees north of the equator you are (click here to work it out)
- The declination of the star you're interested in
Step one is to take the latitude of your location away from 90°, which will give you a number between 0° and 90° (in the northern hemisphere).
As an example, Dallas, Texas is at latitude 33° north (approx). Take 33° away from 90°, and you're left with 57°.
Step two is to check the star’s declination against the number you got for step one.
If the star has a declination which is more than the number you got for step one, then it is circumpolar. If its declination is less than step one's number the star is not circumpolar at your location.
So, for an astronomer in Dallas, any star with a declination greater than 57° is circumpolar in the skies above Dallas.
A star whose declination is less than 57° will set below the horizon for at least a part of its journey around Polaris. Vega, for example, has a declination 39° and so sets below the Dallas horizon, whereas Dubhe with a declination 62° never does, which means Dubhe is circumpolar if you're observing from Dallas.
Try this for yourself using a planisphere, where you can see the constellations which are circumpolar at your latitude.
The simple rule is: the further north you live, the more stars you'll see as circumpolar. At the north pole the whole sky is circumpolar, whilst none of it is at the equator.
You may now be asking 'which constellations are circumpolar from my state?' Well, let’s find out!
What are the Five Circumpolar Constellations?
The five circumpolar constellations are the ones closest to the celestial north pole. In order of declining declination, they are: Ursa Minor (Little Dipper), Cepheus, Camelopardalis, Draco and Cassiopeia. These five constellations are circumpolar in every state except Hawaii and Florida.
The next constellations in order of descending declination are:
- Ursa Major (Big Dipper) - See table below
- Lynx - See table below
- Lacerta - See table below
- Perseus - Only circumpolar in Alaska, North Dakota, Washington, Montana and Minnesota
- Cygnus - Only circumpolar for Alaska, North Dakota, Washington and Montana
Use the chart below to see which constellations are circumpolar from your state.
**In Hawaii, Draco is also not circumpolar
Using the Five Circumpolar Constellations to Navigate the Night Sky
Polaris is the brightest star near the celestial north pole (but only the 48th brightest in the night sky) and your starting point for night sky exploration.
To learn your way around the night sky, begin with the Polaris and work outwards to the five circumpolar constellations.
How to Find Polaris in the Night Sky
Thankfully, the pole star is really easy to find. Just follow these simple steps and you'll see it in no time:
- Face North - wherever you are in the northern hemisphere, Polaris is due north
- Find the Big Dipper and use the 'pointer stars' Merak and Dubhe (click the image to zoom).
The imaginary line starting at Merak and running through Dubhe will lead you to Polaris, which is the next brightest star.
- The distance from Dubhe to Polaris is about five times the distance between Merak and Dubhe.
- If you’re in the mid or southern states, the Big Dipper might not be visible for you tonight. Instead, use his little friend, Ursa Minor, a.k.a The Little Dipper.
- The Little Dipper never sets for anyone living in north America. It's in the sky even during the day... we just can't see it against the brightness of the sun!
You can see in the picture above that the Little Dipper looks like a smaller, rotated version of the Big Dipper.
- Polaris is the last (and brightest) star at the end of Little Dipper's 'handle'
Don't be too harsh on yourself if you don't see Camelopardalis (the giraffe) though.
Its brightest star is only magnitude 4, so it's hard to see against the general background, especially in a light-polluted area.
Interesting Objects In the Five Circumpolar Constellations
You'll never see planets against the backdrop of the circumpolar constellations. The route planets follow across the sky 'the ecliptic', does not pass through any of them.
Yet, there are a handful of Messier objects and double stars to use as year-round telescope testers.
Circumpolar Messier Objects
Cassiopeia and Draco contain the only three Messiers of our circumpolar constellations. You can have a go at finding them on any clear, dark night of the year!
- M103 Open Cluster, mag 7.4, Cassiopeia
- M52 Open Cluster, mag 5.0, Cassiopeia
- M102 Spiral Galaxy, mag 9.9, Draco
Circumpolar Double Stars
Double Stars are a great first step with a telescope and what better ones to find first than those that never set!
Below are a selection of the brightest double stars in circumpolar constellations.
Double Stars in Ursa Minor (Little Dipper) and Camelopardalis
- In Ursa Minor, Polaris itself is a double star. 50x magnification should be enough to resolve the respective bright and faint stars.
- Beta Cam is a binary star, which is two stars orbiting each other. (Double stars just appear close in our skies but could be thousands of light years apart)
Double Stars in Draco
- 16, 17 Draconis easy to separate in binoculars. Can you see the triple star through your telescope?
- Mu Draconis (known as Alrakis) are identical white stars that 120x magnification will resolve
- Nu Draconis also needs a telescope to separate its two white stars
Double Stars in Cepheus
Double Stars in Cassiopeia
Cassiopeia has a wealth of objects to see and is worthy of its own post in the future. For now though try and see:
- Shedar (alpha Cas) is a double star whose primary has an orange color through a telescope. There's a big gap between its primary and secondary stars. The secondary is relatively bright mag 8.9, so is a simple telescope find
- Eta Cas is a binary star. One is a yellow mag 3.5, the other a red mag 7.5
Summary of Circumpolar Constellations
Now you know exactly what a circumpolar constellation is, find out which you can see from your state.
Try to learn these constellations by eye (we'll let you off Camelopardalis though) to help your voyage around the night sky.
To bring more enjoyment to that discovery, find their Messier objects and double stars.
They're available to you whenever the sun is down and the clouds are gone... so there's no excuses!
Product images sourced from Amazon.com